The mechanism of the copper(I)-catalyzed cyclopropanation reaction has been extensively investigated for a medium-size reaction model by means of B3LYP/6-31G(d) calculations. The starting ethylene complex of the N,N'-dimethylmalonaldiimine--copper (I) catalyst undergoes a ligand exchange with methyl diazoacetate to yield a reaction intermediate, which subsequently undergoes nitrogen extrusion to generate a copper--carbene complex. The cyclopropanation step takes place through a direct carbene insertion of the metal--carbene species to yield a catalyst--product complex, which can finally regenerate the starting complex. The stereochemical predictions of a more realistic model (by considering a chiral bis(oxazoline)--copper (I) catalyst) have been rationalized in terms of steric repulsions, showing good agreement with experimental data.

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The four transition structures (TS's) of the reaction between butadiene and acrolein, both uncatalyzed and catalyzed by BF3, have been theoretically studied taking into account electron correlation effects by means of DFT (B3LYP) calculations. In both the uncatalyzed and catalyzed reactions, the endo s-cis is the most stable of the four possible transition structures. In the case of the catalyzed reaction, the inclusion of electron correlation in the search for this transition structure indicates the classical [4+2] reaction path, instead of that corresponding to a [2+4] inverse electron, demands hetero-Diels−Alder reaction, as happens when the Hartree−Fock level of theory is used. The B3LYP/6-31G(d) calculations lead to activation energy values close to those experimentally found. The origin of the endo/exo selectivity, both in the presence and in the absence of the catalyst, is discussed.