The steroidal 1,4-diphosphines 3α- and 3β-diphenylphosphino-2a-(2'-diphenylphosphinoethyl)-5α-cholestanes and their 5H-benzo[b] phosphindole derivatives have been prepared and shown to be useful ligands in asymmetric hydrogenation reactions. Interestingly the 3α- and 3β-derivatives lead to opposing enantioselection preferences when used in these reactions. A steroidal 1,3-diphosphine, 3α-diphenylphosphino-2α-diphenylphosphinomethyl-5α-cholestane, has been prepared as a mixture containing some of the 3β-epimer. The 3α-1,3-diphosphine led to similar enantioselection in hydrogenation reactions as the 3α-1,4-diphosphine, and a model is proposed to explain the sense of the enantioselectivity in the 1,4- and 1,3-diphosphines. A steroidal 1,6-diphosphine has also been prepared but leads to lower optical yields in the hydrogenation reactions. These ligands have been shown to lead to only poor to moderate optical yields when used in asymmetric carbon-carbon bond forming reactions.
Emerging artificial metalloenzymes for asymmetric hydrogenation reactions
