Labile alkoxyamines: past, present, and future

TLDR

Alkoxyamines were discovered in 1974 and have since been extensively studied, notably after Solomon, Rizzardo, and Cacioli patented their key role in nitroxide‑mediated polymerization in 1985. This feature article surveys alkoxyamine applications in radical chemistry, polymerization, and smart materials, and reports new findings on chemically triggered C‑ON bond homolysis that allows controlled alkyl radical generation at room temperature. Alkoxyamines undergo C‑ON bond homolysis to release a persistent nitroxyl radical and a transient alkyl radical. The authors show that this homolysis can be chemically triggered at room temperature, enabling new opportunities for smart materials, novel NMP initiators, and theranostic agents in which the nitroxide facilitates imaging via dynamic nuclear polarization while the alkyl radical targets abnormal cells.

Abstract

Alkoxyamines--per-alkylated derivatives of hydroxylamine R(1)R(2)NO-R(3)--can undergo C-ON bond homolysis to release a persistent nitroxyl radical R(1)R(2)NO˙ and a transient alkyl radical R(3)˙. Although they were considered as an oddity when discovered in 1974, their properties have been extensively studied since the seminal work of Solomon, Rizzardo and Cacioli (Chem. Abstr., 102, 221335q), who patented the key role of alkoxyamines in nitroxide-mediated polymerization (NMP) in 1985. This feature article surveys and assesses the various applications of alkoxyamines: in tin-free radical chemistry, e.g., for the elaboration of carbo- or hetero-cycles, for the development of new reactions, for total synthesis of natural products; in polymerization under thermal conditions (NMP) or photochemical conditions (nitroxide-mediated photo-polymerization, NMP2); and in the design of smart materials. In this feature article, we also describe our recent findings concerning the chemical triggering of the C-ON bond homolysis in alkoxyamines, affording the controlled generation of alkyl radicals at room temperature. Based on these results, we describe herein some new opportunities for applications in the field of smart materials, and of course, some possible developments as new initiators for NMP as well as an entirely new field of application: the use of alkoxyamines as theranostic agents. Indeed, each of the radicals released after homolysis can play an appealing role: the nitroxide, through dynamic nuclear polarization (DNP), can be used for imagery purposes (diagnostic properties), while the alkyl radical can be used to induce cellular disorders in abnormal cells (therapeutic activity).