Oxidative Addition of Methyl Iodide to [RhI(CO){Ph2PCH2P(S)Ph2}]
7. Summary
- Oxidative addition of methyl iodide to [RhI(CO)(dppms)] (1) is ca. 40x faster than to the conventional catalyst for methanol carbonylation, [RhI2(CO)2]-, in CH2Cl2 at 25 oC.
- Extrapolation to 185 oC using the measured activation parameters predicts that addition will be ca. 10x faster for the dppms complex, closely matching the observed enhancement in catalytic activity at this temperature.
- Second order kinetics and a large negative entropy of activation are consistent with a mechanism involving nucleophilic attack by the Rh centre on methyl iodide. The enhanced nucleophilicity of 1 can be explained by the higher electron density on the metal, due to the electron donating dppms ligand.
- Basic ligands, which promote oxidative addition, often stabilise the resulting alkyl complex. For example, addition of MeI to [RhCl(CO)(PnBu3)2] gives a methyl complex which is inert to migratory insertion [Ref. 9]. For the dppms complex, however, the the rhodium methyl complex, 2, is a transient intermediate. Migratory insertion is rapid and gives a stable acetyl complex, 3.
- The dppms ligand, unusually, promotes both oxidative addition and migratory insertion steps in the catalytic carbonylation cycle.
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