KINETICS AND MECHANISM OF CO SUBSTITUTION OF [ETA(5)-CPFE(CO)(3)]PF6 WITH PPH(3) IN THE PRESENCE OF SUBSTITUTED PYRIDINE N-OXIDE

The kinetics of substitution reactions of [eta(5)-CpFe(CO)(3)]PF6 with PPh(3) in the presence of R-PyOs have been studied. For all the R-PyO s (R = 4-OMe, 4-Me, 3,4-(CH)(4), 4-Ph, 3-Me, 2,3-(CH)(4), 2,6-Me(2), 2 -Me),the reactions yield the same product [eta(5)-CpFe(CO)(2)PPh(3)]PF 6, according to a second-order rate law that is first order in concent rations of [eta(5)-CpFe(CO)(3)]PF6 and of R-PyO but zero order in PPh( 3) concentration. These results, along with the dependence of the reac tion rate on the nature of R-PyO, are consistent with an associative m echanism. Activation parameters further support the bimolecular nature of the reactions: Delta H-not equal = 13.4 +/- 0.4 kcal mol(-1), Delt a S-not equal = - 19.1 +/- 1.3 cal k(-1) mol(-1) for 4-PhPyO; Delta H- not equal = 12.3 +/- 0.3 kcal mol(-1), Delta S-not equal = - 24.7 +/- 1.0 cal K-1 mol(-1) for 2-MePyO. For the various substituted pyridine N-oxides studied in this paper, the rates of reaction increase with th e increasing electron-donating abilities of the substituents on the py ridine ring or N-oxide basicities, but decrease with increasing O-17 c hemical shifts of the N-oxides. Electronic and steric factors contribu ting to the reactivity of pyridine N-oxides have been quantitatively a ssessed.