Reversal of Enantioselectivity in the Hydroformylation of Styrene with [2S,4S-BDPP]Pt(SnCl₃)Cl at High Temperature Arises from a Change in the Enantioselective-Determining Step

Deuterioformylation of styrene catalyzed by [(2S,4S)-BDPP]Pt(SnCl₃)Cl at 39 °C gave 3-phenylpropanal (3) and 2-phenylpropanal (2) (n:i = 1.8, 71% ee (S)-2) with deuterium only β to the aldehyde carbonyl and in the formyl group. Small amounts of deuterium were also found in the internal (2.8%), cis terminal (1.4%), and trans terminal (1.3%) vinyl positions of the recovered styrene. Deuterioformylation of styrene at 98 °C gave 3- (3) and 2-phenylpropanal (2) (n:i = 2.3, 10% ee (R)-2) with deuterium both α and β to the aldehyde carbonyl and in the formyl group. Deuterium was also found in the internal (20%), cis terminal (12%), and trans terminal (12%) vinyl positions of the recovered styrene. These deuterioformylation results establish that platinum hydride addition to styrene is largely irreversible at 39 °C but reversible at 98 °C. Hydroformylation of (E)- and (Z)-β-deuteriostyrene at 40 °C, followed by oxidation of the aldehydes to acids, and subsequent derivitization to the (S)-mandelate esters confirmed that 84% of 2-phenylpropanal (2) arises from platinum hydride addition to the si-face of styrene, while 73% of 3-phenylpropanal (3) arises from platinum hydride addition to the re-face of styrene. At 100 °C, the effect of variable H₂ and CO pressure on n:i, % ee, and TOF of hydroformylation of styrene was investigated. The results are consistent with enantioselectivity not being fully determined until the final hydrogenolysis of a platinum acyl intermediate.