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

• 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(2)NH(2) substituent (1). Spectroscopic, structural, thermochemical, and computational studies show that the two amino-phenols are very similar, except that the O center dot center dot center dot N distance (d(ON)) is >0.1 angstrom longer in 2 than in 1. The difference in d(ON) is 0.13 +/- 0.03 angstrom from X-ray crystallography and 0.165 angstrom from DFT calculations. Oxidations of these phenols by outer-sphere oxidants yield distonic radical cations (center dot)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(6)H(4)OMe)(3)(center dot+) (3a(+)) occurs at (1.4 +/- 0.1) x 10(4) M(-1) s(-1) only a factor of 2 slower than the closely related reaction of 1 with N(C(6)H(4)Me)(2)(C(6)H(4)Br)(center dot+) (3b(+)). This difference in rate constants is well accounted for by the slightly different free energies of reaction: Delta G(degrees) (2 + 3a(+)) = +0.078 V versus Delta G(degrees) (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 alpha, Delta ln(k)/Delta 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 center dot center dot center dot H center dot center dot center dot N potential energy surface and the influence of proton vibrational excited states.

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NATURVETENSKAP  -- Kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences (hsv//eng)

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