Promotional effect of CH₃I on hydroxycarbonylation of cycloalkene using homogeneous rhodium catalysts with PPh₃ ligand

The reaction mechanism of the hydroxycarbonylation of cyclohexene with formic acid to form cyclohexanecarboxylic acid using organorhodium complexes was investigated both experimentally and computationally. It was found that excess CH₃I and PPh₃ promote this reaction. The active catalyst species was determined to be a five-coordinated rhodium hydride 16-electron complex derived from a mononuclear rhodium 16-electron complex (Vaska-type complex). PPh₃ and CH₃I played the following roles: i) to promote the formation of the Vaska-type Rh complex from Rh₂Cl₂(CO)₄ (1), ii) to aid the elimination of the PPh₃ ligand from the Vaska-type complex by stabilizing the free energy of formation of a mononuclear three-coordinated rhodium 14-electron complex through the formation of phosphonium iodide ([Ph₃P(CH₃)]I), and iii) to generate the active rhodium hydride species.