Lewis Acid Promoted Reaction of Secondary Phosphines with Carbonyl Compounds: Remarkable Effect of Niobium(V) Chloride in Promoting an Intramolecular Oxidation-Reduction Process

Lewis acid promoted reaction of secondary phosphines and carbonyl compounds is described. Ordinary Lewis acids such as boron trifluoride-diethyl ether complex and titanium(IV) chloride led to the formation of the simple addition product, α-hydroxy phosphine III(Scheme 1), which was isolated as the corresponding oxide III′ formed by facile air oxidation during workup. In sharp contrast niobium(V) chloride promoted a further transformation: the oxygen transfer from the α-carbon to the phosphorus to give the α-deoxygenated phosphine oxides IV (Scheme 1). In the presence of a stoichiometric amount of boron trifluoride-diethyl ether complex, the amount of niobium(V) chloride can be reduced to catalytic quantities without losing the efficiency in yielding phosphine oxide IV. Various phosphine oxides were synthesized in excellent yields by this method.