Dimethylcuprate-Catalyzed Decarboxylative Coupling of Allyl Acetate

Abstract

Allylic alkylation of organocuprates represents an important class of C–C bond forming reactions. A drawback of these reactions is their stoichiometric nature. Although metal-catalyzed decarboxylative coupling reactions offer new opportunities for formation of C–C bonds, catalytic allylic alkylation reactions of organocuprates have not been previously reported. Here, multistage mass spectrometry experiments performed on ion trap mass spectrometers in conjunction with electronic structure calculations are used to demonstrate that the dimethylcuprate anion, [CH3CuCH3]−, can catalyze decarboxylative coupling of allyl acetate in the gas phase via a simple two-step catalytic cycle. In step 1, [CH3CuCH3]− undergoes a cross-coupling reaction with allyl acetate to yield [CH3CuO2CCH3]− as the major product ion. Step 2 involves subjecting the product ion to collision-induced decarboxylation to re-form [CH3CuCH3]−, thereby closing the catalytic cycle. Since the details of step 2 have been previously described (Rijs, N.; Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. J. Am. Chem. Soc. 2008, 130,1069−1079), here the focus is on step 1. Comparison of the reactivity of dimethylcuprate toward allylic susbtrates reveals that while allyl acetate reacts around 70 times more slowly than allyl iodide, it is more selective for cross-coupling. This is rationalized by DFT calculations, which reveal that an increase in kinetic barrier is responsible for both reactivity trends. The lowest energy path was found to involve a stepwise π-oxidative addition proceeding via an η2-C3H5O2CCH3 intermediate and extrusion of a leaving group (LG) anion, followed by a reductive elimination where this LG is recomplexed to the copper center. Finally, DFT calculations were used to shed light on the role of leaving groups LG = CH3CO2–, I–, Br–, Cl– in the allylic alkylation. While selectivity is indeed achieved at the cost of reactivity, the LG effects were more complex than can be accounted for by a simple consideration of the anion proton affinity (APA) of the LG.