Tautomerism of 2-(o-hydroxyphenyl)-azines

Lapachev, V. V.; Stekhova, S. A.; Mamaev, V. P.

doi:10.1007/bf00809676

Cited by 4 publications

(2 citation statements)

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“…With this assumption, the lack of an absorption maximum at 327 nm (ε HOAr - NH 2 (327) = 1.1 M -1 cm -1 ) implies that essentially no zwitterion is present in MeCN solution, that K PT2 < 10 -4 . Similarly, the UV−vis spectrum of HOAr-py shows no peak above 385 nm that would be characteristic of the proton-transferred structure 2 UV−vis spectra of (a) phenol HOAr-NH 2 and (b) phenoxide - OAr-NH 2 in MeCN.…”

Section: Resultsmentioning

confidence: 96%

“…Alternatively, pre-equilibrium proton transfer to yield the zwitterion ( - OAr-BH + ) could be followed by electron transfer (PT2−ET2). Zwitterions such as - OAr-BH + are well known in phenol−base chemistry, particularly when the phenolic portion is highly acidic, as in p -nitrophenols. ,, Rate-limiting proton transfer is ruled out because different oxidants react at different rates and because PT between electronegative elements in general occurs at very fast rates . The defining characteristic of the stepwise mechanisms is the formation of an intermediate with a finite lifetime.…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Concerted Proton−Electron Transfer in the Oxidation of Hydrogen-Bonded Phenols

Rhile

Markle²,

Nagao³

et al. 2006

J. Am. Chem. Soc.

239

397

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Three phenols with pendant, hydrogen-bonded bases (HOAr-B) have been oxidized in MeCN with various one-electron oxidants. The bases are a primary amine (-CPh(2)NH(2)), an imidazole, and a pyridine. The product of chemical and quasi-reversible electrochemical oxidations in each case is the phenoxyl radical in which the phenolic proton has transferred to the base, (*)OAr-BH(+), a proton-coupled electron transfer (PCET) process. The redox potentials for these oxidations are lower than for other phenols, predominately from the driving force for proton movement. One-electron oxidation of the phenols occurs by a concerted proton-electron transfer (CPET) mechanism, based on thermochemical arguments, isotope effects, and DeltaDeltaG(++)/DeltaDeltaG degrees . The data rule out stepwise paths involving initial electron transfer to form the phenol radical cations [(*)(+)HOAr-B] or initial proton transfer to give the zwitterions [(-)OAr-BH(+)]. The rate constant for heterogeneous electron transfer from HOAr-NH(2) to a platinum electrode has been derived from electrochemical measurements. For oxidations of HOAr-NH(2), the dependence of the solution rate constants on driving force, on temperature, and on the nature of the oxidant, and the correspondence between the homogeneous and heterogeneous rate constants, are all consistent with the application of adiabatic Marcus theory. The CPET reorganization energies, lambda = 23-56 kcal mol(-)(1), are large in comparison with those for electron transfer reactions of aromatic compounds. The reactions are not highly non-adiabatic, based on minimum values of H(rp) derived from the temperature dependence of the rate constants. These are among the first detailed analyses of CPET reactions where the proton and electron move to different sites.

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Section: Resultsmentioning

confidence: 96%