Preparation of enantiomerically pure chelate ligands L-2 = XCH2CH(OH)CH2Y from epichlorohydrin - Conformation of their L2Rh(COD)(+) derivatives and enantioselective hydrogenation by L2Rh(COD)(+)

Enantiomerically pure chelate Ligands L-2 = XCH2CH-(OH)CH2Y (1) are obtaine d from epichlorohydrin in a two-step synthesis. X and Y may be different ty pes of R2P donor groups, NR2 or SR donors. The OH function of 1 may be tran sformed into an ether function under specialized conditions. Ligands 1 reac t with [Rh(COD)Cl](2) in the presence of KPF6 to give the coordination comp ounds 2, [L2Rh(COD)]+PF6-, as orange, microcrystalline salts. The structure s adopted by compounds 2 in the solid state have been analysed by X-ray cry stallography in selected cases, and it has been found that the six-membered chelate cycles adopt twist as well as chair conformations depending on the nature of X and Y. In solution, compounds 2 generally show dynamic behavio ur, which is in part due to the conformational flexibility of the six-membe red cycles. In cases where one of the PR2 donor groups contains ortho-subst ituted phenyl substituents, rotational isomerism of these groups is an addi tional dynamic process. For some of these compounds, the nature of the dyna mic behaviour has been analysed by variable-temperature NMR experiments. Co mpounds 2 are found to be precatalysts in the hydrogenation of (Z)-2-acetam idocinnamic acid. The rate of conversion is strongly influenced by the ster ic bulk of the substituents, with smaller substituents leading to higher ra tes. Enantiomeric discrimination is high only for those ligands that contai n ortho-substituted aryl groups at their PR2 donors. The maximum enantiomer ic excess observed was 85% for X = PPh2, Y = P(2-MeOPh)(2).