Photostimulated Reduction of Nitriles by SmI₂

Abstract

Despite their high electron-withdrawing strength, nitriles are not good electron acceptors and therefore are hard to reduce. In this work, using photostimulation in the visible region, we examined the reactivity of aliphatic and aromatic, mono- and dicyano compounds in reaction with SmI(2). A proton donor that complexes efficiently with SmI(2) must be used. Maximum yield was obtained at ca.0.2 M MeOH. Aromatic nitriles were more reactive than aliphatic nitriles, which exhibited negligible yields. Phenylacetonitrile presents an intermediate reactivity. The mechanism of the reaction involves coordination of the SmI(2) to the lone pair of the nitrile nitrogen followed by an inner sphere electron transfer. Surprisingly, m-dicyanobenzene was less reactive than the monocyano derivative benzonitrile. This was traced to the lower ability of the dicyano compound to coordinate to the SmI(2) due to, as was shown by quantum mechanical calculations, its lone pair having an energy significantly lower than that of benzonitrile. It is noteworthy that at the SmI(2) initial concentration used (0.04M), light penetrates only the 0.4 mm outer layer of the reaction mixture. Therefore the photostimulation effect observed was due to irradiation of only 4% of the total reaction volume, implying that under optimal conditions the effect should be 25 times larger.

Reversed Electron Apportionment in Mesolytic Cleavage: The Reduction of Benzyl Halides by SmI2

“…The first is that these two substituents are capable of delocalizing the negative charge from the benzylic position onto themselves, and the second is that they are capable of binding to the samarium cation 10a. 25 Although it is unlikely that the electron‐withdrawing power of the p ‐CN and p ‐CO 2 Me substituents alone is sufficient to overcome the electronegativity of Cl and Br, their pairing with the triply charged samarium cation, apparently results in more efficient negative charge stabilization on the benzylic fragment than by its localization on the nucleofuge. Hence, according to the Bunnett paradigm,24 the mode of cleavage will be reversed.…”

Mechanistic Vistas of Trivalent Nitrogen Compound Reduction by Samarium Diiodide

“…As the focus of this review is ligation of the lone pair on nitrogen to SmI 2 , the question is whether nitriles are also capable of forming a complex with SmI 2 . Table 1 shows the energies of the lone pairs of various nitrogen compounds relative to a simple amine [30] …”

“…and Procter et al . used the SmI 2 −H 2 O−NEt 3 system to reduce nitriles, [33] we used an alternative way to enhance the efficiency of SmI 2 ‐mediated reactions – photoexcitation of SmI 2 [30] with visible light, which generates an excited state of SmI 2 with significantly more energy than the ground state, leading to substantial enhancement in the rate of electron transfer. However, as the electron added to the nitrile resides in a high energy orbital, it may rapidly jump back to the Sm 3+ with no product formation.…”

“…Use of TFE instead of MeOH leads to complete recovery of starting material, demonstrating the necessity of coordinating proton donors to SmI 2 in order to trap a highly reactive radical anion intermediate through a unimolecular protonation. Starting materials, products, and yields of these reactions under photostimulated conditions are given in Scheme 4 [30] …”

Water Electrolysis with a Conducting Carbon Cloth: Subthreshold Hydrogen Generation and Superthreshold Carbon Quantum Dot Formation

“…The photostimulated reduction of various nitriles catalyzed by SmI 2 63 has been reported by the Hoz group 46. They studied the reactivity of aliphatic, aromatic and dicyano compounds using photostimulation in the visible region.…”