Synthesis of the chiral diphosphine ligand 2,3-bis(diphenylphosphino)butane (CHIRAPHOS)

A new synthesis of CHIRAPHOS starting from readily available 2,3-bis(diphenylphosphinyl)-l,3-butadiene has been devised. The key step of the synthesis is the hydrogenation of the diene to rac-2,3-bis(diphenylphosphinyl)butane. This reduction can be conveniently performed by treating 2,3-bis(diphenylphosphinyl)-1,3-butadiene with NaBH4 in THF/CH2C12 mixtures. The racemic diphosphine oxides are subsequently resolved using (2S,3S)-(+)- and (2R,3R)-(-)-2,3-O-dibenzoyltartaric acid [(+)- and (-)- DBTA].