Rhodium(I) mediated arylation of aldehydes with arylboronic acids under base and water free conditions: A computational study

The mechanism for rhodium(I) mediated arylation of aromatic aldehydes by arylboronic acids under base and water free conditions was investigated with DFT methods. The detailed picture resulting from the calculations is that the reaction proceeds via an internal base mechanism, whereby the initially formed alcoholate (obtained by aryl migration to the aldehyde) attacks the electrophilic boron atom of the coordinated arylboronic acid substrate to facilitate the aryl transfer to the metal. Alternative pathways involving B-C oxidative addition or internal proton transfer followed by beta-aryl transfer from thus resulting Rh-O-BPh(OH) moieties are kinetically disfavored. The rhodium atom does not change its oxidation state throughout this whole process, and all steps proceed smoothly within the coordination sphere of rhodium(I). Aldehyde migratory insertion into the Rh-I-aryl bond appears to be the rate limiting step (Delta G(I) = +19.4 kcal mol(-1)) of the catalytic cycle. The subsequent elementary steps involved in the transmetallation process proceed with lower barriers (<+16.3 kcal mol(-1)). The cis-[Rh(OH-CHPh2)(OBOHPh)(PNH2(OH)(2))(2)] species R.S. should be the resting state species under the catalytic conditions according to the DFT calculations.