Non-Linear Effects in Ruthenium-Catalysed Asymmetric Hydrogenation with Atropisomeric Diphosphanes

A strong positive nonlinear effect (asymmetric amplification) was found to take place during asymmetric hydrogenations using chiral atropisomeric diphosphane–ruthenium catalysts. As an example, at atmospheric pressure the use of 50% ee BINAP to prepare [(binap)Ru(Br)2] give rise to a hydrogenated product with 91% ee. The influence of temperature and hydrogen pressure on this effect are presented. These nonlinear effects can be explained on the basis of a hydrogenation mechanism in which diastereomeric dimers as pre-catalytic species are present.