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Redox Chemistry of Selenenic Acids and the Insight It Brings on Transition State Geometry in the Reactions of Peroxyl Radicals
The
redox chemistry of selenenic acids has been explored for the
first time using a persistent selenenic acid, 9-triptyceneselenenic
acid (RSeOH), and the results have been compared with those we recently
obtained with its lighter chalcogen analogue, 9-triptycenesulfenic
acid (RSOH). Specifically, the selenenyl radical was characterized
by EPR spectroscopy and equilibrated with a phenoxyl radical of known
stability in order to determine the O–H bond dissociation enthalpy
of RSeOH (80.9 ± 0.8 kcal/mol): ca. 9 kcal/mol stronger than
in RSOH. Kinetic measurements of the reactions of RSeOH with peroxyl
radicals demonstrate that it readily undergoes H-atom transfer reactions
(e.g., <i>k</i> = 1.7 × 10<sup>5</sup> M<sup>–1</sup> s<sup>–1</sup> in PhCl), which are subject to kinetic solvent
effects and kinetic isotope effects similar to RSOH and other good
H-atom donors. Interestingly, the rate constants for these reactions
are only 18- and 5-fold smaller than those measured for RSOH in PhCl
and CH<sub>3</sub>CN, respectively, despite being 9 kcal/mol less
exothermic for RSeOH. IR spectroscopic studies demonstrate that RSeOH
is less H-bond acidic than RSOH, accounting for these solvent effects
and enabling estimates of the p<i>K</i><sub>a</sub>s in
RSeOH and RSOH of ca. 15 and 10, respectively. Calculations suggest
that the TS structures for these reactions have significant charge
transfer between the chalcogen atom and the internal oxygen atom of
the peroxyl radical, which is nominally better for the more polarizable
selenenic acid. The higher than expected reactivity of RSeOH toward
peroxyl radicals is the strongest experimental evidence to date for
charge transfer/secondary orbital interactions in the reactions of
peroxyl radicals with good H-atom donors