Hydride transfer reactions

The purpose of this research was to examine the structure of the transition states of intermolecular hydride transfer reactions, i.e., whether they are linear or trigonal, by using as a probe hydrogen-deuterium kinetic isotope effects. Three sets of reactions were examined. The first set involved reactions of triarylmethyl cations with triarylmethanes and their deuterated (at central carbon) analogs. These reactions, which are considered to be one-step hydride transfer reactions, gave kH/kD isotope effects in the range 7-9. In view of the fact that the transition states are quite symmetrical with similar force constants f1 and f2, the large kinetic isotope effects exhibited by these reactions support a linear transition state. The data were not accurate enough to determine activation energy parameters and assess the extent of any mechanical tunneling. The second set of reactions involved the less sterically hindered xanthene system by reacting substituted xanthenes with the 9-xanthyl cation and xanthene with the diarylmethyl and triarylmethyl cations. The obtained kH/kD kinetic isotope effects, 3.5-7.5, are consistent with transition states where the angle between donor-hydrogen-acceptor is 120-150 degree on the assumption that the force constants on either side of the hydrogen are the same; it is also possible that the transition states are linear with somewhat unequal force constants on either side of the hydrogen. The third set involved the reactions of triphenylsilane with triarylmethyl cations. No assessment of the structure of the transition states of these reactions could be made because the kinetic isotope effects (1.6-2.2), which are in the range assigned by O'Ferral to cases where the angle between donor-hydrogen-acceptor is about 90 degree, may be due to the expected large differences in the force constants f1 and f2