On the economic application of DuPHOS rhodium(I) catalysts: a comparison of COD versus NBD precatalysts

The effectiveness of cyclooctadiene and norbornadiene precatalysts of the t ype [Rh(DuPHOS)(diolefin)]BF4 in catalytic asymmetric hydrogenation of vari ous prochiral olefins has been examined. In some of the systems studied, th e NBD complex gave rise to the catalytically active species more rapidly th an the corresponding COD complex, as expected. However, as catalyst loading s were reduced to levels more conducive to economic manufacture, the differ ence between the use of COD and NBD precatalysts became increasingly insign ificant. This was conveniently highlighted by the formation of low enantiom eric excess products upon using an equimolar mixture of (S,S) and (R,R) pre catalysts, bearing COD and NBD respectively. With other substrates, the sys tem displayed no induction time for either precatalyst and identical reacti on profiles were observed. (C) 2001 Elsevier Science Ltd. All rights reserv ed.