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 (1 R,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 (1 S,1‘ S )-dimethyl-1,3-benzenedimethanol ( 6 ) followed by nucleophilic attack with LiPPh 2 (BH 3 ) to give [(1 R,1‘ R )-1,3-bis[1-(diphenylphosphino)ethyl]benzene]−bisborane ( 7 ). The bisborane-protected phosphine can be easily handled and purified before deprotecting with HBF 4 ·OMe 2 to give the title compound. [(1 R,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 PdCl 2 (PhCN) 2 and [Pt 2 (μ-Cl) 2 (η 3 -CH 2 C(CH 3 )CH 2 ) 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.