Rates and equilibria of the reactions of tertiary phosphanes and phosphites with benzhydrylium ions.

The kinetics of the reactions of benzhydrylium ions and quinone methides with eight tertiary phosphanes and two phosphites were investigated photometrically. The nucleophilicity parameters N and slope parameters s of these nucleophiles were derived according to the equation log k(20 degrees C) = s(N + E). Correlations of the nucleophilicity parameters N with pK(Ha) and sigma(p) values as well as with the rate constants of reactions with other electrophiles are discussed. In some cases, equilibrium constants for the formation of phosphonium ions were measured, which allow one to determine the Marcus intrinsic barriers of DeltaG(0) (not equal) = 58 kJ mol(-1) for the reactions of triarylphosphanes with benzhydrylium ions. The N parameters [5.51 for P(OPh)3, 10.36 for P(OBu)3, 14.33 for PPh3, 15.49 for PBu3, 18.39 for P(4-Me2NC6H4)3] are compared with the reactivities of other classes of nucleophiles (see, www.cup. uni-muenchen.de/oc/mayr).