Early transition metal promoted reductive CO coupling. Mechanistic details of zirconocene enediolate formation from isotope crossover experiments and molecular orbital studies

Molecular orbital calculations together with an isotope crossover study provide insight into mechanistic details of enediolate‐forming early transition metal promoted reductive CO coupling reactions. It is shown by deuterium labeling, that Bercaw's carbonylation of Cp Zr(CH3)2 (1), which directly leads to the enediolate complex 2, is an exclusively intramolecular process. Both methyl groups and the two CO molecules of 2 assemble at a single Zr center. MO investigations for Cp2Zr(CH3)2 as a model of 1 reveal that such group 4 metallocene‐promoted enediolate‐producing reactions do not involve the proposed intramolecular coupling of two acyl ligands of a bis(acyl) complex generated by double CO insertion, as found for actinide derivatives. Based upon related available experimental evidence and the electronic structure of the monoinsertion product Cp2Zr(COCH3)(CH3), an alternative mechanism for enediolate formation is suggested and tested by extensive MO calculations. The postulated keystep consists of a methyl to acetyl migration in Cp2Zr(COCH3)(CH3) leading to an η2‐acetone intermediate Cp2Zr(η2‐acetone) (30). Interestingly, the methyl‐acetyl bond formation in model calculations requires a prohibitively high energy, unless it is assisted by an additional electron donor at the metal during the transfer of the CH3 group to the acetyl carbon. Using a CO molecule as an incoming model nucleophile, a low energy pathway is found, which can lead from Cp2Zr(COCH3)(CH3) to a zirconaoxirane intermediate 36 (an η2‐acetone complex), carrying an additional stabilizing CO ligand. Subsequent CO insertion in 36 and further rearrangement opens a reaction channel towards the enediolate monomer. The overall mechanistic picture emerging from this experimental/theoretical study is consistent with recent results of the Bercaw group, which has actually isolated the postulated intermediates of type 36. The electronic differences between the systems treated here and corresponding actinide or bis(aryloxy) analogs are briefly discussed.