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%). [CH3AuCH3](-) 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/5DD6-31+G(d) level were conducted on four different mechanisms: (i) S(N)2; (ii) S(N)2'; (iii) oxidative-addition/reductive elimination (OA/RE) via an M(III) eta(3)-allyl intermediate; and (iv) OA/RE via an M(III)' eta-allyl intermediate. For copper, mechanisms (iii) and (iv) are predicted to be competitive. Only the Cu(III) eta(3)-allyl intermediate undergoes reductive elimination via two different transition states to yield either the cross-coupling or the homocoupling products. Their relative barriers are consistent with homo-coupling being a minor pathway. For silver, the kinetically most probable pathway is the S(N)2 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) eta(3)-allyl complex. These results clearly highlight the superiority of organocuprates in allylic substitution reactions.