Silver-catalysed enantioselective addition of O-H and N-H bonds to allenes: A new model for stereoselectivity based on noncovalent interactions

Abstract

The ability of silver complexes to catalyse the enantioselective addition of O-H and N-H bonds to allenes is demonstrated for the first time by using optically active anionic ligands that were derived from oxophosphorus(V) acids as the sources of chirality. The intramolecular addition of acids, alcohols, and amines to allenes can be achieved with up to 73 % ee. The exploitation of a C-H anomeric effect allowed the absolute configuration of a sample of 2-substituted tetrahydrofuran of low ee to be unambiguously assigned by comparison of the chiroptical ORD and VCD measurements with calculated spectra. In the second part of the work, the origin of the stereoselectivity was probed by DFT free-energy calculations of the transition states. A new model of enantiomeric differentiation was developed that was based on noncovalent interactions. This model allowed us to identify the source of stereoselectivity as weak attractive interactions; such dispersive forces are often overlooked in asymmetric catalysis. A new computational approach was developed that represents these interactions as colour-coded isosurfaces that are characterised by the reduced density-gradient profile. Come on allene: The enantioselective addition of O-H and N-H bonds to allenes was achieved by using chiral silver complexes. A new computational method revealed that the stereoselectivity resulted from weak noncovalent interactions. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.