Thiourea-Assisted Iminium Catalysis

Key words

iminium ions - supramolecular chemistry - thioureas

Significance

A thiourea-assisted iminium catalysis has been described. It was found that simple thioureas accelerate previously established reactions of α,β-unsaturated aldehydes with MacMillan’s organocatalyst, presumably by binding to the counteranion of the iminium intermediate. The use of chiral thioureas did not have a significant effect on the enantioselectivity.

Comment

The profound influence of anions on reactivity and selectivity in asymmetric iminium ­catalysis is well established (S. Mayer, B. List ­Angew. Chem. Int. Ed. 2006, 45, 4193). Therefore, thioureas could be expected to influence the activity and selectivity by anion binding during the catalysis (see Review below). In the current report, a mild positive influence of thioureas on reactivity is demonstrated. Development of a chiral thiourea as the only source of asymmetric information remains as the true challenge of this approach.

Review

Z. Zhang, P. R. Schreiner Chem. Soc. Rev. 2009, 38, 1187–1198.