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Early transition metal promoted reductive CO coupling. Mechanistic details of zirconocene enediolate formation from isotope crossover experiments and molecular orbital studies
Author(s) -
Hofmann Peter,
Stauffert Peter,
Frede Martin,
Tatsumi Kazuyuki
Publication year - 1989
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19891220828
Subject(s) - chemistry , intramolecular force , reductive elimination , kinetic isotope effect , metallocene , molecule , nucleophile , transition state , molecular orbital , nucleophilic addition , methyl group , cyclopentadienyl complex , deuterium , computational chemistry , stereochemistry , alkyl , organic chemistry , catalysis , physics , polymer , quantum mechanics , polymerization
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 2 * Zr(CH 3 ) 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 Cp 2 Zr(CH 3 ) 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 Cp 2 Zr(COCH 3 )(CH 3 ), 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 Cp 2 Zr(COCH 3 )(CH 3 ) leading to an η 2 ‐acetone intermediate Cp 2 Zr(η 2 ‐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 CH 3 group to the acetyl carbon. Using a CO molecule as an incoming model nucleophile, a low energy pathway is found, which can lead from Cp 2 Zr(COCH 3 )(CH 3 ) to a zirconaoxirane intermediate 36 (an η 2 ‐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.