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

Abstract

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.

Intrinsic barrier for protonation of radical anions

“…4 Namely, protonation occurs on the site that generate the more stable radical. Therefore, in the first stage we first examined the thermodynamic stability of a series of radicals obtained by protonation of the radical anions.…”

Rate and Mechanistic Investigation of Eu(OTf)2-Mediated Reduction of Graphene Oxide at Room Temperature

“…The second step could be a proton transfer from the water molecules, which are coordinated to Eu(III) (step II in Scheme 2). 49 Next, the second electron transfer results in the formation of a carbanion, followed by a proton transfer from another molecule of bound water (steps III and IV in Scheme 2). The final step involves elimination of a water molecule, which is driven by the gain in stability of the aromatic carbon network in rGO sheets and also, presumably, catalyzed by the Lewis acid {Eu II (H 2 O) n }.…”

Uncovering the Importance of Proton Donors in TmI2‐Promoted Electron Transfer: Facile CN Bond Cleavage in Unactivated Amides

“…Furthermore, deuterium incorporation and kinetic isotope effect studies in the reduction of stilbene, a reaction that is known to proceed through an outer-sphere electrontransfer mechanism, [18] using TmI A detailed examination of different proton donors in the model system (see the Supporting Information) revealed that a much lower concentration of alcohols (10 equiv) is required to enhance the redox potential of TmI 2 in comparison with SmI 2 (100 equiv). [19] This result is consistent with the smaller radial size of Tm II and bodes well for the development of catalytic cycles based on regeneration of the TmI 2 reagent.…”