Recyclable Helical Poly(phenyl isocyanide)-Supported l-Proline Catalyst for Direct Asymmetric Aldol Reaction in Brine

Abstract

A novel helical poly(phenyl isocyanide) bearing Boc protected l-proline pendants (poly-1m) was designed and synthesized. Removed the protecting Boc groups on the l-proline pendants led to the formation of helical polymer poly-2m, which showed high optical activity owing to the preferred right-handed helix of polyisocyanide main chain. Optically active helical poly-2m showed excellent catalytic ability on asymmetric aldol reaction. Helical polymer catalysts exhibited enhanced stereoselectivity in aldol reaction compared to small molecule l-proline. Under the optimized aldol reaction condition, the enantiomeric excess (ee) and diastereomeric ratio (dr) values of the aldol reaction product were respectively up to 90% and > 20/1. Moreover, the helical polyisocyanide catalyst Poly-2m can be easily recovered and reused in the aldol reaction for at least five cycles with maintained its activity and stereoselectivity. Enantioselective aldol reaction catalyzed by poly-2m.