Alkoxyl radicals as O-synthons in self-terminating radical oxygenations: An experimental and theoretical study

摘要

Alkoxyl radicals 9 (XO), generated by photolysis of thiocarbamates 8, were found to initiate and undergo self-terminating radical oxygenations, in which alkynes are transformed into ketones. With this result, we have shown that every major class of organic O-centered radicals can act as O-atom synthon in this sequence, demonstrating the generality of this novel concept in radical chemistry. Theoretical investigations of the terminating homolytic scission of the O-X bond reveal that resonance stabilization in the cleaved radical X (e.g. benzyl, allyl) both lowers the activation barrier, DE, and increases the exothermicity, whereas radical stabilization by inductive effects (as in tert-butyl) only reduces DE, compared to non-stabilized radicals. The experimental results indicate that the final homolytic bond scission is not the only crucial step in this mechanism. but that generation of XO as well as the initial radical addition to the alkyne triple bond must be of similar importance.