Reversed Electron Apportionment in Mesolytic Cleavage: The Reduction of Benzyl Halides by SmI 2

The paradigm that the cleavage of the radical anion of benzyl halides occurs in such a way that the negative charge ends up on the departing halide leaving behind a benzyl radical is well rooted in chemistry. By studying the kinetics of the reaction of substituted benzylbromides and chlorides with SmI 2 in THF it was found that substrates para ‐substituted with electron‐withdrawing groups (CN and CO 2 Me), which are capable of forming hydrogen bonds with a proton donor and coordinating to samarium cation, react in a reversed electron apportionment mode. Namely, the halide departs as a radical. This conclusion is based on the found convex Hammett plots, element effects, proton donor effects, and the effect of tosylate (OTs) as a leaving group. The latter does not tend to tolerate radical character on the oxygen atom. In the presence of a proton donor, the tolyl derivatives were the sole product, whereas in its absence, the coupling dimer was obtained by a S N 2 reaction of the benzyl anion on the neutral substrate. The data also suggest that for the para ‐CN and CO 2 Me derivatives in the presence of a proton donor, the first electron transfer is coupled with the proton transfer.