Dissolving metal reduction of acetylenes: a computational study

The two-electron, two-proton reduction of alkynes to trans-alkenes has been studied computationally using the polarizable continuum regime to model liquid ammonia, the solvent in which such reductions are generally carried out. Two computational approaches have been used. In one, the energies of species (alkyne, radical anion, vinyl radical, vinyl anion, dianion, and alkene) that are implicated as possible participants in the reduction are obtained using high level ab initio single-point computations under the polarizable continuum model (PCM) conditions. In the other approach, the same species are surrounded by ten explicit ammonia molecules before undergoing the same single-point PCM analysis. It has been shown that the two methods provide nearly identical results in terms of relative energies. Other findings include the probable bent nature of the radical anion species in ammonia, the likelihood that the trans stereochemistry of the reduction is determined at the vinyl anion stage, and the elimination of a dianion as a possible species that determines the stereochemical result. Various observations relating the solvent effects of ammonia are made relative to known gas-phase properties of the species studied.