Document Detail

Kinetic effects of increased proton transfer distance on proton-coupled oxidations of phenol-amines.
MedLine Citation:
PMID:  21919508     Owner:  NLM     Status:  MEDLINE    
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···N distance (d(ON)) is >0.1 Å longer in 2 than in 1. The difference in d(ON) is 0.13 ± 0.03 Å from X-ray crystallography and 0.165 Å 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(6)H(4)OMe)(3)(•+) (3a(+)) occurs at (1.4 ± 0.1) × 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)OMe)(2)(C(6)H(4)Br)(•+) (3b(+)). This difference in rate constants is well accounted for by the slightly different free energies of reaction: ΔG° (2 + 3a(+)) = +0.078 V versus ΔG° (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 (Brønsted α, Δ ln(k)/Δ 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.
Todd F Markle; Ian J Rhile; James M Mayer
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Publication Detail:
Type:  Journal Article; Research Support, N.I.H., Extramural; Research Support, Non-U.S. Gov't     Date:  2011-10-11
Journal Detail:
Title:  Journal of the American Chemical Society     Volume:  133     ISSN:  1520-5126     ISO Abbreviation:  J. Am. Chem. Soc.     Publication Date:  2011 Nov 
Date Detail:
Created Date:  2011-10-26     Completed Date:  2012-02-13     Revised Date:  2014-09-16    
Medline Journal Info:
Nlm Unique ID:  7503056     Medline TA:  J Am Chem Soc     Country:  United States    
Other Details:
Languages:  eng     Pagination:  17341-52     Citation Subset:  IM    
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MeSH Terms
Amines / chemistry*
Models, Molecular
Molecular Structure
Phenols / chemistry*
Grant Support
Reg. No./Substance:
0/Amines; 0/Phenols; 0/Protons

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