Selective reduction of aryl halides and .alpha.,.beta.-unsaturated esters with sodium borohydride-cuprous chloride in methanol and its application to deuterium labeling

A reducing system, NaBH 4 -Cu 2 Cl 2 /MeOH, was developed for dehalogenation of aryl halides, conjugate reduction of α,β-unsaturated esters, and deuterium labeling in a chemo- and regioselective manner. These reactions proceeded without reduction of isolated olefins. The Cu 2 Cl 2 is assumed to function as the catalyst which generates a transient species of copper hydride as an active reducing agent, on contact with NaBH 4 . Deuterium-labeling studies indicated that (i) the hydrogen which was transferred to the 4-position of methyl 4-iodobenzoate originates from MeOH and (ii) the hydrogens which were transferred to the α- and β-positions of the conjugated ester originate from MeOH and NaBH 4 , respectively