Mechanistic Insights into the Palladium-Catalysed Asymmetric Phosphination of Cyclohexenyl Triflate

Preliminary results on the mechanism of the asymmetric phosphination involving an achiral alkenyl triflate (cyclohexenyl triflate) and a secondary phosphane-borane, (methyl)-phenylphosphane-borane (1) are reported. A catalytic cycle is proposed based on the variable-temperature P-31 NMR characterization of the individual steps (oxidative addition, transmetallation and reductive elimination). Each likely intermediate involved in this asymmetric C-P coupling reaction has been identified and characterized. Hence, cationic oxidative addition complex 3 has been readily synthesized and engaged in the stoichiometric reaction with highly enantio-enriched secondary phosphane-borane (S-P)-1 enabling to isolate the corresponding diastereomerically pure transmetallation adduct (S, S, R-P)-5. Its structure has been confirmed by X-ray crystallography. Its decomposition by reductive elimination affords the highly enantio-enriched coupling product (S-P)-2. This study also provides information on the origin of the enantioselectivity.