Rate coefficients for intramolecular homolytic substitution of oxyacyl radicals at selenium

Abstract

Ab initio and density functional calculations predict that intramolecular homolytic substitution reactions of oxyacyl radicals at the selenium atom in ω-alkylseleno-substituted radicals proceed via mechanisms that do not involve hypervalent intermediates. When the leaving radical is tert-butyl, energy barriers ( G ‡ ) for these reactions range from 27.1 (G3(MP2)-RAD) kJ mol −1 for the formation of the five-membered cyclic selenocarbonate (6) to 41.5 kJ mol −1 for the six-membered selenocarbonate (8). G3(MP2)-RAD calculations predict rate coefficients in the order of 10 5 -10 8 s −1 and 10 2 -10 5 s −1 for the formation of 6 and 8, respectively, at 298.

The kinetics of carbonyl radical ring closures

“…In recent years, our group has effectively employed computational techniques to provide rate data for intramolecular reactions of radicals that are in good-to-excellent agreement with experimentally derived rate coefficients. [27][28][29][30][31] Taking a similar approach to the challenge at hand, herein we report kinetic parameters relevant to cyclization of systematically substituted hexenoyl and butenyloxyacyl radicals and explicate upon the origins contrasting reactivity in these carbonyl radical additions.…”

The kinetics of alkyl radical ring closures at selenium: formation of selenane

“…This difference in rate constant is to be expected on the basis of the difference in leaving group stability. 31 To provide further comfort in our experimentally determined Arrhenius data, we chose to examine the ring closure of 4 by computational means. In recent years, our group has effectively employed high-level (G3(MP2)-RAD) techniques to provide rate data that are in good-to-excellent agreement with experimentally derived rate coefficients.…”

“…In recent years, our group has effectively employed high-level (G3(MP2)-RAD) techniques to provide rate data that are in good-to-excellent agreement with experimentally derived rate coefficients. [31][32][33][34] For computational expedience, the n-octyl leaving radical in 4 was replaced with the simplest primary alkyl leaving group, namely ethyl. Extensive searching of the B3LYP/6-31G(d) energy surface, as recommended for the G3(MP2)-RAD method, 22 located transition structure 11 (R = Et) for the cyclization of 4 (R = Et) to give selenane 9 (Scheme 5); 11 is depicted in Fig.…”

“…Importantly, the experimental log A numbers from this work are consistent with those obtained experimentally for other intramolecular homolytic substitution reactions at sulfur and selenium, 30 while the G3(MP2)-RAD data are consistent with other calculated log A values. 31,32 These observed differences presumably reflect entropy changes between solution and gas phase reactions.…”

Breaking a bottleneck: Accurate extrapolation to “gold standard” CCSD(T) energies for large open shell organic radicals at reduced computational cost

“…Owing to their high reactivity, the accurate characterization of these chemically interesting species is typically difficult experimentally. Thus, computational modeling is a valuable alternative to understand the electronic structure and chemical reactivity of the fleeting radical intermediates . Unfortunately, an accurate theoretical description of such open shell systems requires an expensive ab initio method (such as the “gold‐standard” coupled‐cluster with single and double and perturbative triple (CCSD(T)) method or an appropriate multireference method) due to problems of delocalization, spin‐contamination and symmetry breaking .…”