Gas-Phase Reactivity of Group 11 Dimethylmetallates with Allyl Iodide

Copper-mediated allylic substitution reactions are widely used in organic synthesis, whereas the analogous reactions for silver and gold are essentially unknown. To unravel why this is the case, the gas-phase reactions of allyl iodide with the coinage metal dimethylmetallates, [CH₃MCH₃]⁻ (M = Cu, Ag and Au), were examined under the near thermal conditions of an ion trap mass spectrometer and via electronic structure calculations. [CH₃CuCH₃]⁻ reacted with allyl iodide with a reaction efficiency of 6.6% of the collision rate to yield: I^– (75%); the cross-coupling product, [CH₃CuI]⁻ (24%); and the homo-coupling product, [C₃H₅CuI]⁻ (1%). [CH₃AgCH₃]⁻ and [CH₃AuCH₃]⁻ reacted substantially slower (reaction efficiencies of 0.028% and 0.072%). [CH₃AgCH₃]⁻ forms I^– (19%) and [CH₃AgI]⁻ (81%), while only I^– is formed from [CH₃AuCH₃]⁻. Because the experiments do not detect the neutral product­(s) formed, which might otherwise help identify the mechanisms of reaction, and to rationalize the observed ionic products and reactivity order, calculations at the B3LYP/def2-QZVP//B3LYP/SDD6-31+G­(d) level were conducted on four different mechanisms: (i) S_N2; (ii) S_N2′; (iii) oxidative-addition/reductive elimination (OA/RE) via an M­(III) η³-allyl intermediate; and (iv) OA/RE via an M­(III) η¹-allyl intermediate. For copper, mechanisms (iii) and (iv) are predicted to be competitive. Only the Cu­(III) η³-allyl intermediate undergoes reductive elimination via two different transition states to yield either the cross-coupling or the homo-coupling products. Their relative barriers are consistent with homo-coupling being a minor pathway. For silver, the kinetically most probable pathway is the S_N2 reaction, consistent with no homo-coupling product, [C₃H₅AgI]⁻, being observed. For gold, no C–C coupling reaction is kinetically viable. Instead, I^– is predicted to be formed along with a stable Au­(III) η³-allyl complex. These results clearly highlight the superiority of organocuprates in allylic substitution reactions.