Alkoxyl Radicals asO-Synthons in Self-Terminating Radical Oxygenations: An Experimental and Theoretical Study

Abstract

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.