Design and Development of Ligands for Palladium-Catalyzed Carbonylation Reactions

Dedicated to Professor Li-Xin Dai on the occasion of his 90^th birthday

Abstract

The palladium-catalyzed carbonylation reaction remains a challenging and significant research field in organic chemistry, and has emerged as a powerful and straightforward protocol for the preparation of various bioactive carbonyl compounds under quite mild reaction conditions. The achievements in this area are correlated to the design and development of versatile ligands that not only facilitate the catalytic transformation, but also provide additional control over the selectivity of the reactions. In this context, a variety of rationally designed ligands with different electronic and steric properties have been synthesized and applied in palladium-catalyzed carbonylation reactions in recent decades. This review focuses mainly on the strategy of ligand design and the results obtained with representative ligands that have different σ-donor properties in the intra- and intermolecular palladium-catalyzed carbonylation reactions of (pseudo)haloarenes with gaseous carbon monoxide and numerous types of nucleophiles. The current limitations and potential trends for further development of palladium-catalyzed carbonylation reactions are also highlighted.

1 Introduction

2 Phosphine Ligands

2.1 Monodentate Phosphines

2.1.1 Triphenylphosphine and Analogues

2.1.2 Di(1-adamantyl)-n-butylphosphine

2.1.3 Biaryl Monophosphines

2.2 Bidentate Phosphine Ligands

2.2.1 Alkyl-Bridged Diphosphines

2.2.2 Ferrocene-Based Bidentate Phosphines

2.2.3 Xantphos and Analogues

2.2.4 BINAP and Analogues

3 N-Heterocyclic Carbenes (NHCs)

4 Other Ligands

4.1 Nitrogen Ligands

4.2 Thiourea-Type Ligands

5 Summary and Outlook

Biographical Sketch

Introduction

The carbonyl group, as a component of carboxylic acids, acyl halides, esters, amides, ureas, ketones and aldehydes, is a useful and important linkage found in a wide variety of bioactive compounds, pharmaceuticals, dyes, and polymers, as well as natural and industrial products.[1] Various methods can be used to introduce a carbonyl group; however, in contrast to classic oxidation, Friedel–Crafts acylation and other reactions, transition-metal-catalyzed cross-coupling reactions not only represent a mild, practical and powerful tool for carbon–carbon or carbon–heteroatom bond formation,[2] but also constitute an atom-economic, efficient and straightforward approach for carbonyl group construction. Since Heck’s seminal reports in 1974,[3] significant efforts have been devoted to the development of transition-metal-catalyzed cross-coupling reactions for this purpose.[1] [4] Among them, the palladium-catalyzed carbonylation has emerged as an indispensable and verstile methodology for the installation of a carbonyl group in the presence of various additional functional groups (Scheme [1]), and it is considered to be one of the most challenging and intriguing research domains in organic chemistry. Furthermore, as an extensively utilized, easily available and inexpensive carbon source, the thermally robust and chemically reactive gas carbon monoxide (CO) has been regarded as the most unique building block for carbonyl group installation. Industry, in particular, has been working toward the development of catalytic protocols, with large-scale applicability in the synthesis of fine chemicals, that use carbon monoxide as a precursor.

Depending on the type of nucleophile (Scheme [1]), transition-metal-catalyzed carbonylation reactions can be categorized into alkoxycarbonylations, thiocarbonylations, aminocarbonylations and carbonylative cross-coupling reactions; however, in the case of non-oxidative carbonylation reactions, they all share the similar catalytic mechanism. The generally accepted catalytic cycle for palladium-catalyzed non-oxidative three-component carbonylation of (hetero)aryl halides or their analogues with carbon monoxide and various nucleophiles is depicted in Scheme [2]. The catalytically active palladium(0) species is generated from the catalyst precursor. Then the organic halide or its analogue undergoes the oxidative addition step with palladium(0) resulting in a palladium(II) intermediate A. The species A further coordinates with carbon monoxide, and migratory insertion leads to formation of an acylpalladium species B. The reactive intermediate B is susceptible to be attacked by a variety of nucleophiles; a reductive elimination follows, to yield the desired carbonylation product and regenerate the palladium(0) species for the next cycle.[5] Within this catalytic transformation, the most important process is the coordination with carbon monoxide: without it, the nucleophilic attack takes place on species A and the cross-coupling products formed by electrophiles and nucleophiles are obtained. However, as an excellent π-acceptor, the π* orbital of carbon monoxide is ready to interact with orbitals of the metal, where the electron may flow from the metal to the carbon monoxide ligand to form a π back-bond.[6] In this manner, the reactivity of palladium(0) towards oxidative addition is reduced owing to the strong binding ability of the carbon monoxide ligand. Moreover, in the presence of a large excess of carbon monoxide, clustering and agglomeration of palladium atoms may occur and non-­active palladium black might be generated.[7] All these possibilities are decisive factors in the interruption of palladium-catalyzed carbonylation reactions.

When a ligand with strong σ-donor property is used, the direct coordination between palladium(0) and carbon monoxide might be avoided, and the electron density on the palladium center might be further enhanced by the robust σ-bonding interaction. This kind of electron-rich ligand not only preserves the activity of palladium(0) intermediates, but also facilitates the further oxidative addition step to form intermediate A and the subsequent coordination with carbon monoxide. Therefore, in recent decades, significant effort has been devoted to the design and development of more general and powerful ligands for this purpose. Until now, a variety of versatile ligands have been synthesized and these have been shown to accelerate a large number of carbonylation reactions.[1] [4] To the best of our knowledge, the design and development of ligands for use in transition-metal-catalyzed carbonylation reactions has not been reviewed so far. In the case of palladium(II)-catalyzed oxidative carbonylation reactions, the oxidants may play a more important role than the ligands in the catalytic transformation, and Lei and co-workers contributed a review paper on this topic area recently.[8] Therefore, in this review we focus mainly on the development of various ligands in palladium-catalyzed carbonylation reactions.

Phosphine Ligands

Among the most utilized ligands in transition-metal-catalyzed coupling reactions, phosphines are the classic and popular choice to accelerate numerous catalytic transformations. In the presence of phosphines, the solubility of metal cations in various organic solvents is significantly enhanced, and the ligand also provides a systematic tunable approach for the catalytic activity of the metal center by altering its steric and electronic characters. For example, the phosphorus atom of R₃P easily coordinates to the palladium center forming a palladium–phosphorus bond, and the process is in dynamic equilibrium. When R groups with stronger electronegativity are attached, the electron density on the phosphorus atom is also increased. The palladium center is rendered more nucleophilic after coordination of an electron-rich phosphine, and this further accelerates the aryl halide oxidative addition step. In addition, phosphorus atom has low-lying empty d-orbitals, which are better able to accept the electron of the metal center and further stabilize low-oxidation-state palladium(0) species better than carbon monoxide (a π-acceptor). Furthermore, the steric properties of the phosphine ligands also play a significant role in the reactivity of the resulting palladium intermediates. When bulky phosphine ligands are coordinated to palladium, some of them easily dissociate since there is steric repulsion among the ligands; only one or two phosphines are still bonded to the metal center, leaving several coordination sites available for substrates. Moreover, the steric properties of the remaining ligands also have a strong impact on the subsequent transformation. In general, bulky phosphine ligands do, indeed, retard the oxidative addition step, but favor the formation of acylpalladium species B and the subsequent reductive elimination step to yield the coupling products.[9] Until now, various phosphine ligands have been applied in palladium(0)-catalyzed carbonylation reactions, and the design and selection of privileged phosphine ligands is of great importance.

Monodentate Phosphines

Triphenylphosphine and Analogues

As a relatively air-stable, colorless crystalline-type compound, triphenylphosphine is used in various organic transformations, especially in transition-metal-catalyzed coupling reactions. With regard to its nucleophilicity and reductive property, triphenylphosphine is ready to coordinate with a number of transition metals, such as palladium, nickel, rhodium, and ruthenium and so on. In the catalytic system containing triphenylphosphine and palladium, triphenylphosphine readily reduces palladium(II) and generates the active palladium(0) species, which can initiate the catalytic process. The nucleophilicity of triphenylphosphine enhances its coordination ability with the metal center, resulting in the formation of the robust palladium–phosphorus coordination bond, which is crucial in influencing the catalytic activity of the resulting catalyst. Taking advantage of these two important properties, triphenylphosphine exhibits excellent performance in palladium(0)-catalyzed carbonylation reactions and significant progress has been achieved in which intra- and intermolecular carbonylative reactions have been fully investigated.

With catalysts containing triphenylphosphine, a large number of heterocycles are readily accessible through intramolecular palladium(0)-catalyzed carbonylative coupling reactions.[10] Although palladium(II) acetate is a popular choice for forming catalytic species in situ with triphenylphosphine, other palladium(II) and palladium(0) sources, such as Pd(TFA)₂, [Pd(η³-C₃H₅)Cl]₂ and Pd₂(dba)₃, are also used as precursors with triphenylphosphine. Of course, several palladium complexes like Pd(PPh₃)₂Cl₂ and Pd(PPh₃)₄ can also be used according to the type of nucleophiles. As one of the most studied carbonylative reactions, the alkoxycarbonylation has received wide-ranging attention and has been applied as a model reaction for the investigation of many designed palladium catalysts.[11]

Halogen-containing saturated and unsaturated alcohols were selected by Ryu[12] and Cho,[13] respectively, as substrates in the palladium-catalyzed carbonylative cyclization to synthesize furanone derivatives (Scheme [3]). Using 10 mol% palladium(II) acetate and 20 mol% triphenylphosphine, Cho and co-workers realized a practical synthetic approach for accessing valuable unsaturated bicyclic furanones from bromo-allylic alcohols via carbonylative cyclization (Scheme [3, b]).

Furthermore, Cho and Kim reported a convenient protocol for alkylidenefuranone preparation by using a catalytic amount of Pd(PPh₃)₂Cl₂, and moderate yields were obtained (38–70%). A variety of cyclic and acyclic β-bromo-α,β-unsaturated ketones were all well-tolerated in this carbonylative cyclization.[14] Enols were also suitable nucleophiles in the intramolecular carbonylative reaction to construct monocyclic and bicyclic enones.[15] Chatani and co-workers described a cogent example of palladium-catalyzed carbonylative reactions by using [Pd(η³-C₃H₅)Cl]₂ and triphenylphosphine for the preparation of bicyclic unsaturated lactones in good to high yields (up to 90%; Scheme [4, a]).[16] In their report, the 2-pyridinyloxy group acts as a good leaving group to generate plausible acylpalladium complexes for the remaining carbonylative transformation, although the mechanism is still ambiguous. Interestingly, even protected phenols can be used as substrates for intramolecular lactonization.[17] Pd(PPh₃)₂Cl₂ was applied by Larock and co-workers in the synthesis of coumestans and their derivatives by way of an iodocyclization with acetoxy-containing 2-(1-alkynyl)anisoles; in this process, the acetoxy group is regarded as the nucleophile (Scheme [4, b]).[18] A number of biaryls were screened in the protocol and biologically interesting aromatic lactones were generated in very good yields (up to 98%).

In addition to hydroxy groups, amines constitute an important type of nucleophile in organic synthesis; therefore, aminocarbonylation is another relevant issue within the realm of this topic.[19] Trost and Ameriks provided a new synthetic approach to access the benzazocine core of FR900482 by way of carbonylative lactamization; an eight-membered heterocycle was constructed by using Pd(PPh₃)₂Cl₂ (Scheme [5, a]).[20] In 2008, Alper and Rescourio extended the aminocarbonylation to carbonylative insertion reactions. With 5 mol% Pd(PPh₃)₄, a series of 3-substituted 3,4-dihydro-2H-1,3-benzothiazin-2-ones were obtained with very high regioselectivity and good yields (56–95%); many different substituents, including primary, secondary alkyl and benzylic groups, were tolerated (Scheme [5, b]).[21] Unexpectedly, aziridines could also be used as nucleophiles in carbonylation reactions.[22] In the presence of Pd₂(dba)₃·CHCl₃ and triphenylphosphine, vinyl aziridines were easily converted into lactams in moderate yields (30–77%), as reported by Aggarwal and co-workers in 2010 (Scheme [5, c]).[23] The authors described an effective synthesis of β-lactams through palladium-­catalyzed carbonylative reactions from β-substituted ­unsaturated aziridines via an isomerization of prescribed lactams.

Similar to the intramolecular case, the intermolecular palladium-catalyzed carbonylation reaction is a fundamental transformation for constructing carbonyl derivatives with high efficiency and selectivity.[24] Among the different carbonylations, alkoxycarbonylation reactions have been studied extensively.[25] Alper and co-workers developed a highly efficient protocol for the preparation of linear esters by way of methoxycarbonylation of aryl olefins; in this case, the bulkier tri(p-tolyl)phosphine was involved (Scheme [6]).[26] The regioselectivity of linear ester products was relatively high (up to 100:0), and various olefins, such as aliphatic alkenes, allylic benzenes, and styrene derivatives, were all well tolerated. In the presence of borates and 5-chloroborosalicylic acid (5-Cl-SA), good to excellent isolated yields (60–92%) were observed. Furthermore, the protocol was readily extended to palladium-catalyzed methoxycarbonylation with ethylenes directly.[27]

In recent years, palladium-catalyzed thiocarbonylation with thiols or thiophenols has been considered as another hot topic and one of the challenging tasks in the field of transition-metal-catalyzed coupling reactions.[28] Alper and co-workers first realized dithiocarbonylation of protected propargyl by using Pd(PPh₃)₄ as a catalyst. The stereoselectivity of the protocol was relative high, and it was successfully extended to a ring-opening thiocarbonylation with thiols and carbon monoxide, and thus afforded the corresponding thioesters in moderate to excellent yields (36–92%; Scheme [7]).[29]

As mentioned above, aminocarbonylative reactions constitute an important issue in the realm of transition-metal-catalyzed reactions.[30] In the presence of palladium(II) acetate and triphenylphosphine, palladium-catalyzed intermolecular double carbonylation reactions with haloarenes and amines were accomplished.[31] Arylboronic acids, along with amines and alcohols, can also serve as nucleophiles in palladium-catalyzed cross-coupling reactions.[32] Ryu and co-workers performed the Suzuki carbonylation of alkyl iodides, producing various ketones in moderate to excellent yields (48–95%; Scheme [8, a]).[33] Notably, however, the alkyl aryl ketones may be produced via reductive elimination of the transmetalation intermediates formed by the acylpalladium species. Furthermore, an alkyl radical may be involved in the transformation. Diarylalkynones[34] and alkynyl aryl ketones[35] are indispensable compounds, and are also readily accessible by using palladium-catalyzed carbonylation reactions. By using Pd(PPh₃)₂Cl₂ and triphenyl phosphite, Beller and co-workers demonstrated the palladium-catalyzed carbonylative reaction of haloarenes with benzynes resulting in 1,4-diarylalk-3-yn-2-ones in moderate yields (45–58%; Scheme [8, b]).[36] In the presence of Pd(PPh₃)₂Cl₂, the carbonylation of alkyl iodides with terminal alkynes was further realized by Ryu and co-workers, with the resulting alkyl alkynyl ketones being obtained in moderate to good yields (40–88%; Scheme [8, c]).[37] These two protocols present efficient approaches to accessing numerous alkynyl ketones, which are key intermediates for diverse biologically active pharmaceuticals and natural products.

The steric and electronic properties of ligands have a significant impact on catalyst activity; hence, altering the substituent groups on phosphorus with different steric and electron-rich properties is a practical strategy to enhance the catalytic activities of the resulting palladium complexes. In addition to the aforementioned tri(p-tolyl)phosphine, other trialkylphosphine ligands, such as tricyclohexylphosphine and tri(tert-butyl)phosphine–­tetrafluoroboric acid complex, can also be used in palladium-catalyzed carbonylative reactions.[38] Using tricyclohexylphosphine and palladium(II) acetate, Beller and co-workers realized the first example of carbonylative Suzuki reactions in pure water (Scheme [9, a]).[38a] Under the optimized reaction conditions, a variety of arylboronic acids were well tolerated with benzyl chlorides and moderate yields were observed (41–78%) under carbon monoxide (10 bar) at 80 °C for 20 hours. Very recently, using a sealed two-chamber carbon monoxide generating system (CO_gen: CO was generated by 9-methylfluorene-9-carbonyl chloride in one chamber in situ, whereas the catalyst and substrates were in the other one) designed by� themselves, Skrydstrup and co-workers achieved a reductive carbonylation of aryl halides with stoichiometric carbon monoxide (Scheme [9, b]).[38b] In the presence of Pd(dba)₂ and tricyclohexylphosphine–tetrafluoroboric acid complex, numerous aryl aldehydes were obtained in moderate to excellent yields (45–98%), and diverse functional groups were well tolerated. Significantly, this carefully designed protocol provided an opportunity for isotopic labeling of the aldehyde group. When tri(tert-butyl)­phosphine–tetrafluoroboric acid complex and [Pd(cinnamyl)Cl]₂ were used, the same research group accomplished a palladium-catalyzed carbonylative Heck reaction of butyl vinyl ether with a variety of aryl halides to afford the 1,3-ketoaldehyde synthons in moderate yields (41–87%; Scheme [9, c]).[38c] The ¹³C-isotopic labeling of aryl methyl ketones was also realized by using the same protocol. Skrydstrup and co-workers successfully extended their work into palladium-catalyzed double carbonylations and the ¹³C-isotopic labeling of phenethylamines.[38d]

Di(1-adamantyl)-n-butylphosphine

As a convincing example of well-matched steric demand and electron-rich character, di(1-adamantyl)-n-butylphosphine (cataCXium^® A, or BuPAd₂)[39] has been optimized as a novel phosphine ligand with broad application not only in various cross-coupling reactions,[40] but also in palladium-catalyzed carbonylation reactions. Using palladium(II) acetate and BuPAd₂, Beller and co-workers reported a reductive carbonylation, even with less-reactive aryl bromides as substrates, under low pressure (5 bar) of carbon monoxide and hydrogen thanks to the strong σ-donor property of the ligand. This protocol constitutes an efficient and general formylation methodology to access a large number of aromatic and heteroaromatic aldehydes in moderate to excellent yields (57–99%; Scheme [10]).[41] With this inspired result in hand, the same research group extended the protocol to alkoxycarbonylation for the preparation of ketones.[42] Under the optimized reaction conditions, alcohols such as n-butanol reacted readily with various (hetero)aryl bromides and carbon monoxide, resulting in excellent selectivity and high yields (up to 99%; Scheme [11, a]).[42a] With similar catalytic systems, the carbonylation of bromoarenes with phenols also proceeded very well to access diverse diaryl ketones in moderate to excellent yields (50–98%).[42b] By using aryl boronic acids as nucleophiles, in 2008, Beller and co-workers further extended their protocol and realized a three-component Suzuki carbonylation to synthesize di­aryl and aryl heteroaryl ketones (Scheme [11, b]).[43] Even with 0.5 mol% catalyst loading, the protocol was tolerant of electron-rich, electron-poor as well as heterocyclic substituents on both sides of the substrates, which were converted into the products in moderate to high yields (45–89%). Furthermore, the application of the protocol in the synthesis of various bioactive compounds, such as Suprofen, was achieved.[43]

When terminal alkynes or amines were utilized as nucleophiles, the Beller research group accomplished Sonogashira carbonylation[44] and aminocarbonylation[45] in the presence of palladium(II) acetate and BuPAd₂. Furthermore, based on their earlier work, Skrydstrup and co-workers described a palladium-catalyzed carbonylative Heck reaction of aryl iodides with various styrene derivatives by using [Pd(cinnamyl)Cl]₂ and BuPAd₂ with their two-chamber system (Scheme [11, c]).[46] In this protocol, electron-rich aryl iodides performed better than their electron-withdrawing and heterocyclic analogues, and up to 81% yields of the desired α,β-unsaturated ketones were obtained; moreover, the protocol has potential application to isotopic labeling of chalcone derivatives. As an extension to this work, Beller and co-workers recently presented a palladium-catalyzed carbonylative coupling of aryl halides with various ketones to produce vinyl benzoates; a catalytic system comprising [Pd(cinnamyl)Cl]₂ and BuPAd₂ was used.[47] Under the optimized reaction conditions, a variety of aryl iodides and ketones were well tolerated and the corresponding products were obtained in moderate yields (44–88%; Scheme [11, d]). Notably, aryl bromides were also found to be suitable substrates for this transformation.

From a synthetic point of view, palladium-catalyzed carbonylation reactions provide access to various intermediates for the synthesis of bioactive compounds, pharmaceuticals, natural products and advanced materials. Beller and co-workers utilized the palladium–BuPAd₂ catalyst system to accomplish the synthesis of the anti-inflammatory drugs Ketoprofen and Suprofen by way of an efficient palladium-catalyzed carbonylative Suzuki reaction and subsequent hydroxycarbonylation with 3- or 4-(vinylphenyl)boronic acid as a nucleophile (Scheme [12]).[48] Similar catalytic systems were used in the syntheses of other important compounds, such as 2-arylbenzoxa­zinones,[49] 2-aryloxazolines,[50] ureas,[51] 2-aminbenzoxazinones,[52] 1,3-diketones,[53] quinazolinones[54] and brom­-hexine derivatives,[55] thus confirming the general utility of the palladium–BuPAd₂ and analogous catalytic systems.

Despite the significant achievements obtained with the use of ligand BuPAd₂, other substrates like allylic alcohols are considered as one of the most challenging and intriguing research tasks in the palladium-catalyzed carbonylative reactions. Recently, Beller and co-workers extended their work to this field, and reported the first general and efficient approach for the direct carbonylation of allylic alcohols with aliphatic alcohols.[56] Under the optimized reaction conditions, a lot of substituted primary aliphatic alcohols and allylic alcohols were readily transferred to β,γ-unsaturated esters with excellent regioselectivity and yields (46–82%; Scheme [13]). Xantphos was also found to be a suitable ligand for this purpose; this will be addressed in the section covering diphosphine ligands below.

Biaryl Monophosphines

Since the first report by Buchwald and co-workers,[57] very bulky and electron-rich mono[dialkyl(biaryl)phosphine]s, such as dicyclohexyl(2′-methylbiphenyl-2-yl)phosphine (MePhos, 1), dicyclohexyl[2′-(dimethylamino)biphenyl-2-yl]phosphine (DavePhos, 2), (2-biphenyl)di-tert-­butylphosphine (JohnPhos, 3), 2-dicyclohexyl(2′,​6′-​­dimethoxybiphenyl-​2-​yl)​phosphine (SPhos, 4), dicyclohexyl(2′,4′,6′-triisopropylbiphenyl-​2-​yl)phosphine (XPhos, 5) and dicyclohexyl(2′,6′-diisopropoxybiphenyl-​2-​yl)phosphine (RuPhos, 6) (Figure [1]), have been widely used in a great number of transition-metal-catalyzed carbon–carbon, carbon–nitrogen, and carbon–oxygen bond-formation reactions that can be carried out on kilogram scale.[58] The electronic and steric characteristics of these types of ligands are all favorable toward accelerating the rates of oxidative addition, transmetalation, and reductive elimination steps in the catalytic cycle of palladium-catalyzed carbonylative reactions. On the basis of their early outstanding contributions to this area, Alper and Zheng tried to explore vinyl ketones as nucleophiles and thereby realized the synthesis of functionalized enolic 2-acyl-3,4-dihydronaphthalen-1(2H)-ones with diethyl(2-iodo­aryl)malonates. In the presence of palladium(II) acetate and Xphos, the stereoselective carbonylation and subsequent cyclization were achieved in moderate to excellent yields (55–96%, Scheme [14]).[59]

With JohnPhos as catalyst, Alper and Zeng further achieved a highly efficient and practical protocol to synthesize 1,4-benzothiazepin-5-ones by using a similar strategy (Scheme [15]).[60] In the presence of 4 mol% palladium(II) acetate and JohnPhos, the reactions were carried out either in tetrahydrofuran with triethylamine or in dioxane with potassium carbonate under carbon monoxide (34.5 bar) at 100 °C. The corresponding heterocyclic products were isolated in good yields (up to 95%). Different electronic and steric properties of the substituents on various thiophenols and N-tosyl aziridines were all well tolerated.

In light of the superior performances of Buchwald’s mono[dialkyl(biaryl)phosphine] ligands in the palladium-catalyzed carbonylation reactions, other heterocyclic mono[dialkyl(biaryl)phosphine]s with sterically hindered and electron-rich properties have been designed and introduced in carbonylation reactions. In 2009, a series of novel bulky pyrrole- and imidazole-based monophosphine ligands (such as 7 and 8) were developed by the Beller research group (Figure [2]).[61] In the case of ligand 7, the phophorus and nitrogen atoms may both coordinate to the same palladium center and further affect the catalytic outcomes.[62]

With these novel heterocyclic ligands in hand,[61a] [62] [63] Beller and co-workers present a general and efficient formula to produce chalcones through the Heck carbonylative reactions with substituted styrenes in the presence of [Pd(cinnamyl)Cl]₂ and ligand 7 (Scheme [16, a]).[64] Under quite mild reaction conditions, isolated yields of up to 90% were observed with various aryl iodides and bromides containing substituents with diverse electronic properties. Moreover, the same catalytic system was applied in the straightforward synthesis of pyrazolines and pyrazoles in 38–79% isolated yields.[65] Later, the same research group also proved that the catalyst derived from palladium(II) bromide and ligand 8 was suitable for Heck-type carbonylative reactions of aryl bromides and vinyl ethers.[66] Under carbon monoxide (5 bar) and nitrogen (75 bar), various 1-aryl-3-alkoxyprop-2-en-1-ones were obtained in 30–75% yields, and the protocol tolerated other styrenes well. In addition, new double Heck-type carbonylative reactions of aryl halides and terminal alkenes with potential applicability in the selective synthesis of 4-arylfuranones were demonstrated (39–87%, Scheme [16, b]).[67] Unfortunately, when less active aryl bromides with stronger electron-withdrawing groups were used, only low conversions were observed.

Ligands like (2-aminophenylbisadamantyl)phosphine (DalPhos) developed by Stradiotto and co-workers are other important members of the family of bulky and electron-rich mono[dialkyl(biaryl)phosphine]s and also exhibit high catalytic activities in various transition-metal-catalyzed reactions.[68] By using palladium(II) acetate and the ligand (2-pyridinylphenyl-bisadamantyl)phosphine [Pyr-DalPhos (9); Figure [2]], Stradiotto, Beller and co-workers carried out aminocarbonylation reactions with gaseous ammonia (Scheme [17]).[69] After careful optimization of the reaction conditions, many bromoarenes were used in testing the substrate scope, resulting in isolated yields of up to 89% in the presence of both carbon monoxide (2 bar) and ammonia (2 bar). All kinds of heterocyclic substrates were well tolerated in this catalytic system. Besides ammonia, various primary and secondary amines all could be used as nucleophiles in the catalytic system to give moderate yields of corresponding amides.

Bidentate Phosphine Ligands

Although sterically bulky and electron-rich monophosphines exhibit very high activities to accelerate both palladium-catalyzed intra- and intermolecular carbonylation reactions with different nucleophiles, significant progress has been achieved.[1a] [4] [70] Unfortunately, in more challenging palladium-catalyzed carbonylative reactions like three-component-coupling carbonylative reactions of alkynes,[71a] thiols,[71b] [c] and inactive aryl bromides,[71d] [e] it is still difficult to achieve satisfactory outcomes using monophosphine ligands; this may be caused by intrinsic defects of monophosphines. Thus, diverse phosphine ligands have been designed to overcome this drawback. Among them, bidentate phosphines have received considerable attention and significant results have been achieved.[72]

In diphosphine ligands, two phosphorus atoms connected by a suitable linkage can chelate the same metal center (M), forming P–M–P bonds, which offer an additional opportunity for tuning the activities of the final catalysts. In 1990, Casey and Whiteker first introduced the concept of the ‘natural bite angle’ (the chelation angle of P–M–P), which is determined solely by the skeleton of diphosphine ligand.[73] The steric and electronic properties are still two important issues that influence the nature of ‘bite angle’, which dramatically affect the catalytic activity of the catalyst containing the bidentate phosphine ligands.[5] Therefore, the sensible correlation ‘nature’ of bite angle with the metal center provides significant insights into the design of bidentate phosphine ligands. Suitable configuration of the ‘natural bite angle’ imparts an appropriate dynamic chelating environment, which may further benefit the rate and selectivity enhancements of the reactions. In order to reach this goal, numerous new types of bidentate phosphine ligands with diverse linking groups have been developed in the past two decades and some of them exhibit high catalytic activity and selectivity in palladium-catalyzed carbonylative reactions. With a proper linkage, two phosphorus atoms can adopt a chelating geometry to minimize steric repulsion between the catalyst and substrates, and further accelerate the formation of acylpalladium species B (Scheme [2]). Furthermore, as an extra advantage of bidentate ligands, one phosphorus atom can dissociate from the metal and behave as a monodentate phosphine ligand. The freed-up vacant metal coordination site is then favorable for the subsequent transmetalation.[72b] According to the linker, four main types of bidentate phosphine ligand are covered in this review.

Alkyl-Bridged Diphosphines

As one type of the most utilized bidentate phosphine ligands, ligands like 1,3-bis(dicyclohexylphosphino)propane (dcpp; 10a), 1,3-bis(diphenylphosphino)propane (dppp; 10b) and 1,4-bis(diphenylphosphino)butane ­(dppb; 10c), have been studied extensively in the palladium-catalyzed carbonylation (Figure [3]). Due to their tunable flexibility (two phosphorus atoms are linked via alkyl chains of different lengths), good to excellent results have been achieved.[74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] The length of the alkyl chain and the electronic properties of the phosphorus substituents constitute two major influences on the activities of the catalysts.

Buchwald and co-workers demonstrated a general approach for the palladium-catalyzed aminocarbonylation of inexpensive and less active aryl chlorides by using bidentate phosphine ligands (Scheme [18]).[75a] In the presence of palladium(II) acetate and dcpp·2HBF₄, the reactions converted smoothly, and various desired amides were produced in moderate to excellent yields (65–98%) under 1 bar carbon monoxide. A series of electron-deficient and electron-rich chloroarenes were well tolerated in this protocol. Importantly, sodium phenoxide must be involved as a basic additive, as it plays a crucial role in this trans­formation. Based on this breakthrough achievement, ­Buchwald’s research group subsequently extended their methodology to the alkoxycarbonylation of more challenging substrates, such as inactive and water-sensitive aryl tosylates and aryl mesylates with various alcohols, and good to excellent yields (75–97%) were obtained.[76]

Ionic liquids (ILs) have attracted considerable attentions for their excellent characteristics, including non-volatility, high stability, and recyclability. In the case of palladium-catalyzed carbonylation reactions, ILs have been involved as recyclable reaction media in place of normal organic solvents and have provided excellent outcomes. Ye and Alper first applied ILs like 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide ([BMIM][NTf₂]) in the palladium-catalyzed carbonylation of 2-vinylphenols, 2-aminostyrenes, and 2-allylanilines to selectively synthesize five-, six- or seven-membered-ring lactones and lactams in the presence of Pd₂(dba)₃·CHCl₃ and dppb (52–94% yields, Scheme [19]).[77] Several other research groups successfully extended the protocol to the syntheses of 3-methylene-2,3-dihydro-1H-quinolin-4-ones,[78] various unsaturated seven-membered-ring lactams[79] and 2-diarylethanone.[80] With dppp, Li, Alper and Yu first presented a general and mild approach for the construction of 2-substituted acrylamides through palladium-catalyzed aminocarbonylation reactions in ILs (Scheme [20]).[81] In the presence of palladium(II) acetate, dppp and [BMIM][NTf₂], alkynes reacted with various primary and secondary amines to yield the desired amides in excellent regioselectivity and up to 85% yields. Notably, the catalytic system could be recovered and reused at least five times without loss of any catalytic activity. Applying dppp and [BMIM][NTf₂], Alper and co-workers also developed a nice example of palladium-catalyzed carbonylation of various alkynes and 1,3-diketones to produce highly substituted endocyclic enol lactones in moderate yields (32–74%).[82]

In 2008, Worlikar and Larock demonstrated a one-pot carbonylation process to access 2-substituted isoindole-1,3-diones by using palladium(II) acetate and dppp in toluene.[83] Beller and co-workers also applied [Pd(cinnamyl)Cl]₂ and dppp in the palladium-catalyzed carbonylative Heck reaction of aryl and alkenyl triflates with aromatic olefins under 10 bar of carbon monoxide (Scheme [21, a]).[84] With the optimized reaction conditions, a wide substrate scope was screened and various valuable chalcones were produced in moderate to excellent yields (40–95%). With benzooxazoles instead of olefins, Beller and co-workers accomplished a direct synthesis of (hetero)aryl ketones in the presence of [Pd(cinnamyl)Cl]₂ and dppp (Scheme [21, b]).[85] With this protocol, other heteroarenes reacted with a variety of iodoarenes possessing electron-donating and electron-withdrawing substituent groups. Although the yields were slightly lower (40–75%), the protocol constituted the first carbonylative cross-coupling reaction through C–H activation of non-preactivated heteroarenes. Recently, Xiao and co-workers realized the synthesis of carboxylic acid anhydrides via palladium-catalyzed carbonylative reactions with palladium(II) acetate and dppp under 1 bar carbon monoxide in N,N-dimethylformamide (Scheme [21, c]).[86] Various iodoarenes were well accommodated and isolated yields of up to 95% were obtained. Interestingly, half an equivalent of water was required in the reaction containing the water-sensitive anhydrides. Using their two-chamber system, Skrydstrup and co-workers described an example of palladium-catalyzed carbonylative α-arylation of aryl bromides with a variety of ketones in the presence of Pd(dba)₂ and dppp (Scheme [21, d]).[87] With 2.2 equivalents of sodium bis(trimethylsilyl)amide (NaHMDS), electron-rich, electron-withdrawing and heterocyclic substrates were all well tolerated by the catalytic system and afforded the corresponding 1,3-diketones in moderate to good yields (15–89%).

Ferrocene-Based Bidentate Phosphines

Another important type of bidentate phosphine ligands incorporate ferrocene. Since their discovery in the 1950s,[88] ferrocene and its derivatives have been widely investigated in almost all fields of organometallic chemistry. Because of their unique sandwich framework, they represent a new family of bidentate phosphine ligands, which play an important role in transition-metal-catalyzed cross-coupling reactions.[89] Importantly, this rigid sandwich structure imparts additional special characteristics on the diphosphine, thus further affecting the activity of the resulting metal catalyst. Firstly, the skeleton of ferrocene is adequately rigid to provide a suitable steric space environment for the metal to coordinate with ligands and substrates. Secondly, the cyclopentadienyl anion (Cp) rings with partial negative charge are easily modified by various donor groups, and further increase the electron density of the coordination P atoms, which can behave as better σ-donors. Thirdly, ferrocene is quite robust and the oxygen- and moisture-sensitivities of the resulting ligands are decreased. All of these properties of ferrocene-based bidentate phosphine ligands meet the requirements for palladium-catalyzed carbonylation reactions.

In consideration of their rigid ‘natural bite angle’ (P–M–P angle), a series of research projects were conducted to optimize the chelating angle of ferrocene-based bidentate phosphine ligands. To date, various 1,1′- and 1,2-disubstituted ferrocenyl phosphines have been synthesized and have revealed very good activities in palladium-catalyzed carbonylation reactions (Figure [4]). Beller and co-workers applied Pd(PhCN)₂Cl₂ and 1-[2-(dicyclohexylphosphanyl)ferrocenyl]ethyldicyclohexylphosphane (Josiphos, 11a) in the carbonylation of less active aryl chlorides (Scheme [22, a]).[90] After detailed optimization of the reaction conditions, better results (up to 91% isolated yields) were observed with 12a than with other flexible bidentate phosphine ligands with alkyl linkages. In the presence of Pd(PhCN)₂Cl₂ and dppf, the same research group accomplished the palladium-catalyzed carbonylation of unprotected bromoindoles with diverse nucleophiles (such as secondary amines, alcohols and even water) in moderate to excellent yields (67–99%, Scheme [22, b]).[91] With this defined protocol, a direct one-pot synthesis of amphetamine derivatives was also fulfilled. Haddad and co-workers utilized Pd(dppf)₂Cl₂ as a catalyst precursor and realized a convergent synthesis of the quinolone that constitutes the key substructure of the protease inhibitor BILN 2061, using palladium-catalyzed carbonylative cyclization reactions.[92]

Inspired by these exciting results, Cai and co-workers found that the palladium-catalyzed alkoxycarbonylation of inert aryl p-fluorobenzene sulfonates or tosylates proceeded very well when 1-[2-(diethylphosphanyl)ferrocenyl]ethyldi(tert-butyl)phosphane (11b) and palladium(II) acetate were applied (Scheme [23, a]).[93] The protocol was well tolerant of electron-rich and electron-poor aryl arenesulfonates with various alcohols and resulted in aryl carboxylic esters in moderate to good yields (61–96%). With microwave irradiation, Roberts and co-workers further described a simple and practical protocol for the preparation of various acyl sulfamides by way of palladium-catalyzed carbonylation in the presence of Pd(dppf) (Scheme [23, b]).[94] With 10 mol% catalyst loading, electron-rich, electron-poor and heterocyclic aryl halides reacted easily with a series of sulfamides and yields of up to 92% of the acyl sulfamides were obtained.

Subsequently, Beller and co-workers explored the aminocarbonylation of (hetero)aryl halides with tough ammonia gas as a nucleophile in the presence of palladium(II) acetate and dppf (Scheme [24, a]).[95] Under carbon monoxide (2 bar) and ammonia (2 bar), various primary amides were obtained in moderate to excellent yields (30–98%), and electron-rich, electron-deficient and heterocyclic substrates were well tolerated. Furthermore, the same catalytic system was applied successfully in the synthesis of phthalazinones via the carbonylative coupling of 2-bromobenzaldehydes with hydrazines and yields of up to 85% were found.[96] Recently, with dppf as the ligand, a novel and efficient palladium-catalyzed double carbonylation has been demonstrated by Li and co-workers (Scheme [24], b,c).[97] Under the optimal reaction conditions of carbon monoxide (40 bar) in tetrahydrofuran at 60 °C, various primary and secondary amines were investigated with indoles in the presence of a mixture of cesium carbonate and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU); the desired products were obtained in moderate yields (50–87%) after 36 hours (Scheme [24], b). With a slight alteration of the reaction conditions (change the base to potassium carbonate and increase the temperature to 100 °C under 1 bar CO), mono-carboxylation was also achieved in up to 90% yields tolerating the substrates containing electron-rich and electron-poor substituents (Scheme [24], c).

With bulky alkyl groups installed at the phosphorus atoms, electron-rich bidentate dppf analogues such as 1,1′-bis(diisopropylphosphino)ferrocene (diPrpf, 12b) and 1,1′-bis(di-tert-butylphosphino)ferrocene (dtbpf, 12c), are obtained (Figure [4]). Butler and co-workers introduced these ligands, and demonstrated their potential applicability in palladium-catalyzed carboxylation reactions.[98a] In 2012, Skrydstrup and co-workers presented a novel approach to access various 1,3-diketones in the presence of Pd(dba)₂ and diPrpf (Scheme [25, a]).[98b] Under the mild reaction conditions, electron-rich iodoarenes performed better than their electron-poor analogues and afforded 1,3-diketones in 65–94% yields. Heterocyclic substrates were also suitable under the optimized reaction conditions. Furthermore, the same research group extended their work to the palladium-catalyzed hydroxycarbonylation of (hetero)aryl halides with substoichiometric carbon monoxide in combination with an acyl–palladium(II) complex and ligand 12c (Scheme [25, b]).[98c] The protocol was tolerant of electron-rich and electron-poor as well as heterocyclic substrates to generate the corresponding products in moderate to excellent yields (53–98%). This work represented the first example of a palladium-catalyzed carbonylation with potassium formate and a substoichiometric amount of carbon monoxide generated by an acyl–palladium(II) precatalyst in situ. Notably, ligand 12c also played a crucial role in the transformation.

Xantphos and Analogues

From the results observed with ferrocenyl bidentate phosphine ligands, rigid backbones do enforce constrained geometry of ligands and reveal quite satisfactory outcomes. Other rigid heterocyclics like xanthene also constitute fascinating platforms to fabricate new types of bidentate phosphine ligands. In order to exploit their unique structural characteristics, various ligands such as bis[2-(diphenylphosphino)phenyl] ether (Dpephos, 13a) and dimethylbisdiphenylphosphinoxanthene (Xantphos, 13b) were developed (Figure [5]). Dpephos was first synthesized by the Taube research group in 1985,[99a] and since then has been widely applied in many transition-metal-catalyzed reactions. Roulland and co-workers described a new approach for the preparation of (Z)-α-chloroacrylates in the presence of Pd₂(dba)₃ and Dpephos (Scheme [26, a]).[99b] Under 1 bar carbon monoxide, the protocol was suited to various 1,1-dichloroalk-1-enes and produced the desired products with high chemo- and stereoselectivity, and yields of up to 91% were obtained. Subsequently, ­Skrydstrup and co-workers demonstrated a general protocol for the preparation of various thioesters by way of palladium-catalyzed thiocarbonylation in the presence of palladium(II) acetate and Dpephos (Scheme [26, b]).[99c] Under the optimized reaction conditions, (hetero)aryl iodides containing electron-rich and electron-poor substituents reacted readily with a number of thiols and yields of up to 99%, along with high chemoselectivity, were observed for the desired products.

In contrast to Dpephos, Xantphos, which was first synthesized by Haenel and co-workers in 1995,[100] is more rigid and exhibits higher catalytic activity in several cross-coupling reactions.[101] The same research group realized the palladium-catalyzed carbonylation of aryl triflates and aryl iodides with N-hydroxysuccinimide in the presence of palladium(II) acetate and Xantphos. Under the mild conditions, a series of N-hydroxysuccinimido esters were readily produced in isolated yields of up to 94% (Scheme [27, a]).[102] By using palladium(II) acetate and Xantphos, Buchwald and co-workers presented a convenient route to synthesize Weinreb amides through palladium-catalyzed aminocarbonylation under very low (1 bar) carbon monoxide pressure (Scheme [27, b]).[103] Bromoarenes with electron-poor or electron-rich substituents and other diverse functional groups were compatible and afforded the desired products in 65–97% yields. The same research group then expanded the substrate scope to those including many other nucleophiles, such as primary, cyclic and acyclic secondary amines, anilines and methanol.[104] Using the same catalyst precursor, Prandi and co-workers successfully realized the palladium-catalyzed aminocarbonylation of lactam-, lactone-, and thiolactone-derived triflates under 1 bar carbon monoxide, and yields of up to 94% were obtained.[105] Beller and co-workers then accomplished the palladium-catalyzed carbonylative Sonogashira reaction of aryl triflates with terminal aryl acetylenes by using [Pd(cinnamyl)Cl]₂ and Xantphos under carbon monoxide (10 bar) at 110 °C in toluene (Scheme [27, c]).[106] The substrate scope was generally broad, various electron-rich and electron-poor aryl triflates were well accommodated to afford moderate yields (50–83%) of the corresponding products, which can be further applied in the preparation of several bioactive compounds. By using Xantphos and [Pd(cod)Cl₂], Skrydstrup and co-workers recently developed a practical protocol to synthesize 1,3-diketones via palladium-catalyzed carbonylative α-arylation of acetylacetone with aryl bromides, and yields of up to 89% were observed.[107]

Besides reductive carbonylation, Xantphos was also applicable in the palladium-catalyzed oxidative carbonylation reaction, in which the direct activation of a carbon–hydrogen bond with palladium(II) and subsequent insertion by carbon monoxide are considered extremely challenging.[4] To date, significant progress has been achieved in this research area. For example, Huang and co-workers achieved a palladium-catalyzed oxidative carbonylation via non-directed C_sp ³–H activation by using Pd(Xantphos)Cl₂ as a catalyst precursor (Scheme [28, a]).[108] Under the optimized reaction conditions, ethanol and various benzylic substrates with diverse electronic and functional groups were suitable starting materials in the synthesis of a series of substituted 2-phenylacetic esters and analogues in 26–74% yields. The drawback of this protocol is that it was not applicable for sterically hindered substrates and only low conversions were observed in those cases examined. Alper and Zeng extended their work to palladium-catalyzed oxidative carbonylation reactions (Scheme [28, b]).[109] Under carbon monoxide (20.7 bar) and in the presence of Pd(MeCN)₄(BF₄)₂ and Xantphos, the direct palladium-catalyzed carbonylation of indoles with a variety of alkynes via C–H activation at the C₃-position was realized. Different linear α,β-unsaturated ketones containing indole fragments were produced in 36–77% yields. However, the transformation of alkynes with electron-withdrawing groups was completely inhibited.

BINAP and Analogues

Owing to the broad utility of asymmetric catalysts, including industrial applications in pharmaceutics, the design and synthesis of chiral ligands have significantly accelerated the development of this field in recent decades. Atropisomeric C ₂-symmetric phosphine ligands like 2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (BINAP, 14a) have attracted considerable attention and have been revealed to have wide-ranging utility in numerous transition-metal-catalyzed reactions (Figure [6]).[110] By using Pd(BINAP)Cl₂, Murry and co-workers demonstrated an example of palladium-catalyzed methoxycarbonylation of various heterocyclic chlorides (Scheme [29, a]).[111] Under the optimized reaction conditions, heterocyclic chlorides reacted smoothly with methanol and were converted into the corresponding alkyl esters in moderate to excellent yields (50–99%). In addition, substrates such as unprotected bromoanilines and bromoanisoles were also accommodated by the protocol and the desired products were obtained in good yields, which may be a reflection of the unique ‘natural bite angle’ of BINAP. With slight structural modification of the privileged skeleton of ­BINAP, a variety of novel atropisomeric bidentate phosphines were synthesized and also investigated in the ­palladium-catalyzed carbonylations. For example, [(5,6),(5′,6′)-bis(1,2-ethylenedioxy)biphen-yl-2,2′-diyl]-bis(diphenylphosphine) (SYNPHOS^®, 14b) was developed by Chan[112] and Genêt[113] for use in asymmetric transformations. Lei and co-workers first applied SYNPHOS^® in the palladium-catalyzed oxidative carbonylation of organoindium reagents with carbon monoxide and alcohols by using desyl chloride as an oxidant (Scheme [29, b]).[114] In the presence of Pd(MeCN)₂Cl₂ and SYNPHOS^®, a large number of alkyl and aryl indium reagents were investigated and the corresponding products were provided in moderate to good yields (60–96%).

N-Heterocyclic Carbenes (NHCs)

Although the development of various phosphine ligands has greatly accelerated progress in the field of palladium-catalyzed carbonylative reactions, their applicability is still hampered by the air-sensitive property of the phosphines. Furthermore, the π-acceptor character of phosphine results in strong bonding between carbon monoxide and the metal center, which might reduce the activity of the palladium–phosphine catalysts towards oxidative addition of aryl halides or pseudo-halides. Thus, the design and synthesis of new robust ligands remain intriguing research topics in the field of palladium-catalyzed carbonylation reactions.

As a strong σ-donors and weak π-acceptors, N-heterocyclic carbenes (NHCs) are robust ligands, and are widely used in transition-metal-catalyzed cross-coupling reactions.[115] Unlike phosphines, NHCs can act as powerful, neutral two-electron donors and readily form stable metal–carbon bonds. Generally, NHCs are strong σ-donors almost without metal-to-ligand π-back-bonding ability. Experiments and theoretical calculations[116] further indicate that NHCs are stronger σ-electron donors than most electron-rich phosphines. Therefore, NHCs significantly increase the electron density on the metal center, thereby facilitating the oxidative addition step and accelerating the catalytic transformation. In contrast to the phosphine ligands, which allow for variation through the substituents on the phosphorus atoms, NHCs provide a unique and independent approach to tuning their activity, in that the flanking nitrogen substituents and the size of the aromatic ring are responsible for the steric and electronic properties of the resulting NHC ligands.[117] Furthermore, the coordination topologies of phosphine and NHCs towards metal centers are completely different from each other. In general, when coordinated with metal, the three substituents on phosphorus form a cone-like geometry, away from the metal. However, the nitrogen substituents of NHCs can produce a pocket surrounding the metal center, thus exhibiting a much stronger influence on the activity of the resulting catalyst.

Since the pioneering work by Arduengo[118a] and ­Herrmann,[118b] considerable attention has been paid to the design and development of diverse NHCs and related metal complexes (Figure [7]). As the most extensively investigated examples, NHCs derived from imidazolium salts have been shown to be versatile in various cross-coupling reactions. Ryu and co-workers reported the first example of carbonylation of aryl iodides by using palladium–NHC complex 15a in a micro-flow system (Scheme [30, a]).[119] However, the substrate scope was relatively limited due to the rather small steric bulk around the palladium center, and only 72–92% isolated yields were observed for these few cases. Taking into account that the steric bulk around the palladium center would facilitate the reductive elimination step in the catalytic cycle, Nolan and co-workers developed Pd–NHC 15b in 2002.[115a] The isopropyl groups are considerably bulky and further increase the catalytic activity and applicability of the resulting catalyst.[115b] In the presence of complex 15b under 1 bar carbon monoxide, Orellana and co-workers accomplished the aminocarbonylation of aryl and alkenyl iodides with pyrrole in moderate to excellent yields (43–93%; Scheme [30, b]).[120] Although the pressure of carbon monoxide was relatively low, 10 mol% catalyst loading had to be used in the protocol. Furthermore, other amines were not investigated. Inspired by the design strategy represented by Pd–NHC 15b, Organ and co-workers introduced the bulkier Pd–NHC 15c, which also exhibited broad applicability in cross-coupling reactions.[121] Martin and co-workers first realized the palladium-catalyzed carbonylation of sterically hindered ortho-disubstituted aryl iodides with Pd–NHC 15c (Scheme [30, c] and d).[122] Under the mild reaction conditions, various aryl boronic acids were readily converted into biaryl ketones in moderate to excellent yields (33–98%). Furthermore, a carbonylative Negishi coupling of sterically hindered ortho-disubstituted aryl iodides with alkynyl nucleophiles was also explored and proceeded very well in the presence of Pd–NHC 15c, which offers a new synthetic approach for lutiolin.[123]

In comparison to the extensive studies on the steric bulk of NHCs, much less attention has been paid to their activity enhancement by alteration of the electronic properties.[124] Fusing additional aromatic rings to the imidazole is a practical way to adjust these properties.[125] Tu and co-workers developed a series of metal–NHC complexes and explored their potential applications in catalysis and material aspects,[126] and found that the less studied ylidenes obtained from benzimidazolium salts behaved differently. As a result of this, robust Pd–NHC 15e, incorporating a π-extended, sterically bulky imidazolium salt, was developed and was found to exhibit extremely high activities towards aminations and Suzuki–Miyaura cross-coupling reactions.[124] With allyl instead of pyridine as the throw-away ligand, Pd–NHC 15f was successfully applied in the aminocarbonylation of (hetero)aryl iodides with various amines under atmospheric pressure of carbon monoxide (Scheme [31]).[127] With catalyst loading at 0.5 mol%, the products were afforded in yields of up to 99%; the reaction tolerated diverse electronic and steric substituents on both substrates. Furthermore, a concise synthesis of the anticancer drug tamibarotene was accomplished by incorporating the same protocol.[127]

Other Ligands

In addition to the numerous phosphine and N-heterocyclic carbene ligands, there are still other types of ligands based on pyridines, oxazolines and thioureas that are also useful in palladium-catalyzed carbonylative reactions.

Nitrogen Ligands

There are only a few bidentate nitrogen ligands that have been used in palladium-catalyzed carbonylation reactions (Figure [8]). Among them, 1,10-phenanthroline (Phen, 16a) has been shown to be versatile in organometallic chemistry. Several research groups[128] have explored the activity of Phen in the palladium-catalyzed carbonylation. In 2008, Alper and co-workers successfully extended the protocol to include the reductive carbonylation of nitroarenes to urethanes, and yields of up to 96% were obtained (Scheme [32, a]).[128c] After their introduction by Evans and Corey,[129] C ₂-symmetric bis(oxazoline)s (Box1, 16b) have received considerable attention and are now regarded as versatile chiral bidentate ligands that exhibit excellent selectivity in various asymmetric catalyses.[130] In 2010, Kato and co-workers investigated the activity of Box1 in the palladium-catalyzed methoxycarbonylation of alkynols (Scheme [32, b]).[131] In the presence of Pd(TFA)₂ and Box1 under 1 bar carbon monoxide, various homopropargyl and propargyl alcohols were screened as substrates with methanol, and were successfully converted into the corresponding lactones in 73–94% yields; this is indicative that the protocol is applicable in the synthesis of several important natural products, such as dihydrokawain, tetronic acids, and β-methoxyacrylate antibiotics. Inspired by the C ₂-symmetric bis(oxazoline)s, Sasai and co-workers developed a spirobis(isoxazoline) and derivatives.[132] They then used spirobis(isoxazoline) ligand i-Pr-SPRIX (16c) and [Pd(MeCN)₄](BF₄)₂ to carry out an enantioselective intramolecular oxidative aminocarbonylation of alkenylureas (Scheme [32, c]).[133] A variety of alkenylureas were readily converted into the desired products in 30–89% yields under mild reaction conditions (1 bar CO, –40 °C).

Thiourea-Type Ligands

Concerning the strong binding ability of sulfur to transition metals, Yang and co-workers developed several thiourea ligands, based on the benzimidazolethione skeleton, for application in palladium-catalyzed reactions (Figure [9]).[134] In the presence of thiourea 17a and palladium(II) acetate, palladium-catalyzed Suzuki carbonylative reactions were accomplished, and various arylboronic acids were successfully coupled with aryldiazonium salts or aryl iodides and resulted in high yields (up to 98%) of the desired ketones (Scheme [33]).[134a] With these results in hand, the same research group then developed bidentate N,S-ligands 17b and 17c. However, only moderate activity was observed in the palladium-catalyzed alkoxycarbonylation reactions with these ligands.[134b] [c]

Summary and Outlook

In summary, this review outlines the achievements in palladium-catalyzed carbonylation reactions by presenting various representative rationally designed, versatile ligands with diverse electronic and steric properties. In combination with the generally accepted mechanism of transition-metal-catalyzed carbonylation reactions with gaseous carbon monoxide, the strategy and principle of ligand design to accelerate the transformation and avoid the other side reactions are explicitly illustrated. The recent achievements in several categories of ligands, including monophosphines, bidentate phosphines, NHCs, as well as other nitrogen-containing and thiourea ligands, along with their potential applications, are compiled to support these arguments. Except for the phosphine ligands, the examples of palladium-catalyzed carbonylation reactions with robust ligands are still limited; in particular, there is no example of these ligands being used for palladium-catalyzed oxidative carbonylation reactions. All these are indicative of the immature situation of this area at the current time. In addition, the fact that high carbon monoxide pressure, high catalyst loading, and tedious air-sensitive handling are required in most cases reveals that there is still a long journey to develop more general and powerful ligands for palladium-catalyzed carbonylative reactions.

In addition to the rational design and development of versatile ligands, several other factors, including additives, reaction media, and protocol also affect the carbonylation transformation, and are briefly mentioned in this review. The combination of designed, privileged ligands with microwave assistance or ionic liquids presents another opportunity to drive further developments of palladium-catalyzed carbonylation reactions.

During the preparation of this submission, Beller, Wu and co-workers further extended their studies on palladium-catalyzed carbonylation from a three- to a four-component process.[135] The concise one-pot synthetic protocol to produce 4(3H)-quinazolinones via the palladium-catalyzed four-component carbonylative cross-coupling was successfully realized by using palladium(II) acetate and BuPAd₂. Although the pressure of carbon monoxide was relatively high, the efficiency of this novel protocol to construct bioactive and complicated products in a straightforward manner clearly indicates the intriguing and challenging goals for ligand design in palladium-catalyzed carbonylative reactions.

Acknowledgment

Financial support from the National Natural Science Foundation of China (No. 91127041 and 21172045), the Changjiang Scholars and Innovative Research Team in University (IRT1117), the Shanghai Shuguang Program and Pujiang Program, the Shanghai Leading Academic Discipline Project (B108) and the Department of Chem­istry Fudan University is gratefully acknowledged.

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