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

• Published in: Organometallics 1996-10, Vol.15 (22), p.4687-4694
• Main Authors: Scrivanti, Alberto, Beghetto, Valentina, Bastianini, Alessandra, Matteoli, Ugo, Menchi, Gloria
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om960429s

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 = (R)-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 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).