Asymmetric hydroformylation of styrene catalyzed by platinum(II)-alkyl complexes containing atropisomeric diphosphines

The complexes [Pt(CH3)Cl(P-P)] 1-3 (1, P-P = (S)-6,6′-(dimethoxybiphenyl)-2,2′-diylbis-(diphenylphosphine) ((S)-MOBIPH); 2, P-P = OR)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl ((R)-BINAP); 3, P-P = (2S,3S)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-(diphenylphosphino)-butane ((S,S)-DIOP)) in the presence of SnCl2 catalyze the asymmetric hydroformylation of styrene. The reaction proceeds under mild conditions (50°C, P(H2) = P(CO) = 50 atm) to give the desired branched aldehyde with moderate regioselectivity. Good enantioselectivities (up to 75%) have been obtained using [Pt(CH3)Cl{(S)-MOBIPH}]. The influence of solvent, temperature, P(H2), and P(CO) has been studied. An impressive influence of the solvent has been observed: using [Pt(CH3)Cl{(R)-BINAP}], the chirality of 2-phenylpropanal obtained 3, in toluene or in tetrahydrofuran is opposite to that of 2-phenylpropanal produced in dichloromethane or acetone. Using [Pt(CH3)Cl{(S)-MOBIPH}] or [Pt(CH3)Cl{(R)-BINAP}], an unusual increase of the rate and enantioselectivity of the reaction with increasing P(CO) is observed. In order to get information on the reaction mechanism, the carbonylation of [Pt(CH3)(SnCl3){(S)-MOBIPH}] (4) has been studied. This reaction carried out at room temperature and atmospheric pressure affords an equilibrium mixture containing the cationic alkyl complex [Pt(CH3)(CO){(S)-MOBIPH}]+[SnCl3]- (6) and the neutral acyl species [Pt(COCH3)(SnCl3){(S)-MOBIPH}] (7). The carbonylation of [Pt(CH3)(SnCl3){(R)-BINAP}] (5) proceeds in the same fashion to give [Pt(CH3)(CO){(R)-BINAP}]+[SnCl3]- (8) and [Pt(COCH3)(SnCl3){(R)-BINAP}] (9).