Water-soluble metal complexes and catalysts: X. BIPHLOPHOS, a novel axially chiral diphosphine ligand: Preparation and application in the asymmetric rhodium-catalyzed hydroformylation of styrene

A stereospecific synthesis of both enantiomers of the new axially chiral diphosphine ligand 2,2′-bis(diphenylphosphinomethyl)-4,4′,6,6′-tetrachloro-1, 1′-biphenyl 5 called BIPHLOPHOS, starting from the readily available pure enantiomers of 4,4′,6,6′-tetrachlorodiphenic acid, was developed. The (-)-enantiomer was then tested in the homogeneous rhodium-catalyzed asymmetric hydroformylation of styrene. The ligand was used to prepare the complex IrH(CO)(PPh₃)L₂ (L₂=5) 6, the crystal structure of which was determined and compared with the related complex RhH(CO)(PPh₃)(BISBI). Additionally, ligand 5 was sulfonated by classical methods to yield a uniform reaction product. This is in sharp contrast to the analogous ligands BISBI and NAPHOS, where sulfonation yields a complicated mixture of products. The sulfonated ligand is excellently water-soluble and shows activities and regioselectivities in the hydroformylation of styrene comparable to the well-investigated ligand BISBI.