Gas-phase synthesis of the homo and hetero organocuprate anions [MeCuMe]-, [EtCuEt]-, and [MeCuR]-

The homocuprates MeCuMe]- and EtCuEt]- were generated in the gas phase by double decarboxylation of the copper carboxylate centers MeCO2CuO2CMe]- and EtCO2CuO2CEt]-, respectively. The same strategy was explored for generating the heterocuprates MeCuR]- from MeCO2CuO2CR]- (R = Et, Pr, iPr, tBu, allyl, benzyl, Ph). The formation of these organocuprates was examined by multistage mass spectrometry experiments, including collision-induced dissociation and ion-molecule reactions, and theoretically by density functional theory. A number of side reactions were observed to be in competition with the second stage of decarboxylation, including loss of the anionic carboxylate ligand and loss of neutral alkene via beta-hydride transfer elimination. Interpretation of decarboxylation of the heterocarboxylates MeCO2CuO2CR]- was more complex because of the possibility of decarboxylation occurring at either of the two different carboxylate ligands and giving rise to the possible isomers MeCuO2CR]- or MeCO2CuR]-. Ion-molecule reactions of the products of initial decarboxylation with allyl iodide resulted in C-C coupling to produce the ionic products ICuO2CR]- or MeCO2CuI]-, which provided insights into the relative population of the isomers, and indicated that the site of decarboxylation was dependent on R. For example, MeCO2CuO2CtBu]- underwent decarboxylation at MeCO2- to give MeCuO2CtBu]-, while MeCO2CuO2CCH2Ph]- underwent decarboxylation at PhCH2CO2- to give MeCO2CuCH2Ph]-. Each of the heterocuprates MeCuR]- (R = Et, Pr, iPr, allyl, benzyl, Ph) could be generated by the double decarboxylation strategy. However, when R = tBu, intermediate MeCuO2CtBu]- only underwent loss of tBuCO2-, a consequence of the steric bulk of tBu disfavoring decarboxylation and stabilizing the competing channel of carboxylate anion loss. Detailed DFT calculations were carried out on the potential energy surfaces for the first and second decarboxylation reactions of all homo- and heterocuprates, as well as possible competing reactions. These reveal that in all cases the first decarboxylation reaction is favored over loss of the carboxylate ligand. In contrast, other reactions such as carboxylate ligand loss and beta-hydride transfer become more competitive with the second decarboxylation reaction.