Solution studies of the asymmetric hydrogenation catalyst system derived from the [rhodium (1,2-bis(diphenylphosphino)-1-cyclohexylethane)] moiety

Abstract

A solution 31P NMR study of the asymmetric hydrogenation catalyst derived from the [rhodium(1,2-bis(diphenylphosphino)-1-cyclohexylethane)] system was initiated to gain further insight into the solution chemistry of this catalyst. In polar, weakly coordinating solvents, the solution behavior is similar to that of previously studied hydrogenation catalysts. However, since this ligand imparts enhanced solubilities to the cationic rhodium complexes, it was possible to study the catalytic intermediates in non-donor solvents such as CH 2Cl 2. When the catalyst precursor complex [Rh(norbornadiene)-(Cycphos)](PF 6) is hydrogenated in CH 2Cl 2, a dimeric structure results. The 31P NMR of this complex formed in CD 2Cl 2 is complex and is interpreted as a mixture of different isomeric dimeric complexes. When the dimeric complex [Rh(Cycphos)] 2(PF 6) 2 is isolated as a crystalline solid and redissolved in CD 2Cl 2, the resultant 31P NMR spectrum is unusual. The spectrum is interpreted as arising from a single dimeric isomer showing novel long-range, non-bonded RhP couplings. This result and its implications to the mechanism of action of these chiral catalysts is discussed.