Effect of Basic Site Substituents on Concerted Proton–Electron Transfer in Hydrogen-Bonded Pyridyl–Phenols

Separated concerted proton–electron transfer (sCPET)
reactions
of two series of phenols with pendent substituted pyridyl moieties
are described. The pyridine is either attached directly to the phenol
(HOAr-pyX) or connected through a methylene linker (HOArCH2pyX) (X = 4-NO2, 5-CF3, 4-CH3, and 4-NMe2). Electron-donating and -withdrawing substituents have a substantial
effect on the chemical environment of the transferring proton, as
indicated by IR and 1H NMR spectra, X-ray structures, and
computational studies. One-electron oxidation of the phenols occurs
concomitantly with proton transfer from the phenolic oxygen to the
pyridyl nitrogen. The oxidation potentials vary linearly with the
pKa of the free pyridine (pyX), with slopes
slightly below the Nerstian value of 59 mV/pKa. For the HOArCH2pyX series, the rate constants ksCPET for oxidation by NAr3•+ or [Fe­(diimine)3]3+ vary primarily with the thermodynamic driving
force (ΔG°sCPET), whether ΔG° is changed by varying the potential of the oxidant
or the substituent on the pyridine, indicating a constant intrinsic
barrier λ. In contrast, the substituents in the HOAr-pyX series affect λ as well as ΔG°sCPET, and compounds with electron-withdrawing substituents
have significantly lower reactivity. The relationship between the
structural and spectroscopic properties of the phenols and their CPET
reactivity is discussed.