A new method to prepare functional phosphines through steady-state photolysis of triarylphosphines†

Abstract

The steady-state photolysis of triarylphosphine, Ar₃P, was carried out using a xenon lamp or a high-pressure mercury lamp under an argon atmosphere in a solvent containing a functional group, CH₃X. Gas chromatograph-mass spectroscopic analysis on the photolysis showed that a phosphine to which the functional group from the solvent is incorporated, Ar₂PCH₂X, was formed in a moderate yield, along with tetraaryldiphosphine, Ar₂PPAr₂. The product, Ar₂PCH₂CN, from the photolysis in acetonitrile (X=CN) was isolated by column chromatography. In the photolysis in other solvents tried here (ethyl acetate, acetone, 2-butanone, and 3,3-dimethyl-2-butanone), Ar₂PCH₂X formed in the reaction mixture was so labile on a silica-gel column that it was treated with S₈ powder to convert to the corresponding phosphine sulfide, Ar₂P(=S)CH₂X. The resulting phosphine sulfide was isolated by column chromatography. The isolated products in these reactions, Ar₂PCH₂CN and Ar₂P(=S)CH₂X, were characterized by ¹H, ¹³C, and ³¹P NMR spectroscopy, IR spectroscopy, and elemental analysis or high-resolution mass spectroscopy. The formation of Ar₂PCH₂X as well as Ar₂PPAr₂ is explained by homolytic cleavage of a P-C bond of Ar₃P in the photoexcited state. This reactivity of Ar₃P in the photoexcited state is in sharp contrast to that exerted under aerobic conditions, where Ar₃P in the photoexcited state donates readily an electron to oxygen producing the radical cation, Ar₃P^·+. This photoreaction, which affords a functional phosphine, Ar₂PCH₂X, in one-pot with generating very small amounts of unidentified side products, has potential for use in preparing functional phosphines.