Imidazol(in)ium-2-carboxylates as Latent, Thermally Activated Organocatalysts for Transesterification Reactions

Abstract

1,3-Disubstituted imidazol(in)ium-2-carboxylates (ImCO 2 ) were used as precatalysts for transesterification reactions of unactivated ethyl benzoate with monohydric alcohols, dihydric alcohols, and amino alcohols. Highly active N -heterocyclic carbenes (NHCs) are usually not used directly as catalysts because of their air and moisture sensitivity. Here they were liberated in situ by thermal decarboxylation of ImCO 2 at designated temperatures. The results showed that the yield of the reactions reached up to 95% within 3 h using only 0.5 mol% catalyst. No active enol esters, molecular sieves, or a 20-fold excess of alcohol were used to drive the reaction to completion. It was proved that unsaturated imidazolium-2-carboxylates have a better activity than saturated species in this type of reaction. Different types of substituting groups of ImCO 2 exhibited varied transesterification activity. 采用咪唑 (啉)-2-羧酸盐 (ImCO 2 ) 作为催化剂前驱体, 通过温度控制脱除 ImCO 2 的 2-位羧酸根, 再原位释放出高活性的 N -杂环卡宾 (NHCs), 进而催化转酯反应. 研究了 NHCs 在苯甲酸乙酯与一元醇、二元醇及氨基醇反应体系中的催化活性. 结果表明, 在该体系中不需要选用活化酯为原料, 无需加入分子筛或加大醇酯比提高产率, 仅用少量催化剂 (0.5 mol%), 在较短时间内 (3 h), 即可使反应转化率达到 95%. 1,3-Disubstituted imidazol(in)ium-2-carboxylates (ImCO 2 ) are air- and water-stable species and efficient precursors of N -heterocyclic carbenes (NHCs), which were found to be effective catalysts for transesterification and acylation reactions.