Efficient Transesterification/Acylation Reactions Mediated by N-Heterocyclic Carbene Catalysts

Abstract

Imidazol-2-ylidenes, a family of N-heterocyclic carbenes (NHC), are efficient catalysts in the transesterification involving numerous esters and alcohols. Low catalyst loadings of aryl- or alkyl-substituted NHC catalysts mediate the acylation of alcohols with enol acetates in short reaction times at room temperature. Commercially available and more difficult to cleave methyl esters react with primary alcohols in the presence of alkyl-substituted NHC to efficiently form the corresponding esters. While primary alcohols are selectively acylated over secondary alcohols with use of enol esters as acylating agents, methyl and ethyl esters can be employed as protective agents for secondary alcohols in the presence of the more active alkyl-substituted NHC catalysts. The NHC-catalyzed transesterification protocol was simplified by generating the imidazol-2-ylidene catalysts in situ.

Highly efficient carbene and polycarbene catalysis of the transesterification reaction

“…Under identical experimental conditions, the following series for the efficiencies emerged for the carbene catalysts that were examined: 2d (0) < 1a (1,0) < 1b 1,5) < 7 (1,8) < 8a (3,7) < 8b (6,2) < 9 (6,0) < 10 (6,7) < 2a (9,3) < 3b (10,0) < 3a (10,3) < 4 (12,0) < 5 (12,6) < 6 (12,9) < 3c = 2b (13,3) < 2c = 11 (13,4).…”

“…Since stable carbenes are highly basic compounds (pKa 21-28) they have also been suggested as catalysts for the transesterification reaction. [2][3][4][5][6][7][8] The latter reaction is typically carried out at room temperature in the presence of 0.5-5.0 mol% of carbenes and molecular sieves. The efficiencies of the catalysts can be estimated on the basis of the values of TON and TOF.…”

“…2 However, larger values of 200 and 2300 respectively have been observed for the reactions of vinyl esters due to the irreversibility of the acetaldehyde release process. 4 The transesterification reaction serves as an important tool for the production of biodiesel fuel. In the latter case the reaction is carried out in an excess of alcohol.…”

Synthesis and properties of heteroaromatic carbenes of the imidazole and triazole series and their fused analogues

“…397,398 The carbene was combined with stoichiometric amounts of the required reagents in the presence of molecular sieves. [399][400][401] In the first case, catalysis with imidazol-2-ylidenes in the presence of primary alcohols, resulted values for TON and TOF of 17-19 and 1 h ) but proceeds irreversibly due to the release of acetaldehyde.…”

Organocatalysis: Fundamentals and Comparisons to Metal and Enzyme Catalysis

“…In 1994, Bakhiar and Smith found that acyl transfers could be promoted by stoichiometric amounts of stable carbenes [113]. In 2002, Nolan's [51] and Hedrick's [52] research groups reported that N-heterocyclic carbenes (NHC) such as 17 and 18 (imidazol-2-ylidenes) are efficient catalysts in the transesterification of esters and alcohols [53,114], as exemplified by Scheme 11. N-Heterocyclic carbenes act as catalysts for living ring-opening polymerizations of lactones, generating polyesters [115,116] (see e.g., Scheme 4).…”