Quasiphosphonium intermediates. Part II. Protonation of trialkyl phosphites and the mechanism of their dealkylation by hydrogen chloride

Abstract

Variable temperature ³¹P n.m.r. studies of solutions of trineopentyl phosphite and hydrogen chloride in dioxan and ether show that rapidly reversible protonation occurs at room temperature. At –40° or below, the site of protonation is identified as the phosphorus atom [δ–24·9 p.p.m. from 85% H₃PO₄(J_PH 834 ± 6 Hz)]. Although dealkylation of less sterically hindered phosphites occurs rapidly above –50°, the protonated form of tri-n-butyl phosphite can be obtained at –70°. Reversible formation of dialkyl phosphorochloridite, which appears to require initial protonation at oxygen, also takes place to a small extent. The kinetics of the first dealkylation of the trialkyl phosphites (RO)₃P (R = Me, Et, Prⁿ, or Buⁿ) by hydrogen chloride in dioxan have been measured at 25°. For tri-n-butyl phosphite the dealkylation was shown to be first order in phosphite and second order in hydrogen chloride, if the former is in excess ([graphic omitted]10 : 1 molar). The results are consistent with rapid reversible protonation of the phosphite, followed by slow nucleophilic displacement of the alkyl group by either the hydrogen dichloride anion or a second molecule of hydrogen chloride. With more nearly equal concentrations of reactants it is not possible to distinguish between processes which are second and third order in hydrogen chloride. Variations in the observed overall third-order rate constant with changing reactant ratios might be due to changes in the order in hydrogen chloride.