CO2 fixation by alkylzinc amides:: A quantum chemical study motivated by recent experimental results

Herein the insertion of CO2 into the Zn-N bond of alkylzinc amides of the general formula RZnNR'2 (R = H, CH3, CF3, R' = CH3, iPr, H, F, C6H5) is discussed on the basis of quantum chemical calculations. Several possible reaction pathways were considered, starting from monomeric as well as dimeric alkylzinc amides. Special consideration is given to the influence of the electronic properties of the amido group on the reaction energy. The mechanisms were studied in detail starting with the model compounds HZnNH2 and H3CZnN(CH3)(2) as well as the dimer [HZnNH2](2). The reaction of monomeric compounds with CO2 first leads to an intermediate featuring a Zn-N-C-O four-membered ring. This first step requires an activation energy Delta G of ca. 60 kJ.mol(-1) and turns out to be the rate -determining step. In the second step, which requires a much smaller activation energy, the carbamate product is formed. Reactions of dimeric alkylzinc amides are subject to a significantly higher activation barrier (with a Delta G value of 103 kJ-mol(-1)), with a four-membered ring as the transition- state structure. The activation energy is slightly higher than the energy required for decomposition of the dimer into two monomers, and therefore it is plausible that the alkylzinc amides react as monomers with CO2. The calculations for the dimer reaction again gave evidence for the formation of an intermediate exhibiting in this case a sixmembered ring. The barrier to formation of the carbamate from this intermediate is very small. The results of this work contribute to the knowledge Of CO2 fixation by alkylzinc amides and might shed some light on enzymatic reactivity. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).