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

摘要

To test the effect of varying the proton donor-acceptor 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 CPh_2NH_2 substituent (1). Spectroscopic, structural, thermochemical, and computational studies show that the two amino-phenols are very similar, except that the O • • • N distance (d_(ON)) is >0.1 A longer in 2 than in 1. The difference in d_(ON) is 0.13 ± 0.03 A from X-ray crystallography and 0.165 A from DFT calculations. Oxidations of these phenols by outer-sphere oxidants yield distonic radical cations ·OAr-NH_3~+ by concerted proton-electron transfer (CPET). Simple tunneling and classical kinetic models both predict that the longer donor-acceptor 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(C_6H_4OMe)_(3~·+) (3a~+) occurs at (1.4 ± 0.1) × 10 M~(-1)s~(-1) , only a factor of 2 slower than the closely related reaction of 1 with N(C_6H_4OMe)_2(C_6H_4Br)~(·+) (3b~+). This difference in rate constants is well accounted for by the slightly different free energies of reaction: △G° (2 + 3a~+) = +0.078 V versus AG° (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 (Bronsted α, △ ln(k)/△ ln(K_(eq))). These results show that the simple tunneling model is not a good predictor of the effect of proton donor-acceptor 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 • • • H • • • N potential energy surface and the influence of proton vibrational excited states.