Unraveling the Importance of Noncovalent Interactions in Asymmetric Hydroformylation Reactions.

For catalytic asymmetric hydroformylation (AHF) of alkenes to chiral aldehydes, though a topic of high interest, the contemporary developments remain largely empirical owing to rather limited molecular insights on the origin of enantioselectivity. Given this gap, herein, we present the mechanistic details of Rh-(,)-YanPhos-catalyzed AHF of α-methylstyrene, as obtained through a comprehensive DFT (ω-B97XD and M06) study. The challenges with the double axially chiral YanPhos, bearing an -benzyl BINOL-phosphoramidite and a BINAP-bis(3,5--Bu-aryl)phosphine, are addressed through exhaustive conformational sampling. The C-H···π, π···π, and lone pair···π noncovalent interactions (NCIs) between the -benzyl and the rest of the chiral ligand limit the -benzyl conformers. Similarly, the C-H···π and π···π NCIs between the chiral catalyst and α-methylstyrene render the -face binding to the Rh-center more preferred over the -face. The transition state (TS) for the regiocontrolling migratory insertion, triggered by the Rh-hydride addition to the alkene, to the more substituted α-carbon is 3.6 kcal/mol lower than that to the β-carbon, thus favoring the linear chiral aldehyde over the achiral branched alternative. In the linear pathway, the TS for the hydride addition to the -face is 1.5 kcal/mol lower than that to the -face, with a predicted of 85% for the aldehyde (expt. 87%). The energetic span analysis reveals the reductive elimination as the turnover determining step for the preferred linear aldehyde. These molecular insights could become valuable for exploiting AHF reactions for substituted alkenes and for eventual industrial implementation.