Reversal of Enantioselectivity in the Hydroformylation of Styrene with [2<i>S,</i>4<i>S</i>-BDPP]Pt(SnCl<sub>3</sub>)Cl at High Temperature Arises from a Change in the Enantioselective-Determining Step

Deuterioformylation of styrene catalyzed by [(2<i>S</i>,4<i>S</i>)-BDPP]Pt(SnCl<sub>3</sub>)Cl at 39 °C gave 3-phenylpropanal (<b>3</b>) and 2-phenylpropanal (<b>2</b>) (<i>n</i>:<i>i</i> = 1.8, 71% ee (<i>S</i>)-<b>2</b>) 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%), <i>cis</i> terminal (1.4%), and <i>trans</i> terminal (1.3%) vinyl positions of the recovered styrene. Deuterioformylation of styrene at 98 °C gave 3- (<b>3</b>) and 2-phenylpropanal (<b>2</b>) (<i>n:i</i> = 2.3, 10% ee (<i>R</i>)-<b>2</b>) with deuterium both α and β to the aldehyde carbonyl and in the formyl group. Deuterium was also found in the internal (20%), <i>cis</i> terminal (12%), and <i>trans</i> 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 (<i>E</i>)- and (<i>Z</i>)-β-deuteriostyrene at 40 °C, followed by oxidation of the aldehydes to acids, and subsequent derivitization to the (<i>S</i>)-mandelate esters confirmed that 84% of 2-phenylpropanal (<b>2</b>) arises from platinum hydride addition to the <i>si</i>-face of styrene, while 73% of 3-phenylpropanal (<b>3</b>) arises from platinum hydride addition to the <i>re</i>-face of styrene. At 100 °C, the effect of variable H<sub>2</sub> and CO pressure on <i>n:i</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.