Hydroxylated HMPA Enhances both Reduction Potential and Proton Donation in SmI₂ Reactions

Abstract

HMPA is known to increase the reduction potential of SmI2. However, in many cases, the transferred electron returns from the radical anion of the substrate back to the Sm(3+). This could be avoided by an efficient trapping of the radical anion: e.g., by protonation. However, bimolecular protonation by a proton donor from the bulk may be too slow to compete with the back electron transfer process. An efficient unimolecular protonation could be achieved by using a proton donor which complexes to SmI2, in which case the proton is unimolecularly transferred within the ion pair. A derivative of HMPA in which one of the methyl groups was substituted by a CH2CH2OH unit was synthesized. Cyclic voltammetry studies have shown that it resembles HMPA in its ability to enhance the reduction potential of SmI2, and reactivity studies show that it has also efficient proton shift capabilities. The various aspects of this additive were examined in the reactions of SmI2 with three substrates: benzyl chloride, methyl cinnamate, and anthracene.

High‐Affinity Proton Donors Promote Proton‐Coupled Electron Transfer by Samarium Diiodide

“…Another facet of coordination that may influence the mechanism of substrate reduction is the alteration of the reducing power (ease of oxidation) of Sm II ions upon ligation of proton donors . Addition of cosolvents and additives are known to have an impact on the redox potential of SmI 2 through the production of a thermodynamically more powerful reductant or through stabilization of the Sm III state . When water is employed as an additive, large amounts (1000 equiv based on [SmI 2 ]) are required to significantly influence the redox potential .…”

“…[10] Addition of cosolvents and additives are known to have an impact on the redox potential of SmI 2 through the production of athermodynamically more powerful reductant or through stabilization of the Sm III state. [11] When water is employed as an additive,l arge amounts (1000 equiv based on [SmI 2 ]) are required to significantly influence the redox potential. [12] Additionally, glycols have al imited impact on the redox potential of SmI 2 with dg providing amodest change of 0.13 V(3kcal).…”

Quantitation of the Interactions of Alcohols and Amines with SmI2: Pros and Cons of VIS and NMR Spectroscopies

“…Another example is complexation with proton donors. Due to internal protonation within the ion pair, which enables unimolecular protonation of short-lived radical anions, reduction of substrates that otherwise cannot be accomplished with SmI 2 is enabled . Different proton donors can yield different products, and intensive ligation can change the reaction mechanism from an inner sphere to an outer sphere electron transfer by hampering the direct contact between the substrate and SmI 2 .…”

Lithium Bromide/Water as Additives in Dearomatizing Samarium‐Ketyl (Hetero)Arene Cyclizations

“…It was found that the best yields in this type of transformation were achieved with lithium bromide and water as additives (72 equiv of each of the components) 9. The standard conditions with hexamethylphosphoramide (HMPA)10 and tert ‐butanol as promoters afforded only low yields of compounds 2 . Intrigued by these results and envisioning a chance to avoid the use of carcinogenic and mutagenic HMPA11 in other samarium‐ketyl reactions, we began to investigate typical ketone (hetero)arene cyclizations under these new conditions.…”