Synthesis and X-ray Crystal Structures of Palladium(II) and Platinum(II) Complexes of the PCP-Type Chiral Tridentate Ligand (1R,1′R)-1,3-Bis[1-(diphenylphosphino)ethyl]benzene. Use in the Asymmetric Aldol Reaction of Methyl Isocyanoacetate and Aldehydes

Enantiomerically pure (1R,1′R)-1,3-bis[1-(diphenylphosphino)ethyl]benzene (4) has been obtained for the first time. The key step involves in situ tosylation of chiral (1S,1′S)-dimethyl1,3-benzenedimethanol (6) followed by nucleophilic attack with LiPPh2(BH3) to give [(1R,1′R)1,3-bis[1-(diphenylphosphino)ethyl]benzene]-bisborane (7). The bisborane-protected phosphine can be easily handled and purified before deprotecting with HBF4‚OMe2 to give the title compound. [(1R,1′R)-2,6-bis[1-(diphenylphosphino)ethyl]phenyl]chloropalladium(II) (9) and the platinum(II) analogue (8) were synthesized through reaction of the chiral PCP-type ligand with PdCl2(PhCN)2 and [Pt2(μ-Cl)2(η-CH2C(CH3)CH2)2], respectively, and their X-ray crystal structures were determined. Removal of chloride with AgOTf provided an active catalyst species for the asymmetric aldol reaction of methyl isocyanoacetate and aldehydes.