Investigation of the rhodium-catalyzed hydroboration of NHC-boranes: the role of alkene coordination and the origin of enantioselectivity.

The mechanism of the intramolecular enantioselective rhodium(i)-catalyzed hydroboration of NHC-boranes is investigated by experimental reactivity measurements and molecular electronic structure calculations, within the framework of the Density Functional Theory and the Random Phase Approximation methods. The crucial role of alkene coordination and the origin of enantioselectivity are discussed. Two possible mechanisms are considered, via either prior hydride migratory insertion or boron migratory insertion. The minimum energy computed pathway leads to the enantiomer experimentally observed, therefore supporting the hydride migratory insertion mechanism. Calculations of the final steps of the catalytic cycle, i.e. regeneration of the catalyst and release of the product, give us further insights into the mechanism and rationalize the experimental results.