title: Reversed Electron Apportionment in Mesolytic Cleavage: The Reduction of Benzyl Halides by SmI2.
abstract: 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 SmI2 in THF it was found that substrates para-substituted with electron-withdrawing groups (CN and CO2 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 SN 2 reaction of the benzyl anion on the neutral substrate. The data also suggest that for the para-CN and CO2 Me derivatives in the presence of a proton donor, the first electron transfer is coupled with the proton transfer.
authors.affiliation: Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900 (Israel).
authors.forename: Offir
authors.initials: O
authors.lastname: Yitzhaki
authors.affiliation: Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900 (Israel). shoz@biu.ac.il.
authors.forename: Shmaryahu
authors.initials: S
authors.lastname: Hoz
keywords: electron transfer; kinetics; radicals; reduction; samarium