Guidelines for the Use of Proton Donors in SmI2 Reactions: Reduction of α-Cyanostilbene

The reduction of a series of alpha-cyanostilbenes with SmI(2) was studied in THF in the presence of various proton donors. No reaction occurred in the presence of the alcohols TFE, i-PrOH and t-BuOH. In the presence of MeOH, water and ethylene glycol the reactions occurred; however in the presence of water and ethylene glycol they were too fast for kinetic determinations (tau(1/2) < 1 ms). Reactions with MeOH were first order in SmI(2) and first order in the substrate. The order in MeOH varies as a function of its concentration and the plot of log k vs log [MeOH] is sigmoidal. Comparison of the kinetic isotope effect and the incorporation isotope effect suggests that, counterintuitively, protonation of the radical anion takes place on the carbon beta to the cyano group. It is concluded that proton donors that form complexes with SmI(2) expand the range of substrates that can be reduced by SmI(2). This is due to their proximity to the radical anion as it is formed. This short-lived radical anion cannot be efficiently trapped by a proton donor from the bulk medium. A protocol is herein suggested as to when proton donors which complex to SmI(2), e.g. MeOH, water and ethyleneglycol should be used, and when it is recommended to use noncomplexing proton donors, e.g. TFE, i-PrOH and t-BuOH, to induce reaction.