Recyclable heterogeneous and low-loading homogeneous chiral imidazolidinone catalysts for α-alkylation of aldehydes

Abstract

Two polystyrene-supported and six homogeneous MacMillan imidazolidinone catalysts were prepared and tested for the asymmetric α-alkylation of propanal with benzodithiolylium tetrafluoroborate. The chiral imidazolidinone was linked to polystyrene through the N-3 atom or through the phenyl ring and their catalytic activity was compared with that of their unsupported precursors. This comparison has allowed us to find an unsupported catalyst that displays high catalytic activity down to 5 or 2 mol-% at room temperature with a high level of enantioselectivity also when used with hexanal and 3-phenylpropanal. In addition, one of the heterogeneous materials was revealed to be highly recyclable for at least eight cycles with no loss in efficiency working at the same levels of activity and enantioselectivity as the unsupported counterpart. Recycling drive: Is it possible to have a supported chiral imidazolidinone catalyst that is recyclable without loss in activity or a homogeneous counterpart catalytically active at low loading? We have investigated this topic using the α-alkylation of aldehydes as a benchmark reaction (see figure). Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.