The chemistry of heteroarylphosphorus compounds. Part 11. The effects of 3-furyl and 3-thienyl substituents at phosphorus on the rate and course of alkaline hydrolysis of phosphonium salts and on the decomposition of phosphonium betaines

Abstract

Both benzyltri-(3-furyl)- and benzyltri-(3-thienyl)-phosphonium bromides (3; X = O or S, R = PhCH₂) undergo alkaline hydrolysis with preferential loss of the benzyl group, indicating that the latter is better able to support the forming carbanionic centre than are the 3-heteroaryl groups, in contrast to the situation for the corresponding 2-heteroaryl isomers. A comparison of the rates of hydrolysis of a series of methyltri(heteroaryl)phosphonium salts shows that the relative reactivities are in the order 2-furyl(10⁹) > 2-thienyl(3 × 10⁶) > 3-furyl(46) > 3-thienyl-(33) > phenyl(1 × 10^–2), indicating that the 3-heteroaryl substituents are much less strongly electron-withdrawing than the 2-isomers. The phosphonium betaine derived from the reaction of tri-(3-furyl)phosphine with styrene oxide decomposes to form styrene and the phosphine oxide, whereas the corresponding 3-thienyl derivative follows an alternative reaction pathway with the formation of the vinylphosphine oxide (9; Ar = 3-thienyl) and the rearrangement product (8; Ar = 3-thienyl).