Part III. COD versus NBD precatalysts. Dramatic difference in the asymmetric hydrogenation of prochiral olefins with five-membered diphosphine Rh-hydrogenation catalysts

Induction periods in the asymmetric hydrogenation of prochiral olefins with five-membered chelates of the type [Rh(PP)(diolefin)]BF4 originate from the parallel-running hydrogenation of the prochiral substrate and the diolefin that enters the system as a constituent of the precatalyst. Reactivities towards the most commonly used diolefins COD or NBD can differ by three powers of ten. X-ray crystal structure analyses of [Rh((S,S)-Et-DuPHOS)(NBD)]BF4 and of [Rh((R,R)-Et-DuPHOS)(COD)]BF4 imply that a recently discussed relation between the sense of rotation of the diolefin in the precatalyst (clockwise or anticlockwise twist) is not very likely. (C) 2001 Elsevier Science B.V. All rights reserved.