Ruthenium-Catalyzed Asymmetric Hydrohydroxyalkylation of Butadiene: The Role of the Formyl Hydrogen Bond in Stereochemical Control

Abstract

The catalyst generated in situ from RuH$_2$(CO)(PPh$_3$)$_3$, ($S$)-SEGPHOS, and a chiral phosphoric acid promotes asymmetric hydrohydroxyalkylation of butadiene and affords enantioenriched $\alpha$-methyl homoallylic alcohols. The observed diastereo- and enantioselectivities are determined by both the chiral phosphine and chiral phosphate ligands. Density functional theory calculations (M06/SDD-6-311G(d,p)−IEFPCM(acetone)//B3LYP/SDD-6-31G(d)) predict that the product distribution is controlled by the kinetics of carbon−carbon bond formation, and this process occurs via a closed-chair Zimmerman−Traxler-type transition structure (TS). Chiral-phosphate-dependent stereoselectivity arising from this TS is enabled through a hydrogen bond between the phosphoryl oxygen and the aldehyde formyl proton present in TADDOL-derived catalysts. This interaction is absent in the corresponding BINOL-derived systems, and the opposite diastereo- and enantioselectivity is observed. Additional factors influencing the stereochemical control are determined.We are grateful to The English-Speaking Union (Lindemann Trust Fellowship), the National Institutes of Health-NIGMS (RO1-GM069445), and the National Science Foundation (CHE-1361104) for financial support. Computational resources were provided by the UCLA Institute for Digital Research and Education (IDRE) and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation (OCI-1053575)