Abstract
The effectiveness of cyclooctadiene and norbornadiene precatalysts of the type [Rh(DuPHOS)(diolefin)]BF 4 in catalytic asymmetric hydrogenation of various prochiral olefins has been examined. In some of the systems studied, the NBD complex gave rise to the catalytically active species more rapidly than the corresponding COD complex, as expected. However, as catalyst loadings were reduced to levels more conducive to economic manufacture, the difference between the use of COD and NBD precatalysts became increasingly insignificant. This was conveniently highlighted by the formation of low enantiomeric excess products upon using an equimolar mixture of ( S, S) and ( R, R) precatalysts, bearing COD and NBD respectively. With other substrates, the system displayed no induction time for either precatalyst and identical reaction profiles were observed.
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