KINETICS OF REACTION OF ETHYL DIPHENYLPHOSPHINATE WITH PHENYLMAGNESIUM BROMIDE

The reaction of ethyl diphenylphosphinate and phenylmagnesium bromide has been investigated in tetrahydrofuran (THF). Infrared and phosphorus nmr spectral evidence has been obtained for the rapid formation of a moderately strong 1:1 complex between the phosphoryl oxygen atom of the phosphinate and the magnesium atom of the Grignard reagent. The Keq for the formation of this complex is estimated to be about 6 x 103]. mole-1 or greater. The exchange of ethyl diphenylphosphinate in the complex is fast on the nmr time scale. Subsequent slow reorganization of this complex to product has been found to follow first-order kinetics and to be concentration independent over the range that was investigated (0.03-0.5 M). Below 0.03 M the results suggest that dissociation of the complex becomes appreciable. The Ea (13.4 kcal/mole) and ∆S* (–33.2 eu) for the reorganization of the complex to product were calculated from the average first-order rate constants obtained at four temperatures. The phosphorus nmr spectrum of the product of the reorganization step indicates a relatively strong complex between the phosphoryl oxygen atom of triphenylphosphine oxide and magnesium bromoethoxide and not a pentacovalent phosphorane as has been suggested. Tributylphosphine oxide accelerates the rate of reaction of ethyl diphenylphosphinate with phenylmagnesium bromide, whereas magnesium bromoethoxide retards the reaction. Phosphorus nmr spectral evidence for complex formation is presented. The rate of reaction of ethyl diphenylphosphinate with phenylmagnesium bromide in THF is about five times that in diethyl ether. All of these observations appear consistent with the proposed mechanism. Spectral and kinetic data have been obtained for the more complex reaction of diphenylmagnesium with ethyl diphenylphosphinate. © 1969, American Chemical Society. All rights reserved.