Catalytic Enantioselective Alkylations with Allylic Alcohols

Abstract

Allylic alcohols have recently risen to prominence as a valuable synthetic alternative to classic activated alkylating agents in asymmetric catalysis. The intrinsic drawbacks that limited their employment in catalytic enantioselective transformations, until recently, have been efficiently addressed providing elegant solutions. This short review intends to summarize the most salient and recent outcomes in this emerging research area, highlighting the scope and limitations of the most selective catalytic methodologies.

1 Introduction

2 Redox Metal Catalysis

2.1 C-C Bond-Forming Reactions

2.2 C-N and C-O Bond-Forming Reactions

3 Electrophilic Activation of C=C

3.1 C-C Bond-Forming Reactions

3.2 C-N and C-O Bond-Forming Reactions

4 Substitution via Carbocation Intermediates

5 Conclusions

‘…Chemical synthesis has now reached an extraordinary level of sophistication, but there is vast room for improvements…’ [¹]

1 Introduction

The search for cost- and environmental-impact-containing synthetic methodologies is a current challenge for the organic chemical community.

In this direction, besides the development of unprecedented chemical transformations, the replacement of ‘old chemistries’ with more enabling synthetically accessible strategies (i.e. the substitution of hazardous chemicals with the most sustainable alternatives), constitutes a parallel channel of effort. This is in line with Article 12 of the Regulation (Ec) No 1907/2006 of the European Parliament, that specifically mentions: ‘An important objective of the new system to be established by this Regulation is to encourage and in certain cases to ensure that substances of high concern are eventually replaced by less dangerous substances or technologies where suitable economically and technically viable alternatives are available….’. [²]

In the realm of stereoselective carbon-carbon bond-forming processes, the asymmetric allylic alkylation (AAA) of prochiral or racemic substrates has been a pillar of organic synthesis for several decades, enabling rapid and direct access to complex polyfunctionalized compounds, comprising stereochemically defined stereogenic centers [Scheme  [¹] (a)]. [³]

The well-established Tsuji-Trost-type nucleophilic substitution of activated allylic fragments is among the most potent and direct stereoselective tool for the functionalization of stabilized ‘soft’ as well as unstabilized ‘hard’ carbon- and heteroatom-based nucleophiles (Scheme  [²] , path a). [4] Besides this straightforward methodology, stereoselective metal, as well as metal-free, catalytic approaches involving, carbocationic intermediates (Scheme  [²] , path b) or reaction channels comprising ‘soft’-metal-assisted olefin activation (Scheme  [²] , path c) are reliable synthetic entries towards the realization of asymmetric nucleophilic substitutions. As a consequence, the portfolio of chemical manipulations towards the realization of stereochemically defined allylic stereogenic centers is rich, allowing rapid access to chemical diversity and complexity.

Recent developments in the field concern the replacement of conventional activated leaving groups (X) such as: acetates­, carbonates, phosphates, and halides with more synthetically reliable alcohols [Scheme  [¹] (b)]. [5] The latter discoveries have brought several advantages into the field that are related to step economy (most of the aforementioned leaving groups are obtained from alcohols), cost efficiency (alcohols are largely available at reasonable prices), and environmental impact (water would results as the only stoichiometric byproduct of the catalytic transformation). On the other hand, alcohols are by far the more poorly reactive electrophilic species, with the resulting need for high loading of catalysts, high reaction temperatures, and/or addition of external activators.

Despite this, the additive-free palladium-catalyzed allylic substitution with alcohols was pioneered in 2002 by Ozawa­ and co-workers, [6] however, the afore-cited aspects substantially retarded both the discovery and the subsequent consolidation of allylic alcohols as enabling reaction partners of catalytic and enantioselective alkylation procedures.

The intrinsic inertness of alcohols forced scientists to study and develop innovative combinations of metal-based and metal-free catalytic systems in the arsenal of asymmetric synthesis. Of particular relevance is the successful combination of chiral organocatalysts and σ- or π-acid metal species that frequently proved perfect compatibility and synergetic action­. [7]

This short review provides comprehensive coverage of the recent use of simple and unactivated alcohols in asymmetric nucleophilic allylic substitutions, with particular emphasis on methodologies that combine prochiral or chiral but racemic substrates and chiral promoting agents. It should be emphasized that many examples of the stereospecific displacement of unactivated allylic alcohols under reagent control (i.e., use of stereodefined reagents with achiral promoters) [8] have been described, however, such an approach is beyond the scope of this review.

2 Redox Metal Catalysis

In the present short review, the term ‘redox metal catalysis’ is utilized to address asymmetric allylic substitutions comprising low-valent-transition-metal catalysis. The reaction profile has been extensively investigated with activated allylic leaving groups, and allylic alcohols are considered to have analogous steps. Firstly, the late-transition-metal species would insert oxidatively into the C-OH bond leading to key electrophilic metal-η^³-complexes. Upon inner- or outer-sphere nucleophilic attack, the re-formed η^²-adduct would release the product with the concomitant regeneration of the catalytic active metal species (Scheme  [³] ). At the present, only low-valent-metal catalysis based on chiral iridium(I), palladium(0), and ruthenium(II) complexes have been utilized in the allylic alkylation with alcohols, under a redox regime.

2.1 C-C Bond-Forming Reactions

Preliminary evidence on the suitability of simple allylic alcohols in enantioselective allylic alkylation reactions were documented by Helmchen and co-workers in 2004. [9] In this comprehensive study, regio- and stereoselective Ir(I)-catalyzed alkylation of the sodium carbanion of di­methyl malonate (2a) was carried out in the presence of trans-cinnamyl alcohol (1a), leading to the corresponding branched product b- 3aa in moderate yield (63%) and good enantiomeric excess (81%). Phosphine-oxazoline ligand A proved to be the ligand of selection. What clearly emerged from a comparative study with several alkylating reagents is that the replacement of the more reactive acetate with 1a caused a marked drop in the branched/linear ratio (2.7:1, with respect to 19:1) whereas the resulting enantiomeric excess was nearly unchanged (Scheme  [4] ).

Tamaru and co-workers elegantly documented the capability of simple allylic alcohol (1b) in the Friedel-Crafts-type alkylation of 1H-indoles in the presence of catalytic amount of [Pd(PPh₃)₄] (5 mol%) and with triethylborane (30 mol%) as the additive. [¹0] In line with these findings, Trost and co-workers reported an unprecedented catalytic asymmetric allylic alkylation of 3-substituted indoles 4 in the presence a chiral [Pd(0)] complex featuring ligand B (Scheme  [5] ). [¹¹]

Synthetically accessible reaction rates were guaranteed through the addition of a stoichiometric amount of 9-hexyl-9-borabicyclo[3.3.1]nonane [9-BBN(C₆H₁₃)] that is likely to be involved in the alcohol activation. As a matter of fact, the in situ formed borane-alcohol 6 could undergo subsequent oxidative addition with the chiral [Pd(0)] complex, [¹²] triggering a classic [Pd(0/II)] catalytic cycle that led to the desired indolines 5 in optimal yields and enantiomeric excesses up to 90%. Finally, the synthetic versatility of the catalytic procedure was clearly exemplified by adopting the asymmetric allylic alkylation as the key step in the stereoselective synthesis of (-)-esermeth­ole (Scheme  [5] ).

Very recently, List and Jiang opened up new horizons in the catalytic enantioselective alkylation of α-branched aldehydes, by combining palladium (i.e., [Pd(PPh₃)₄], 1.5 mol%) and organocatalysis (Brønsted acid catalysis, 3 mol%) along with a sterically congested primary amine (enamine catalyst, 40 mol%). [¹³] In Scheme  [6] , the example of racemic 2-phenylpropanal (7a) is shown. The newly generated all-carbon quaternary stereocenter was obtained with high enantiomeric excess (94%). In this regard, the predominant formation of the E-configured enamine in the condensation between the primary amine and the α-branched aldehyde accounted for these findings. The BINOL-based chiral phosphoric acid C-H was hypothesized to bring in proximity both in situ formed enamine (H-bonding interaction) and the η^³-allyl-Pd intermediate 9. Moreover, the multiple role of catalyst C-H was also extended to the plausible activation of the allylic alcohol 10, favoring the subsequent oxidative insertion by [Pd(PPh₃)₄] (Scheme  [6] ).

2.2 C-N and C-O Bond-Forming Reactions

Catalytic allylic amination with alcohols is a highly desirable, rapid, and atom-economic approach towards synthetically flexible allylamines. [¹4] Iridium(I/III) catalysis has already showed remarkable pertinence to boost the functionalization of both primary and secondary anilines via intermolecular enantioselective condensation with primary allylic alcohols. [¹5] Hartwig and co-workers elegantly demonstrated the efficiency of chiral phosphoramidite ligand D in controlling both regio- and the stereochemistry of the process when Nb(OEt)₅ or triphenylborane were used as alcohol activators in stoichiometric (120 mol%) or catalytic (8 mol%) amounts, respectively. In particular, high branched/linear ratios (constantly higher than 95:5) and excellent enantiomeric excesses were obtained in both circumstances. Two exemplificative cases for 4-methoxyaniline (11a) and trans-cinnamyl alcohol (1a) are presented in Scheme  [7] . Interestingly the presence of 4 Å molecular sieves (MS) proved to be essential to achieve synthetically relevant chemical yields.

Furthermore, the protocol was also efficiently extended to primary (i.e., BnNH₂, p-MeOBnNH₂) and secondary (i.e., morpholine) aliphatic amines, which provided the desired mono-N-allylated compound in acceptable to good optical as well as chemical yields, despite their strong Lewis basicity.

An unresolved issue in the area of enantioselective allylic amination reactions is the use of ammonia as a nucleophilic partner. Accordingly, Carreira and co-workers pioneered the field by describing the use of sulfamic acid (H₂NSO₃H, 13) as an ammonia equivalent, in the preparation of primary allylamines with alcohols. [¹6] Spectroscopic investigations revealed that the use of N,N-dimethylform­amide as reaction media led to the formation in situ of a Vilsmeier-type intermediate 15 with sulfamic acid delivering in solution one equivalent of ammonia. Therefore, the secondary allylic alcohol is hypothesized as reacting with 15 yielding the activated adduct 16 that can smoothly undergo the subsequent oxidative addition step with the iridium(I) complex. The final stage of the catalytic cycle (not shown in Scheme  [8] ) would involve the expected trapping of the η^³-iridium complex intermediate by the delivered ammonia. Although the largest part of the investigation was addressed to the development of the not-stereoselective variant, preliminary evidence for the catalytic and enantioselective transformation of racemic secondary allylic alcohols to the corresponding primary amines were reported in the presence of chiral phosphoramidite ligand E (6 mol%, Scheme  [8] ). Subsequently, the same team reported the stereospecific substitution of optically active secondary allylic alcohols with 13 in the presence of analogous [Ir(I)-(P,alkene)] complexes with the assistance of the iodide anion. [8a]

Enantioselective allylic etherification has been recently object of growing attention as a rapid entry to valuable, stereodefined building blocks in organic synthesis. [4] [¹5b]

An elegant approach for the direct utilization of unactivated allylic alcohols as electrophilic precursors was documented by Carreira and co-workers though the asymmetric dynamic kinetic resolution (DKR) of racemic secondary alcohols with primary alkyl alcohols 17. [¹7] Once again, the chiral E-iridium(I) complex emerged as a competent promoter for the preparation of optically active unsymmetrical allyl ethers 18. Crucial was the selection of the Brønsted acid as the co-catalyst. Here, the choice was dictated by the narrow range of pK _a (3.4-3.9) that originated a suitable additive for the activation of the secondary alcohol. Allyl ethers 18 were routinely obtained with exceptionally high enantiomeric ratios (up to 99.5:0.5) in the presence of an excess of aliphatic co-reactant­ 17, in order to minimize the side formation of symmetrical allyl ethers (Scheme  [9] ).

Intramolecular catalytic asymmetric allylation of alcohols has been matter of discussion also through ruthenium(II) catalysis. In particular Kitamura and co-workers reported on the efficiency of a cationic [CpRu] complex F, containing pyridine-2-carboxylic acid as ligand, in promoting the dehydrative condensation of variously functionalized primary alcohols and 1b. [¹8] The enantioselective variant was subsequently reported by the same team simply by replacing the pyridine-2-carboxylic acid derivative with a new class of synthetically flexible 6-(2-chloro-1-naphthyl)-5-methylpyridine-2-carboxylic acid derivative (R = H, allyl) ligand G. [¹9]

A wide range of five- and six-membered cyclic ethers 20 were isolated in high yields (87-98%) and excellent enantiomeric ratios (up to 99:1), whereas seven-membered rings were unrealizable under the best conditions. The main strength of the methodology is the exceptional substrate/catalyst ratio that can be efficiently employed (up to 10000). Finally, a redox-mediated donor-acceptor mechanism, involving a [Ru^II/Ru^IV] couple is invoked by the authors (Scheme  [¹0] ).

Furthermore, the impact of low-valent ruthenium catalysis on the direct enantioselective manipulation of allylic alcohols was examined very recently. [²0a] Highly stereoselective inter- as well as intramolecular C-C and C-X bond-forming allylic alkylation reactions were presented in the presence of chiral Ru(II) complexes based on newly designed Naph-diPIM ligands H. The unique performances guaranteed by H were rationalized in terms of: high scaffold rigidity, extended π-system and strong σ-donating character of the pyrrolo units. In Scheme  [¹¹] the synthesis of isochromane 20b is shown. Interestingly, a substrate/catalyst ratio of 1000 was sufficient to isolate 20b in 98% yield and >99:1 er, when catalytic amounts of 4-toluenesulfonic acid were utilized to speed up the process via the electrophilic activation of 19b.

3 Electrophilic Activation of C=C

Differently from redox-‘active’ low-valent metals (i.e. [Ru(II)], [Pd(0)], and [Ir(I)]), ‘softer’ transition metal species with marked π-acid character have been shown to promote allylic alkylation methodologies via conventional electrophilic activation of the carbon-carbon double bond, under η^²-complexation pathway. In this direction, the isohypsic nature of Au(I) and Hg(II) salts/complexes, their unique carbophilicity and moderate heterophilicity makes these metals candidates for promoters of allylic alkylation reactions in the presence of scarcely reactive alcohols. [²¹]

In principle, the late-transition-metal-assisted reaction profile exemplified in Scheme  [²] (c) can occur through a concerted or stepwise mechanistic event. More frequently it involves the initial addition of the nucleophilic species to the olefin unit leading to a β-hydroxy organometallic intermediate 21, with the subsequent β-elimination to restore the carbon-carbon double bond (Scheme  [¹²] ). It should be emphasized that concomitant metal-oxygen coordination cannot be ruled out a priori (i.e. σ-Lewis acid mediated mechanism) even if the poor oxophilicity of late-transition-metal species should favor the η^²-[M/olefin] interaction. [²²]

3.1 C-C Bond-Forming Reactions

In this direction, our group has recently discovered the efficiency of chiral binuclear gold(I) complexes in promoting the intramolecular allylic alkylation of indoles with simple allylic alcohols, providing a wide range of polycyclic scaffolds such as tetrahydrocarbazoles and tetrahydrocarbolines 23. Scheme  [¹³] shows a representative methodology for the synthesis of 2-vinyltetrahydrocarbolines 23 starting from readily available (Z)-indolyl alcohols 22. Under best conditions, the use of (R,R)-MeO-biphep(AuOTf)₂ (10 mol%) triggered the corresponding ring-closing process in good yields and enantiomeric excesses (up to 94%). [²³a,b] Such a protocol represents one of the first examples of catalytic enantioselective Friedel-Crafts-type allylic alkylation of arenes with alcohols under additive-free conditions. [²4]

The key role of the electrophilic Au(I)-ene activation was demonstrated by the influence of the configuration of the alkene over the overall chemical as well as stereochemical profile of the process. Interestingly, the methodology was extended to the preparation of 4-vinyltetrahydrocarbolines 25 simply by building up the linkage between the indole and the hydroxy group at the C2 position of the indole core. [²³c]

3.2 C-N and C-O Bond-Forming Reactions

Over the past few years, the use of mercury(II) salts in organic synthesis has received substantial interest, with particular concern to the electrophilic activation of π-systems. [²5] Yamamoto and co-workers documented the hydroarylation reaction of alkynes and allylic amino-alkylations with loading of mercury(II) triflate as low as 0.1 mol%. Accordingly, the team reported also the first enantioselective mercury(II)-catalyzed intramolecular allylic amination of primary alcohols 26 in the presence of chiral binaphane ligand J (Scheme  [¹4] ). [²6]

Under optimal conditions a large range of 2-vinylindolines 27 were obtained in excellent yields (>90%) and enantiomeric excesses up to 99%. Conceivable rational for the observed stereochemical outcome was finally provided by considering the selective activation of the carbon-carbon bond of the allylic alcohol by the [(P-P)Hg]^²+ complex.

Enantioselective dehydrative intramolecular oxaallylic alkylation was then described by our group for the synthesis of 2-vinylmorpholines under gold catalysis. In particular, the chemical know-how gained in the Friedel-Crafts-type alkylation of indoles (Scheme  [¹³] ) was successfully applied to readily available and configurationally pure allylic diols 28. [²7] A survey of reaction conditions unrevealed the efficiency of binuclear cationic gold complexes carrying DTBM-segphos K (2.5 mol%) as the chiral ligand (enantiomeric excesses up to 95%). Experimental evidence emphasized the impact of the gold counterion (AgNTf₂ as the scavenger salt of election) as well as C=C configuration over the final products. In particular, nearly complete inversion of stereoinduction was recorded by reacting (Z)- or (E)-28b in the presence of (S)-K (Scheme  [¹5] ).

Interestingly, the methodology proved to be complementary with respect to classic palladium-catalyzed Tsuji-Trost approach, because when mono-alcohol acetates or carbonates were subjected to the best conditions, the corresponding morpholine products were obtained in poor yield.

4 Substitution via Carbocation Intermediates

The nucleophilic substitution of alcohols via an S_N1 or carbocationic mechanism is a highly desirable theme in organic synthesis with growing implications in asymmetric catalytic transformations. [5c]

In particular, tailor-designed allylic alcohols have been utilized in the enantioselective α-allylic alkylation of aldehydes and for the synthesis of 2H-chromenes via intramolecular C-O bond-forming process. In both situations, chiral organocatalysis played a pivotal role.

In the first case, Cozzi and co-workers have documented the installation of substituted allylic C3-units, deriving from 1,1,3-triarylallyl cations, at the α-position of enolizable aldehydes. [²8] The reaction generally required catalytic amounts of indium(III) bromide (20 mol%) to ensure the formation of the stabilized allylic cation, whereas free MacMillans’ II generation imidazolidinone catalyst L (20 mol%) was the chiral secondary amine of choice for the in situ enamine formation. Diastereoselectivity was generally moderate to good (up to 5:1 syn/anti), and exceptionally high levels of stereocontrol were reached when the more reactive racemic naphthyl allylic alcohol derivative (±)-1e was condensed with excess phenylacetaldehyde (7b) (dr anti/syn 20:1, ee anti 99%). In this case, the use of a Lewis acid co-catalysis was not mandatory (Scheme  [¹6] ).

Not only, covalent, but also contact ion-pair catalysis [²9] has been efficiently employed in metal-free asymmetric allylic substitution reactions with alcohols. Rueping and co-workers described, in 2008, an unprecedented stereoselective approach to functionalized atropoisomeric bisindolylmethanes 31 in moderate enantiomeric excess (er up to 81:19). In particular, tertiary allylic alcohols 30 underwent nucleophilic substitution by a second unit of indole ring in the presence of catalytic amount of N-triflylphosphoramide M-H. [³0] Mechanistically, although the final allylic substitution was part of a cascade 1,4-1,2-addition of N-methylindole to β,γ-unsaturated α-keto esters (not shown), the authors provided evidence on the role of contact ion-pair catalysis 32 in the key stereodiscriminating event of the allylic alkylation of indoles (Scheme  [¹7] ). [³¹]

Very recently, the methodology was further consolidated in the preparation of 2H-1-benzopyran derivative 34, by means of chiral 3,3′-disubstituted BINOL-based Brønsted acid catalyzed intramolecular asymmetric allylic alkylation with phenols 33. In this case, partially reduced BINOL-based N-triflylphosphoramide N-H was utilized to trigger the initial formation of the allylic carbocation through a dehydrative reaction of 33. [³²] The consequent formation of tight contact ion-pairs 35 between the positively charged intermediate and N ^- is responsible of creating a preferential nucleophilic attack of the phenol group towards one of the two prochiral faces of the secondary cation species 35. Enantiomeric excesses in the range 84-96% were recorded (Scheme  [¹8] ).

5 Conclusions

In conclusion, the rapid growth of interest in the use of alcohols in enantioselective allylic alkylations emphasizes the concrete potential of alcohols in this field of research. The serious limitations that have slowed down its development, with respect to conventional and more reactive alkylating reagents, have forced scientists to develop new combinations of chiral catalysts and chiral catalysts/additives. In particular, unusual metal/metal and metal/organocatalytic systems have been efficiently adopted in both inter- as well as intramolecular transformations in order to bypass the intrinsic inertness of alcohols as an electrophile agents.

However, the game has only just begun. The use of allylic alcohols in catalytic enantioselective substitution reactions appears limited only by scientific creativity and there is much more still to come in this intriguing scenario.

Acknowledgment

Acknowledgment is made to Progetto FIRB ‘Futuro in Ricerca’ Innovative sustainable synthetic methodologies for C-H activation processes, (MIUR, Rome), Università di Bologna, and Fondazione del Monte di Bologna e Ravenna.

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Official Journal of the European Union, REGULATION (EC) No 1907/2006, L 396.

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Official Journal of the European Union, REGULATION (EC) No 1907/2006, L 396.