Effect of basic site substituents on concerted proton-electron transfer in hydrogen-bonded pyridyl-phenols

Abstract

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 (HOArCH(2)pyX) (X = 4-NO(2), 5-CF(3), 4-CH(3), and 4-NMe(2)). Electron-donating and -withdrawing substituents have a substantial effect on the chemical environment of the transferring proton, as indicated by IR and (1)H 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 pK(a) of the free pyridine (pyX), with slopes slightly below the Nerstian value of 59 mV/pK(a). For the HOArCH(2)pyX series, the rate constants k(sCPET) for oxidation by NAr(3)(•+) 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.

Figure 1

Variation of thermochemical and structural parameters in HOArCH₂pyX (blue ) and HOAr-pyX (red ) versus the estimated pK_a of the pyridinium (see Ref. 46).

Figure 2

ORTEPs of HOArCH₂pyCH₃ (left) and HOAr-pyCH₃ (right).

Figure 4

Brønsted plots for (a.) reactions of HOArCH₂pyX with [N(C₆H₄X)₃]^+·, and (b.) reactions of HOAr-pyX with [Fe(R₂bpy)₃]³⁺ and [Fe(Me_xphen)₃]³⁺.

Figure 5

Brønsted plots for reactions of (a.) HOArCH₂pyCH₃ with [N(C₆H₄X)₃]^+·, (b.) HOArCH₂pyH with [N(C₆H₄X)₃]^+·, and (c.) HOArCH₂pyX with [N(C₆H₄OMe)(C₆H₄Br)₂]^+· (points on dashed blue line) and [Fe(Me₂phen)₃]³⁺ (points on solid red line).

Figure 6

Intrinsic kinetic rate constants k° (Table 3) plotted vs. the pK_a of the pyridine (Table 1), for reactions of HOArCH₂pyX and HOAr-pyX with different oxidants. The estimated error on log(k°) is ± 0.3.

Figure 7

Kohn-Sham molecular orbital diagrams for the HOMOs of (a.) HOAr-pyNMe₂, (b.) HOAr-pyNO₂, and (c.) HOArCH₂pyNMe₂.

Scheme 1

sCPET reactivity of phenols with pendent bases, and the phenols with pendent pyridines used in this study, HOAr-pyX and HOArCH₂pyX.