Hydroxycarbonylation of Alkenes with Formic Acid Catalyzed by a Rhodium(III) Hydride Diiodide Complex Bearing a Bidentate Phosphine Ligand

An improved catalytic system was
developed for the Rh complex-catalyzed
hydroxycarbonylation of alkenes using formic acid (HCOOH). Ligand
screening revealed DPEphos, a bidentate phosphine ligand, to be effective,
and the corresponding rhodium­(III) hydride diiodide complex, RhHI2(CO)­(DPEphos) (2), was applied as a catalyst.
Complex 2 showed high activity and selectivity in the
hydroxycarbonylation of alkenes using HCOOH without any additives
and was applicable to a variety of alkene substrates. Analysis of
the catalytic cycle using density functional theory calculations was
used to elucidate the reaction steps. Initially, an alkyl group was
formed by the insertion of an alkene into Rh–H with subsequent
acyl group formation via the insertion of a CO ligand, similar to
the reaction steps suggested for Rh-catalyzed alkene hydroformylation.
The latter part of the catalytic cycle involved the reductive elimination
of acyl iodide and subsequent hydrolysis to form a carboxylic acid,
analogous to the reaction steps suggested for the synthesis of acetic
acid from methanol and CO using a rhodium complex catalyst with hydroiodic
acid as an auxiliary catalyst. These results indicate that the iodine
atom in complex 2 acts as an auxiliary catalyst during
the additive-free hydroxycarbonylation of alkenes using HCOOH.