Hydroxylated HMPA enhances both reduction potential and proton donation in SmI₂ reactions

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 Sm³⁺. 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.