Observations regarding the mechanisms of O-atom transfer from metal nitro ligands to oxidizable substrates

Abstract

New experimental evidence is presented related to the mechanism of O atom transfer from metal nitro complexes to alkenes and carbon monoxide. Oxidation of alkenes by the nitro complex (py) CoTPPNO/sub 2/ in the presence of Pd(CH/sub 3/CN)/sub 2/Cl/sub 2/ as a cocatalyst is found to give product distributions that do not exactly match those expected for the simple intermolecular bimetallic O atom transfer originally proposed. Pyridine, nitro, and nitrosyl ligand exchange between the cobalt and palladium are found to be facile. In one plausible revised mechanism, the bimetallic alkene oxidation could therefore proceed via nitro group transfer to palladium, oxidation via the same metallacycles as for the monometallic Pd(CH/sub 3/CN)/sub 2/ClNO/sub 2/ catalyst, and transfer of the resulting nitrosyl ligand back to cobalt. Several other complex mechanisms cannot be ruled out. Other systems were also investigated, but no evidence could be found for a bimetallic open-chain intermediate formed by intermolecular addition of a metal nitro complex to a coordinated alkene. A literature claim for intermolecular O atom transfer from metal nitro groups to carbon monoxide based on isotopic double-labeling experiments is incomplete due to the absence of suitable control experiments. Attempts to carry out the requisite controls were hamperedmore » by experimental limitations, but the results obtained again show that there is no compelling evidence for intermolecular O atom transfer from a metal nitro group. 42 references, 2 tables.« less