Guidelines for the use of proton donors in SmI₂ reactions: Reduction of α-cyanostilbene

The reduction of a series of α-cyanostilbenes with SmI₂ 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 (τ_1/2 < 1 ms). Reactions with MeOH were first order in SmI₂ 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 β to the cyano group. It is concluded that proton donors that form complexes with SmI₂ expand the range of substrates that can be reduced by SmI₂. 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 SmI₂, 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.