Diarylprolinol silyl ether catalyzed asymmetric friedel-crafts alkylation of indoles with α,β-unsaturated aldehydes: Enhanced enantioselectivity and mechanistic investigations

Abstract

A highly enantioselective Friedel-Crafts alkylation of indoles with α,β-unsaturated aldehydes with excellent enantioselectivities (up to >99%ee) has been developed, and this improved method offers substantial advantages over traditional approaches, not only by avoiding the use of acids or bases, but also in terms of the higher level of stereoselectivity. In addition, we have demonstrated through a plausible mechanism that the role of silicon in the diarylprolinol silyl ethers (Jørgensen-Hayashi organocatalyst) not only serves as a bulky group to induce steric repulsion but also serves as a Lewis acidic promoter to accelerate the reaction between the secondary amine and the substrate (α,β-unsaturated aldehyde). A highly enantioselective Friedel-Craftsalkylation of indoles with α,β-unsaturated aldehydes with excellent enantioselectivities (up to >99%ee) has been developed, and this method offers substantial advantagesover traditional approaches, not only by avoiding the use of acids or bases, but also in terms of the high level of stereoselectivity. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.