Reactions of SmI₂ with Olefins:  Mechanism and Complexation Effect on Chemoselectivity

Abstract

Radical anions of activated olefins may serve as good models for the transition state of Michael addition reactions. 1,2 In our search for a suitable reducing agent to effect the generation of these radical anions 3 our attention was drawn to SmI 2 , which is well-known to have exceptional qualities as a single electron transfer reductant. 4,5 Our initial studies revealed, however, that SmI 2 displays an extremely interesting and multifaceted mechanistic chemistry of which very little is well understood. 6 We believe therefore that an exploration of the mechanistic chemistry of SmI 2 is justified on its own merits. Equation 1 outlines the reactions studied.In a competition experiment, a mixture of MA and DP in THF was reacted with SmI 2 in the presence of a proton donor. 7,8 The product ratio was found to depend on the concentration of the latter. At low proton donor concentration, the selectivity is relatively high and approaches the thermodynamic stability ratio of the two radical anions. 9 As the proton donor concentration increases, the selectivity decreases, finally reaching a constant value. For MeOH and trifluoroacetic acid (TFA), at high concentration, the product ratio approaches unity ( Figure 1). These results are consistent with a mechanism where the protonation competes with the equilibration of the radical anions. At the lower proton donor concentration range, the lifetime of the radical anions is long enough to permit equilibration resulting in a relatively high selectivity. As the proton donor concentration increases, the equilibration is suppressed, thus leading to a lower selectivity. Consistent with the suggested mechanism, the plateau for the much stronger acid TFA is achieved at a concentration (0.25 M) much lower than that of MeOH (2.5 M).Surprisingly, for PrOH and MeOD, proton donors having kinetic acidity lower than MeOH, the plateau level is achieved at a product ratio below unity (0.7 and 0.8, respectively). The absence of selectivity at the plateau region for TFA and MeOH indicates a fast and unselective reaction of SmI 2 with the two substrates. 10 The lack of dependence of the selectivity on the "external" concentration of the various proton donors at the plateau regions, combined with the fact that for MeOD and PrOH plateau is achieved below unity, suggests that protonation occurs internally, probably within a triple complex such as ROH‚SmI 2 ‚MA •-(or ROH‚SmI 2 ‚DP •-). 11 The triple complexes can either undergo equilibration (and gravitate to their thermodynamic distribution ratio) or undergo an internal protonation which will "lock" the product distribution. The height of the plateau is determined by a competition between two processes: internal protonation within the triple complex and equilibration of the radical anions of the two substrates. In the case of MeOH this protonation is fast enough to prohibit any appreciable equilibration. Slowing down the protonation rate by using acids with lower kinetic acidity (MeOD and PrOH) permits a certain extent of equilibration. The ...

Urea derivatives enhance the photocatalytic activity of dye-modified titanium dioxide

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Transformation of gem‐Dicyanoethenes by Samarium: Direct Formation of Indenes or Direct Decyanation with in Situ Disilylation

“…1,1-Dicyano-2,2-diarylethylenes, like diaryl ketones, were reported to be able to form stable radical anions as a result of the similarity between the C=C(CN) 2 and C=O groups. [11] On the basis of this property, the reductive crosscoupling reaction of 1,1-diaryl-2,2-dicyanoethylenes or 1,1-diaryl-2-cyano-2-ethoxycarbonylethylenes with aromatic nitriles mediated by samarium diiodide were successfully realized to prepare polysubstituted 3H-pyrroles. [12] On the basis of the previous work and the results presented in this paper, the following tentative reaction pathway is proposed as a rational mechanism (Scheme 2).…”

“…First, radical anion intermediate A is formed by electron transfer from samarium to substrate 1. [11] Structure A would be in resonance with structure B, which reacts with the TMSCl electrophile and then accepts another electron from samarium to afford anion C. Anion C would be in resonance with D, the carbon atom of which attacks the C=C=N moiety, possibly in a Michael-type fashion. Silylation at the N atom forms E, which aromatizes (possibly by two 1,5-H shifts) to produce final product 2a.…”

Intrinsic barrier for protonation of radical anions

“…1). 1 Incorporation isotope effect was one of the mechanistic exploration tools used in of this study. The experiment was conducted in two different manners.…”