Revealing the Similarities of alpha,beta-Unsaturated Iminiums and Acylazoliums in Organocatalysis

摘要

The secondary amine-catalyzed reactions proceeding via alpha,beta-unsaturated iminiums and the N-heterocyclic carbene (NHC)-catalyzed transformations taking place via alpha,beta-unsaturated acylazoliums are the two widely used electrophilic intermediates in organocatalysis. Over the last two decades, these two intermediates are extensively utilized for the enantioselective construction of valuable molecules. Both intermediates are generated by the covalent binding of catalysts to the substrates leading to LUMO activation of alpha,beta-unsaturated carbonyls. A variety of soft nucleophiles are known to add to the alpha,beta-unsaturated iminiums and acylazoliums in a conjugate fashion, and in many cases, striking similarity in reactivity has been observed. Having said this, there are few cases where these intermediates exhibit difference in reactivity. This Minireview is aimed at highlighting the resemblances in reactivity between alpha,beta-unsaturated iminiums and acylazoliums thereby shedding light on the unnoticed parallels of the two intermediates in organocatalysis.