Chiral chelate phosphanes: XI. Application of cyclopentane-based C2 chiral bis(phosphane) ligands C5H8(PR2)2 to PtSn-catalyzed styrene hydroformylation

Treatment of [(1,5-C8H12)PtCl(X)] (X=Cl, CH3, CH2CMe3) with C2 chiral cyclopentane-1,2-diyl-bis(phosphanes) C5H8(PR2)2, either as racemic mixtures or as resolved enantiomers, afforded the chelate complexes [C5H8(PR2)2Pt(Cl)(X)] (X=Cl: R=Ph (1), N-pip (2), OPh (3); X=CH3: R=Ph (4), N-pip (5), OPh (6); X=CH2CMe3: R=Ph (7), N-pip (8), OPh (9); ‘N-pip’=N(CH2)5), (+)-[(1R,2R)-C5H8{P(OPh)2}2PtCl2] [(R,R)-3], (−)-[(1S,2S)-C5H8{P(OPh)2}2PtCl2] [(S,S)-3], (−)-[(1R,2R)-C5H8(PPh2)2Pt(Cl)(X)], and (+)-[(1S,2S)-C5H8(PPh2)2Pt(Cl)(X)] (X=CH3: (R,R)-4, (S,S)-4; X=CH2CMe3: (R,R)-7, (S,S)-7). Reacting 4, 6, and 7 with AgO3SCF3 led to triflate derivatives [C5H8(PR2)2Pt(X)(OSO2CF3)] [X=CH3: R=Ph (11), OPh (12); X=CH2CMe3: R=Ph (13)] with covalently bonded OSO2CF3 ligands. The unusual Pt2 complex [μ-Cl{C5H8(PPh2)2PtCH3}2]O3SCF3 (14) containing an unsupported single PtClPt bridge was also isolated. In the presence of SnCl2, complexes 1, 3, 4, 6, 7, and 9 are catalysts for the hydroformylation of styrene forming 2- and 3-phenylpropanal together with ethylbenzene. Except for 1, they also catalyze the consecutive hydrogenation of the primary propanals to alcohols. High regioselectivities towards 2-phenylpropanal (branched-to-normal ratios ≥91:9) were obtained in hydroformylations catalyzed by 3 and 4, for which the influence of varied CO/H2 partial pressures, catalyst-to-substrate ratios and different reaction temperatures and times on the outcome of the catalytic reaction was also studied. When tin-modified complexes (R,R)-3, (S,S)-3, and (S,S)-4 were used as optically active Pt(II) catalysts, an only low stereoselectivity for asymmetric hydroformylation (e.e.<18%) was observed. The PtSn complexes [C5H8(PR2)2Pt(CH3)(SnCl3)] [R=Ph (15), OPh (17)], resulting from SnCl2 insertion into the PtCl bonds of 4 or 6, undergo rapid degradation in solution, forming mixtures composed of [C5H8(PR2)2Pt(X)(Y)] with R=Ph or OPh and X/Y=Cl/SnCl3 (16, 18), Cl/Cl (1, 3), and SnCl3/SnCl3 (19, 20), respectively. In the presence of SnCl2, triflate complex 11 also becomes a catalyst for styrene hydroformylation and consecutive hydrogenation of the aldehydes to alcohols. The crystal structures of 11 complexes — 2, 5, 7, 8, 9, 10 (the previously prepared [C5H8{P(N-pip)2}2Pt(CH2CMe3)2]), 13, 14, 16, (R,R)-3, and (S,S)-3 — were determined by X-ray diffraction.