Kinetic Effects of Increased Proton Transfer Distance on Proton-Coupled Oxidations of Phenol-Amines

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• 作者：Todd F. Markle ; Ian J. Rhile ; James M. Mayer
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：November 2, 2011
• 年：2011
• 卷：133
• 期：43
• 页码：17341-17352
• 全文大小：1051K
• 年卷期：v.133,no.43(November 2, 2011)
• ISSN：1520-5126

文摘

To test the effect of varying the proton donor鈥揳cceptor distance in proton-coupled electron transfer (PCET) reactions, the oxidation of a bicyclic amino-indanol (2) is compared with that of a closely related phenol with an ortho CPh₂NH₂ substituent (1). Spectroscopic, structural, thermochemical, and computational studies show that the two amino-phenols are very similar, except that the O路路路N distance (d_ON) is >0.1 脜 longer in 2 than in 1. The difference in d_ON is 0.13 卤 0.03 脜 from X-ray crystallography and 0.165 脜 from DFT calculations. Oxidations of these phenols by outer-sphere oxidants yield distonic radical cations ^{鈥?/sup>OAr鈥揘H₃+ by concerted proton鈥揺lectron transfer (CPET). Simple tunneling and classical kinetic models both predict that the longer donor鈥揳cceptor distance in 2 should lead to slower reactions, by ca. 2 orders of magnitude, as well as larger H/D kinetic isotope effects (KIEs). However, kinetic studies show that the compound with the longer proton-transfer distance, 2, exhibits smaller KIEs and has rate constants that are quite close to those of 1. For example, the oxidation of 2 by the triarylamminium radical cation N(C₆H₄OMe)₃鈥? (3a+) occurs at (1.4 卤 0.1) 脳 104 M鈥? s鈥?, only a factor of 2 slower than the closely related reaction of 1 with N(C₆H₄OMe)₂(C₆H₄Br)鈥? (3b+). This difference in rate constants is well accounted for by the slightly different free energies of reaction: 螖G掳 (2 + 3a+) = +0.078 V versus 螖G掳 (1 + 3b+) = +0.04 V. The two phenol-amines do display some subtle kinetic differences: for instance, compound 2 has a shallower dependence of CPET rate constants on driving force (Br酶nsted 伪, 螖 ln(k)/螖 ln(K_eq)). These results show that the simple tunneling model is not a good predictor of the effect of proton donor鈥揳cceptor distance on concerted-electron transfer reactions involving strongly hydrogen-bonded systems. Computational analysis of the observed similarity of the two phenols emphasizes the importance of the highly anharmonic O路路路H路路路N potential energy surface and the influence of proton vibrational excited states.}