The mechanism of cyclopropanation catalysed by Cu(I) complexes has been investigated by calculation using a series of diazoalkanes containing inductive electron donating (methyl) and withdrawing (CF3) substituents and a range of metal fragments (Cu+, [(DAB)Cu]+, ClCu and (triflate)Cu). Copper–diazoalkane complexes exist as an equilibrium of C- and N-bonded isomers. Catalysis occurs through lowering of the activation energy for rate determining C–N bond cleavage of the C-bonded isomer; this is most marked for (triflate)Cu. Direct reaction of the copper–carbene complex occurs to yield stable cupracyclobutanes in all but one case. Associative substitution of the cupracyclobutane by diazoalkane completes the catalytic cycle.
