Factors Governing Reactivity and Selectivity in Hydrogen Atom Transfer from C(sp3)-H Bonds of Nitrogen-Containing Heterocycles to the Cumyloxyl Radical

A kinetic study of the hydrogen atom transfer (HAT)reactions from nitrogen-containing heterocycles (secondary and tertiarylactams, 2-imidazolidinones, 2-oxazolidinones, and succinimides) to thecumyloxyl radical has been carried out employing laserflash photolysiswith ns time resolution. HAT occurs from the C-H bonds that are alpha tonitrogen, activated by hyperconjugative overlap with the N-C???O pi system. In the lactam series, the second-order HAT rate constant (kH)was observed to decrease by a factor of similar to 4 going from thefive- and six-membered ring derivatives to the eight-membered ones, a behavior thatwas rationalized on the basis of a reduced extent of hyperconjugativeactivation associated to the greaterflexibility of the larger ringscompared to the smaller ones. In thefive-membered-ring substrateseries, thekHvalues were observed to increase by >3 orders of magnitude on going from succinimide to 2-imidazolidinones, abehavior that was explained in terms of the divergent contribution of hyperconjugative activation and deactivating electronic effectsdetermined by ring functionalities. The results are discussed in the framework of the development of HAT-based C-H bondfunctionalization procedures.

Automated summary

A kinetic study was conducted to investigate the hydrogen atom transfer (HAT) reactions from nitrogen-containing heterocycles to the cumyloxyl radical. The study revealed that HAT occurs through hyperconjugative overlap between the C-H bonds and the N-C???O π system. The size of the ring and the functional group have significant effects on the reaction rate.