How intramolecular coordination bonding (ICB) controls the homolysis of the C–ON bond in alkoxyamines

Abstract

Because the C–ON bond homolysis rate constant k_d is an essential parameter of alkoxyamine reactivity, it is especially important to tune k_d without a major alteration of the structure of the molecule. Recently, several approaches have become known, e.g., protonation of functional groups and formation of metal complexes. In this paper, coordination reactions of [Zn(hfac)₂(H₂O)₂] with a series of new SG1-based alkoxyamines affording complexes with different structures are presented. The k_d values of the complexed forms of the alkoxyamines were compared to those of free and protonated ones to reveal the contribution of the electron-withdrawing property and structure stabilization. Together with previously published data, this work provides clues to the design of alkoxyamines that can be effectively activated upon coordination with metal ions. Furthermore, our results provide insight into the mechanism underlying the influence of complexation on the reactivity of alkoxyamines. This led us to describe different types of coordination: intramolecular in nitroxyl fragment, intramolecular in alkyl fragment, intramolecular between alkyl and nitroxyl fragment, and intermolecular one. All of them exhibit different trends which are dramatically altered by changes in conformation.