Computational Evidence for the Existence of Cu~(III) Intermediates in Addition and Substitution Reactions with Dialkykuprates

摘要

Dialkykuprates are widely used nucleophilic agents in organic synthesis. Whether these species function as electron transfer agents or react nucleophilicalh has been debated at length, yet the mechanism remains unresolved. A potential problem with thenucleophilic mechanisms proposed by Corey for additions to unsaturated bonds, and by Goering et al. and Kraft for substitution reactions, is that in both cases a Cu~(III) intermediate is involved. To date, there is no direct experimental evidence for the existence of trialkylcopper structures. We now demonstrate computationally that trimethykopper, Cu(CH_3)_3, indeed corresponds to an energy minimum when the copper center is stabilized by donors such as water or ammonia. Even in the absence of such electron donors. Cu(CH_3)_3 corresponds to a minimum at some of the levels of theory employed. These findings support the intermediacy of trialkylcopper species in the presence of ether solvents and/or nitrogen ligands, which are commonly used in dialkylcuprate reactions. We also show that Cu(CH_3)_3 and the inorganic CuF_3 molecule have similar geometries: however, the copper center in the former has a much smaller natural charge than in the latter. The copper charge in Cu(CH_3)_3 is, in fact, similar tothat in the Cu~(II) organocopper species [CuCH_3]~ + and Cu(CH_3)_2.