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Synlett 2012(3): 357-366  
DOI: 10.1055/s-0031-1290101
ACCOUNT
© Georg Thieme Verlag Stuttgart ˙ New York

Recent Advances in Transition-Metal-Catalyzed Esterification

Fang Luoa, Changduo Panb, Jiang Cheng*a
a College of Chemistry & Materials Engineering, Wenzhou University, Wenzhou 325000, P. R. of China
Fax: +86(577)56998939; e-Mail: jiangcheng@wzu.edu.cn;
b Wenzhou Institute of Industry & Science, Wenzhou 325000, P. R. of China

Further Information

Publication History

Received 11 July 2011
Publication Date:
09 December 2011 (online)

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Biographical Sketches

Fang Luo was born in Sichuan, P. R. of China, in 1985. She received her Bachelor’s degree from Inner Mongolia Normal University, P. R. of China, in 2004. She recently obtained her Master’s degree from Wenzhou University, P. R. of China, in 2011.
Changduo Pan was born in Zhejiang, P. R. of China, in 1983. He received his Master’s degree from Wenzhou University, P. R. of China, in 2009. He currently works in Wenzhou Institute of Industry & Science.
Jiang Cheng was born in Zhejiang, P. R. of China, in 1974. He received his Bachelor’s and Master’s degrees and PhD from Nanjing University. After he had obtained his PhD under the supervision of Prof. Zhaoguo Zhang and Jianhua Xu in 2004, he worked at Wenzhou University and was promoted to professor in 2010. His research interests include the development of transition-metal-catalyzed es­terification, cyanation and C-H bond activation.

Abstract

In this account, we summarized our recent progress in transition-metal-catalyzed esterification via different pathways, including lactonization, Chan-Lam reaction, oxidative esterification and C-H functionalization.

1 Introduction

2 Esterification of Aldehyde with Aryl Boron

2.1 Aromatic Esterification Reaction between Aldehydes and Arylboronic Acids

2.2 Cascade Aryl Addition-Lactonization of Phthalaldehyde with Aryl Boron

2.3 Cascade Aryl Addition-Lactonization of Phthalaldehydo­nitrile with Arylboronic Acids

3 Esterification of Carboxylic Acid via Chan-Lam Reaction

4 Bis-esterification of Cyclic Anhydrides with Alkoxysilanes

5 Oxidative Esterification of Aldehydes or Benzylic Alcohols with Phenols

6 Ester Formation via C-H Functionalization

6.1 Benzoxylation of C-H Bonds with Carboxylic Acids

6.2 Benzoxylation of 2-Arylpyridine sp² C-H Bonds with ­Anhydrides

6.3 Benzoxylation of 2-Arylpyridine sp² C-H Bonds with Acyl Chlorides

7 Conclusion

Key words

esterification - transition metals - catalysis - esters - C-H functionalization

1 Introduction

The ester functionality is ubiquitous as the structure of important natural and synthetic molecules and the need for ester compounds will never lessen. [¹] Benzoate derivatives are important building blocks in the synthesis of natural and pharmacological compounds. [²] Usually, these compounds are prepared via Fischer esterification [³] or transesterification [4] reactions, which normally involve strong acidic or basic conditions limiting the reaction scope (Scheme  [¹] ). [5] The Baeyer-Villiger oxidation [6] reaction may suffer from low regioselectivity (Scheme  [¹] ). In the past few years, oxidative esterification has widely received attention and has become an economical alternative to traditional ester synthesis. [7] Meanwhile the transition-metal-catalyzed esterification also has received increasing attention. [8] In this account we summarized our recent progress in transition-metal-catalyzed esterification to access benzoate derivatives via different pathways.

2 Esterification of Aldehyde with Aryl Boron

2.1 Aromatic Esterification Reaction between Aldehydes and Arylboronic Acids

Scheme 1 Traditional esterification reactions

In 2007, we reported the palladium-catalyzed addition of arylboronic acids to aldehydes to access carbinol derivatives. It enjoyed high prestige thanks to the advantages of boronic acids, such as low toxicity, stability to air and moisture, and good functional group tolerance. [9] Next, we found diaryl ketone derivatives were obtained when K2CO3 was replaced by Cs2CO3 as the base. [¹0] Our interest in the development of organoboron reactions led us to continually explore the potential reactions between organoboron reagents and aldehydes. Sequentially, we demonstrated a palladium-catalyzed aromatic esterification reaction between aldehydes and organoboronic acids under an air atmosphere (Scheme  [²] , eq. 1). [¹¹] To understand the mechanism more clearly, labeling studies were conducted using ¹8O2. The result showed that dioxygen took part in the reaction and was crucial for this transformation (Scheme  [²] , eq. 2). In 2010, Gois developed an efficient iron/NHC-catalyzed aerobic oxidative aromatic esterification of aldehydes with boronic acids. [8g]

2.2 Cascade Aryl Addition-Lactonization of Phthalaldehyde with Aryl Boron

Scheme 2 Palladium-catalyzed aromatic esterification reaction between aldehydes and arylboronic acids

In recent years, transition-metal-catalyzed addition of arylboronic acids to aldehydes was well developed by other groups and us. [9] [¹²] Based on the previous elegant works, we envisioned that after the addition of arylboronic acids to one carbonyl of phthalaldehyde, an intramolecular esterification could be achieved. Thus, we would develop a novel and facile strategy to obtain phthalide, which is present in a large number of natural products and biologically active compounds (Figure  [¹] ). [¹³] With this in mind, we initiated our investigation by examining the reaction of phthalaldehyde and phenylboronic acid using rhodium as the catalyst. After several rounds of optimization, we found that the best results were accomplished by using [{Rh(cod)Cl}2] (5 mol%) and dppb (5 mol%) as the catalyst, and K2CO3 (2 equiv) in dry 1,2-dichloroethane at 65 ˚C. [¹4] Having identified the optimal reaction parameters, the scope of arylboronic acids was investigated and the 3-aryl and alkenyl phthalides were obtained in moderate to good yields (Scheme  [³] ).

Figure 1 Selected examples of 3-substituted phthalides with reported biological activities

Scheme 3 Rhodium-catalyzed cascade aryl addition-lactonization of phthalaldehyde with arylboronic acids

Further study showed that the aryl addition to phthalaldehyde could lead to product 1, which may act as an intermediate. 2-[Hydroxy(phenyl)methyl]benzaldehyde (1) is in equilibrium with 2, [¹5] and when it is subjected to the reaction conditions, 3 was isolated in 84% yield (Scheme  [4] ). This result confirmed that our initial notion was practicable.

Scheme 4 The reaction of a possible intermediate under standard conditions

Based on the rhodium-catalyzed cascade aryl addition-­intramolecular esterification reaction, we successfully developed a palladium-catalyzed addition of arylboronic acids to phthalaldehyde, followed by an intramolecular lactonization to access 3-substituted phthalides (Scheme  [5] ). [¹6] Very recently, Cheng reported such a transformation catalyzed by cobalt. [¹7]

Scheme 5 Palladium-catalyzed cascade aryl addition-lactonization of phthalaldehyde with arylboronic acids

It is well-known that organoboron compounds and particularly boronic acids are useful reagents for C-C bond formation with various electrophiles in the presence of transition metals. [¹8] In spite of the advantages of low toxicity and easy manipulation, boronic acids often dimerize and trimerize to form boronic acid anhydrides and boroxines (which depends on storage water content). [¹9] Recently, due to their superior features such as higher stability and ease of preparation and purification, [²0] potassium organotrifluoroborates have become attractive alternatives to boronic acid derivatives that easily undergo protode­boronation. [²¹] Over the past decade, the transition-metal-catalyzed arylation reactions of aldehydes with organoboronic acids have attracted much attention. [²²] Nevertheless, the employment of potassium organotrifluoroborates in transition-metal-catalyzed 1,2-addition of aldehydes is rare. [²³] Based on the aforementioned works, the rhodium- or palladium-catalyzed reaction of phthalaldehyde with potassium organotrifluoroborates to access 3-substituted phthalides using the transition-metal-catalyzed 1,2-addition of aldehyde as the key step was achieved (Scheme  [6] ). [²4] Compared to the lactonization of phthal­aldehyde with arylboronic acids, the yields were relatively higher.

Scheme 6 Rhodium- or palladium-catalyzed cascade aryl addition-lactonization of phthalaldehyde with potassium organotrifluorobor­ates

2.3 Cascade Aryl Addition-Lactonization of Phthalaldehydonitrile with Arylboronic Acids

Inspired by our aforementioned works, we envisioned the development of the transition-metal-catalyzed reaction of phthalaldehydonitrile with organoboronic acids to access 3-substituted phthalides. However, great challenges are remaining since the cyano group is inert to the insertion of metal species in comparison to C=O, partly due to its low polarity. Moreover, the aromatic nitriles may also have good affinity to transition metals, resulting in the deactivation of the catalyst. For example, PdCl2(RCN)2 (R = Me, Ph) are widely used as Pd catalysts. Larock [²5] and Lu [²6] reported carbopalladation of the nitrile to form an iminopalladium intermediate, which would hydrolyze to ketones, respectively. Murakami demonstrated that organorhodium species could undergo intramolecular nucleophilic addition to nitrile to give the iminorhodium species. [²7] Encouraged by their seminal and our previous work, we developed a rhodium- or palladium-catalyzed cascade aryl addition-lactonization of phthalaldehydonitrile with boronic acids to access 3-substituted phthalides in moderate to good yields (Scheme  [7] ). [²8]

Scheme 7 Rhodium- or palladium-catalyzed cascade aryl addition-lactonization of phthalaldehydonitrile with arylboronic acids

3 Esterification of Carboxylic Acid via Chan-Lam Reaction

The Chan-Lam coupling reaction was widely studied in the last few years because it allows aryl carbon-hetero­atom bond formation via an oxidative coupling of aryl­boronic acids, stannanes or siloxanes with N-H or O-H containing compounds. [²9] However, the scope of the Chan-Lam reaction in C-O bond formation was limited to phenol and aliphatic alcohol, [³0] and the reaction of carboxylic acid as heteroatom nucleophile in the Chan-Lam reaction was less reported. Thus, we conceived carboxylic acid as heteroatom nucleophile to form an ester. Meanwhile, aryl trialkoxysilane was widely used as transmetallation reagent in organic synthesis because of its low cost, easy availability, nontoxic byproducts and stability under many reaction conditions. [³¹] Thus, we realized a copper(II)-catalyzed esterification of arene carboxylic acids with aryl and vinyl trimethoxysilanes, affording aryl and vinyl benzoate derivatives in moderate to good yields (Scheme  [8] ). [³²] Notably, vinyl benzoate derivatives were expediently produced. However, 3 equivalents of AgF were required in such transformation. To overcome this drawback, we developed a Cu(OTf)2-mediated Chan-Lam reaction of carboxylic acids and arylboronic acids using urea as the additive (Scheme  [9] ). [³³] Almost at the same time, Liu reported Cu-catalyzed O-arylation reactions of carboxylic acids with arylboronic acids in the presence of 2 equivalents of Ag2CO3. [8f]

Scheme 8 Copper-catalyzed esterification of arene carboxylic acids with aryl and vinyl trimethoxysilanes

Scheme 9 Copper-mediated esterification of arene carboxylic acids with arylboronic acids

Scheme 10 The esterification reaction using aryl trimethoxysilane

4 Bis-esterification of Cyclic Anhydrides with Alkoxysilanes

Aryl trialkoxysilanes have been widely used as significant transmetallation reagents in organic synthesis. However, only the aryl moiety of aryl trialkoxysilane has been transferred to the organic product and the alkoxyl moiety was discarded as waste, which diminishes the atom economy for such transformations. In 2006, Lerebours and Wolf reported that the methoxy group of phenyltrimethoxysilane was transferred to the aldehyde to form methyl benzoate (Scheme  [¹0] , eq. 1). [³4] However, to the best of our knowledge, examples of the transfer of both the aryl and alkoxy moieties of the aryl trialkoxysilane have never been reported. Recently, we developed a copper(II)-catalyzed aromatic esterification reaction of carboxylic acid with aryl and vinyl trialkoxysilanes (Scheme  [¹0] , eq. 2). [³²] Interestingly, when the acyclic anhydride benzoic anhydride and phenyltrimethoxysilane were subjected to the procedure, methyl benzoate and phenyl benzoate were detected by GC-MS in equal amounts (Scheme  [¹0] , eq. 3). In light of this, we envisioned a bis-esterification of cyclic anhydrides with aryl trialkoxysilanes and vinyl trialkoxysilanes, in which the alkoxy and aryl (or vinyl) esters of dicarboxylic acids are prepared in one pot (Scheme  [¹0] , eq. 4). [³5] After several rounds of optimization, we found the best results and a series of anhydrides with alkoxy­silanes were pursued (Scheme  [¹¹] ).

Scheme 11 Copper-catalyzed esterification of anhydride with aryl or vinyl trimethoxysilane

5 Oxidative Esterification of Aldehydes or Benzylic Alcohols with Phenols

Scheme 12 Palladium/NHC-catalyzed oxidative esterification of aldehydes with phenols

Scheme 13 Tandem benzylic oxidation-oxidative esterification of benzylic alcohols with phenols

Scheme 14 ortho-Benzoxylation of 2-arylpyridines with carboxylic acids

Oxidative esterification has received increasing attention and has become an economical alternative to traditional ester synthesis. [7] However, to develop a facile and versatile procedure on such transformation still remains a highly desired goal for organic chemists. After many trials, we achieved a palladium/NHC-catalyzed oxidative esterification of aldehydes with phenols, which used air as the clean oxidant (Scheme  [¹²] ). [³6]

Alcohols are usually readily available as bulk chemicals. Generally, alcohols could be converted into esters by multiple steps. However, the direct conversion of alcohols into esters in the presence of catalysts represents a green, economic, and sustainable process. [³7] After the elaboration of oxidative esterification of aldehydes with phenols, we developed a tandem benzylic oxidation-oxidative ­esterification of benzylic alcohols with phenols (Scheme  [¹³] ). [³8] During this study, Lei and Beller developed palladium-catalyzed aerobic oxidative esterification of benzylic alcohols, respectively. [³9]

6 Esters Formation via C-H Functionalization

6.1 Benzoxylation of C-H Bonds with Carboxy­lic Acids

Selective functionalization of the C-H bonds has emerged as a powerful tool in organic synthesis because it obviates the tedious multistep prefunctionality. [40] Recently, C-O bond formation via the cleavage of a C-H bond has attracted much attention. [4¹] For example, in 2005, Yu reported a Pd-catalyzed stereoselective oxidation of methyl groups by carboxylic anhydrides. [4²] Subsequently, the same group described an elegant example of Cu(OAc)2-catalyzed oxidative acetoxylation of arene C-H bonds in HOAc/Ac2O using oxygen as a clean oxidant. [4³] However, the reports on such transformations via transition-metal-catalyzed C-H bond cleavage are almost limited to acet­oxylation, and a stoichiometric oxidant is required to fulfill the catalytic cycle. In other words, transition-metal-catalyzed benzoxylation of a C-H bond is not common. [44] Inspired by our previous work of palladium-catalyzed acylations of aromatic C-H bonds using aldehydes, [45] we began an exploratory study on the ortho-benzoxylation of 2-arylpyridines with carboxylic acids. After many trials, we achieved an ortho-benzoxylation reaction of 2-aryl pyridines with carboxylic acids in the presence of [Rh(cod)Cl]2, PCy3˙HBF4, CuI and NMP. [46] The procedure tolerates carbomethoxy, formyl, bromo, chloro, and nitro groups, providing the benzoxylated products in moderate to good yields (Scheme  [¹4] ).

Encouraged by the aforementioned works, we developed a palladium-catalyzed acyloxylation of the benzyl sp³ C-H bond with carboxylic acid employing PhI(OAc)2 as a stoichiometric oxidant. The procedure tolerates a series of functional groups, such as methoxy, chloro, bromo, iodo, vinyl, formyl, phenolic hydroxy, nitro, and cyano groups, providing the acyloxylation products in moderate to good yields (Scheme  [¹5] ). [47]

Scheme 15 Palladium-catalyzed acyloxylation of the benzyl sp³ C-H bond

Scheme 16 Copper-catalyzed ortho-benzoxylation of the 2-arylpyridine sp² C-H bond with anhydride

6.2 Benzoxylation of 2-Arylpyridine sp ² C-H Bonds with Anhydrides

Next, we developed an efficient copper-catalyzed ortho-benzoxylation of the 2-arylpyridine sp² C-H bond with anhydride, affording mono- or diacyloxylation products in moderate to good yields (Scheme  [¹6] ). [48] The employment of inexpensive copper catalysts and O2 as the terminal oxidant provides a significant practical advantage for this transformation.

6.3 Benzoxylation of 2-Arylpyridine sp ² C-H Bonds with Acyl Chlorides

During the study on acylations of aromatic C-H bonds using aldehydes, [45] we envisioned to use acyl chloride as the coupling partner via C-H bond cleavage. We began an exploratory study using 2-o-tolylpyridine and benzoyl chloride. Interestingly, benzoxylation of 2-o-tolylpyridine was found. After several rounds of optimization, the optimized reaction conditions were found as follows: Cu(OAc)2 (20 mol%), and t-BuOK (2 equiv) in toluene under O2 at 145 ˚C (Scheme  [¹7] ). [49] During the reaction, carboxylic anhydride was detected by GC-MS. The acyl chloride may readily form carboxylic anhydride in the presence of a base and moisture. [50] Based on the experimental results and our previous work, [48] we reasoned carboxylic anhydride was the intermediate in this transformation.

Scheme 17 Benzoxylation of 2-arylpyridine sp² C-H Bonds with Acyl Chlorides

Interestingly, when Li2CO3 was used in the procedure instead of t-BuOK, ortho-chlorination of 2-arylpyridine occurred (Scheme  [¹8] ).

Scheme 18 ortho-Chlorination of 2-arylpyridine

7 Conclusion

In this account, we described our recent work in transition-metal-catalyzed esterification of aldehyde, carboxylic acid, anhydrides, acyl chlorides and benzylic alcohols through a variety of ways such as lactonization, Chan-Lam reaction, oxidative esterification and C-H functionalization. Among those, accessing esters via C-H bond cleavage would be an attractive method. We believe that more practical, facile and mild procedures for such transformations will be developed continuously in the near future.

Acknowledgment

We thank all group members who contributed to the studies described in this review. Financial support was provided by the National Natural Science Foundation of China (No. 20972115) and the Natural Science Foundation of Zhejiang Province (Nos. Y4110109 and R4110294).

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Scheme 1 Traditional esterification reactions

Scheme 2 Palladium-catalyzed aromatic esterification reaction between aldehydes and arylboronic acids

Figure 1 Selected examples of 3-substituted phthalides with reported biological activities

Scheme 3 Rhodium-catalyzed cascade aryl addition-lactonization of phthalaldehyde with arylboronic acids

Scheme 4 The reaction of a possible intermediate under standard conditions

Scheme 5 Palladium-catalyzed cascade aryl addition-lactonization of phthalaldehyde with arylboronic acids

Scheme 6 Rhodium- or palladium-catalyzed cascade aryl addition-lactonization of phthalaldehyde with potassium organotrifluorobor­ates

Scheme 7 Rhodium- or palladium-catalyzed cascade aryl addition-lactonization of phthalaldehydonitrile with arylboronic acids

Scheme 8 Copper-catalyzed esterification of arene carboxylic acids with aryl and vinyl trimethoxysilanes

Scheme 9 Copper-mediated esterification of arene carboxylic acids with arylboronic acids

Scheme 10 The esterification reaction using aryl trimethoxysilane

Scheme 11 Copper-catalyzed esterification of anhydride with aryl or vinyl trimethoxysilane

Scheme 12 Palladium/NHC-catalyzed oxidative esterification of aldehydes with phenols

Scheme 13 Tandem benzylic oxidation-oxidative esterification of benzylic alcohols with phenols

Scheme 14 ortho-Benzoxylation of 2-arylpyridines with carboxylic acids

Scheme 15 Palladium-catalyzed acyloxylation of the benzyl sp³ C-H bond

Scheme 16 Copper-catalyzed ortho-benzoxylation of the 2-arylpyridine sp² C-H bond with anhydride

Scheme 17 Benzoxylation of 2-arylpyridine sp² C-H Bonds with Acyl Chlorides

Scheme 18 ortho-Chlorination of 2-arylpyridine