Asymmetric Hydroformylation of Styrene Catalyzed by Platinum(II)−Alkyl Complexes Containing Atropisomeric Diphosphines

The complexes [Pt(CH 3 )Cl(P−P)] 1 − 3 ( 1, P−P = ( S )-6,6‘-(dimethoxybiphenyl)-2,2‘-diylbis(diphenylphosphine) (( S )-MOBIPH); 2, P−P = ( R )-2,2‘-bis(diphenylphosphino)-1,1‘-binaphthyl (( R )-BINAP); 3, P−P = (2 S,3 S )-2,3- O -isopropylidene-2,3-dihydroxy-1,4-(diphenylphosphino)butane (( S, S )-DIOP)) in the presence of SnCl 2 catalyze the asymmetric hydroformylation of styrene. The reaction proceeds under mild conditions (50 °C, P (H 2 ) = P (CO) = 50 atm) to give the desired branched aldehyde with moderate regioselectivity. Good enantioselectivities (up to 75%) have been obtained using [Pt(CH 3 )Cl{( S )-MOBIPH}]. The influence of solvent, temperature, P (H 2 ), and P (CO) has been studied. An impressive influence of the solvent has been observed: using [Pt(CH 3 )Cl{( R )-BINAP}], the chirality of 2-phenylpropanal obtained in toluene or in tetrahydrofuran is opposite to that of 2-phenylpropanal produced in dichloromethane or acetone. Using [Pt(CH 3 )Cl{( S )-MOBIPH}] or [Pt(CH 3 )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(CH 3 )(SnCl 3 ){( 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(CH 3 )(CO){( S )-MOBIPH}] + [SnCl 3 ] - ( 6 ) and the neutral acyl species [Pt(COCH 3 )(SnCl 3 ){( S )-MOBIPH}] ( 7 ). The carbonylation of [Pt(CH 3 )(SnCl 3 ){( R )-BINAP}] ( 5 ) proceeds in the same fashion to give [Pt(CH 3 )(CO){( R )-BINAP}] + [SnCl 3 ] - ( 8 ) and [Pt(COCH 3 )(SnCl 3 ){( R )-BINAP}] ( 9 ).