Oxidative Addition of Methyl Iodide to [RhI(CO){Ph₂PCH₂P(S)Ph₂}]

The chelating P-S ligand, Ph₂PCH₂P(S)Ph₂ (dppms), has recently been shown to be unusually active in promoting the rhodium/iodide catalysed carbonylation of methanol to acetic acid.¹ In this paper we present kinetic data concerning a key reaction in the catalytic cycle, the oxidative addition of methyl iodide to [RhI(CO)(dppms)] (1). The reaction leads to a stable acetyl complex, [RhI₂(COMe)(dppms)] (3) via a methyl intermediate, [RhI₂Me(CO)(dppms)] (2). We present evidence that both oxidative addition and migratory insertion steps are accelerated relative to the conventional Monsanto catalyst, [RhI₂(CO)₂]^-, explaining the enhanced catalytic activity. We also report an X-ray crystal structure of the acetyl product, 3.