Electron paramagnetic resonance spectroscopic studies of the electron transfer reaction of Hantzsch ester and a pyrylium salt

Abstract

The oxidation of Hantzsch ester by a pyrylium cation takes place via electron-proton-electron transfer. The reaction was investigated with EPR spectroscopy using TEMPO and DMPO for inhibition and spin trapping, respectively, of the radicals appearing during the reaction. The present in-depth EPR study of the radical reactions of a NADH analogue indicate a complex electron transfer mechanism in the title reaction.

Scheme 1

Fig. 1. Time dependence of the EPR spectra of DMPO adducts (top) and contour plots of the absorption spectra (bottom). Conditions: [HEH]₀ = 0.04 M, [TMPP]₀ = 0.04 M, [DMPO]₀ = 0.045 M, DMSO, under argon, room temperature, (A): [3]₀ = 0 M, (B): [3]₀ = 0.2 M.

Fig. 2. (A) Fitting of an experimental EPR spectrum (dashed line) with a superposition of first derivative of Lorentzian (dotted line) and Voigt (solid line) functions. Top trace: residuals. (B) Deconvoluted individual EPR spectra. Experimental conditions are the same as for Fig. 1 (A, in the absence of compound 3) and the spectrum was recorded 12 minutes after the onset of HEH oxidation.

Fig. 3. Deconvoluted EPR spectra (left) and corresponding structure of DMPO-adducts (right). (A): DMPO–HE˙; (B): DMPO–TMPP(4)˙ (dotted line), DMPO–TMPP(2,6)˙ (solid line); (C): DMPO–TMPP(4H)˙⁺ (solid line), DMPO–TMPP(2H)˙⁺ (dotted line).

Fig. 4. Time dependence of the concentration of the different spin-trapped radical adducts, measured as the integrated intensity of the corresponding EPR absorption spectra. Conditions: [HEH]₀ = 0.04 M, [TMPP]₀ = 0.04 M, [DMPO]₀ = 0.045 M, DMSO, under argon, room temperature, (A): [3]₀ = 0 M, (B): [3]₀ = 0.2 M. Symbols: DMPO–HE˙ (●), DMPO–TMPPH˙(4) (■), DMPO–TMPP˙(2,6) (▼), DMPO–TMPP(4H)˙+ (♦), DMPO–TMPP(2H)˙+ (▲).

Scheme 2. The proposed reaction mechanism (R = 4-methoxyphenyl; HEBF₄ = oxidized Hantzsch ester tetrafluoroborate salt).