Samarium(II)-mediated pinacol coupling in water: occurrence of unexpected disproportionation and action of low-valent samarium as an active species.

[reaction: see text] Mechanistic studies of one-electron reduction in water using samarium were carried out. Unexpected disproportionation in water was observed via UV-vis spectroscopic analysis. This fact indicates that low-valent samarium species can exist in water. Furthermore, the SmCl(3)-Sm and SmCl(3)-Mg systems were found to act as good one-electron reducing agents in water.