985. The occurrence of 1,2- or 1,4-addition in the reduction of some αβ-unsaturated ketones with metal hydrides

3-Methylcyclohexenone, carvenone, and cholestenone have been reduced with sodium borohydride in three solvents. In "diglyme" and propan-2-ol the percentage of attack at the β-carbon atom (1,4-addition) decreased with increasing steric hindrance at this carbon atom. Reductions in pyridine gave only the products of 1,4-addition. When pyridine, other amines, and triphenylphosphine were present in small amounts during reductions in "diglyme," the amount of 1,4-addition was directly related to the boron basicity of the additive. The results are interpreted in terms of steric requirements of the reducing species, and relative steric hindrance to nucleophilic attack a t the two possible sites in αβ-unsaturated ketones. Lithium aluminium hydride in ether or pyridine gave mainly the products of 1,2-addition, but lithium tetrakis(dihydro-N-pyridyl)aluminate paralleled other dihydropyridine derivatives in giving large amounts of saturated compounds.