Conformational Change with Steric Interactions Affects the Inner Sphere Component of Concerted Proton鈥揈lectron Transfer in a Pyridyl-Appended Radical Cation System

详细信息    查看全文

• 作者：Evan A. Welker ; Brittney L. Tiley ; Crina M. Sasaran ; Matthew A. Zuchero ; Wing-Sze Tong ; Melissa J. Vettleson ; Robert A. Richards ; Jonathan J. Geruntho ; Stefan Stoll ; Jeffrey P. Wolbach ; Ian J. Rhile
• 刊名：Journal of Organic Chemistry
• 出版年：2015
• 出版时间：September 4, 2015
• 年：2015
• 卷：80
• 期：17
• 页码：8705-8712
• 全文大小：433K
• ISSN：1520-6904

文摘

Proton-coupled electron transfer (PCET) model systems combine one-electron oxidants and bases to generate net hydrogen atom acceptors. We have generated two persistent pyridyl-appended radical cations: 10-(pyrid-2-yl)-10H-phenothiazinium (PPT鈥?) and 3-(pyrid-2-yl)-10-methyl-10H-phenothiazinium (MPTP鈥?). EPR spectra and corresponding calculations indicate phenothiazinium radical cations with minimal spin on the pyridine nitrogen. Addition of hindered phenols causes the radical cations to decay, and protonated products and the corresponding phenoxyl radicals to form. The 螖G掳 values for the formation of intermediates (determined through cyclic voltammetry and pKa measurements) rule out a stepwise mechanism, and kinetic isotope effects support concerted proton鈥揺lectron transfer (CPET) as the mechanism. Calculations indicate that the reaction of PPT鈥? + tBu3PhOH undergoes a significant conformational change with steric interactions on the diabatic surface while maintaining the hydrogen bond; in contrast, MPTP鈥? + tBu3PhOH maintains its conformation throughout the reaction. This difference is reflected in both experiment and calculations with 螖GMPTP鈥?猝?/sup> < 螖GPPT鈥?猝?/sup> despite 螖GMPTP鈥?掳 > 螖GPPT鈥?掳. Experimental results with 2,6-di-tert-butyl-4-methoxyphenol are similar. Hence, despite the structural similarity between the compounds, differences in the inner sphere component for CPET affect the kinetics.