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

1,3-Disubstituted imidazol(in)ium-2-carboxylates (ImCO2) 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 ImCO2 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 ImCO2 exhibited varied transesterification activity.

摘要采用咪唑 (啉)-2-羧酸盐 (ImCO2) 作为催化剂前驱体, 通过温度控制脱除 ImCO2 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 (ImCO2) 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.Figure optionsDownload as PowerPoint slide