Effect of Basic Site Substituents on Concerted Proton鈥揈lectron Transfer in Hydrogen-Bonded Pyridyl鈥揚henols

详细信息    查看全文

• 作者：Todd F. Markle ; Tristan A. Tronic ; Antonio G. DiPasquale ; Werner Kaminsky ; James M. Mayer
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：December 20, 2012
• 年：2012
• 卷：116
• 期：50
• 页码：12249-12259
• 全文大小：414K
• 年卷期：v.116,no.50(December 20, 2012)
• ISSN：1520-5215

文摘

Separated concerted proton鈥揺lectron 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 pK_a of the free pyridine (pyX), with slopes slightly below the Nerstian value of 59 mV/pK_a. For the HOArCH₂pyX series, the rate constants k_sCPET for oxidation by NAr₃^鈥? or [Fe(diimine)₃]³⁺ vary primarily with the thermodynamic driving force (螖G掳_sCPET), whether 螖G掳 is changed by varying the potential of the oxidant or the substituent on the pyridine, indicating a constant intrinsic barrier 位. In contrast, the substituents in the HOAr-pyX series affect 位 as well as 螖G掳_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.