Abstract
Because alkoxyamines are employed in a number of important applications, such as nitroxide-mediated polymerization, radical chemistry, redox chemistry, and catalysis, research into their reactivity is especially important. Typically, the rate of alkoxyamine homolysis is strongly dependent on temperature. Nonetheless, thermal regulation of such reactions is not always optimal. This review describes various ways to reversibly change the rate of C–ON bond homolysis of alkoxyamines at constant temperature. The major methods influencing C–ON bond homolysis without alteration of temperature are protonation of functional groups in an alkoxyamine, formation of metal–alkoxyamine complexes, and chemical transformation of alkoxyamines. Depending on the structure of an alkoxyamine, these approaches can have a significant effect on the homolysis rate constant, by a factor of up to 30, and can shorten the half-lifetime from days to seconds. These methods open new prospects for the application of alkoxyamines in biology and increase the safety of (and control over) the nitroxide-mediated polymerization method.
Highlights
Invented as an initiator for nitroxide-mediated polymerization (NMP), [1,2] nowadays alkoxyamines find a wide range of applications including tin-free organic radical chemistry, [3] as initiators for radical cyclization, [4] radical addition reactions, [5,6] creation of self-healing polymers, [7] optoelectronic materials, [8] and encoding systems, [9] and in biomedicine, as theranostic agents
[10] The majority of applications involve the ability of alkoxyamines to undergo C–ON bond homolysis after heating, releasing nitroxide and an alkyl radical
Factors affecting stability of the C–ON bond are important. An example of such significance is given by Chauvin et al, [11] who studied the influence of the initiation rate on the regime of NMP
Summary
Alkoxyamines are adducts of stable nitroxides with C-centered radicals. Invented as an initiator for nitroxide-mediated polymerization (NMP), [1,2] nowadays alkoxyamines find a wide range of applications including tin-free organic radical chemistry, [3] as initiators for radical cyclization, [4] radical addition reactions, [5,6] creation of self-healing polymers, [7] optoelectronic materials, [8] and encoding systems, [9] and in biomedicine, as theranostic agents. [10] The majority of applications involve the ability of alkoxyamines to undergo C–ON bond homolysis after heating, releasing nitroxide and an alkyl radical. Factors affecting stability of the C–ON bond are important An example of such significance is given by Chauvin et al, [11] who studied the influence of the initiation rate on the regime of NMP. This review properties, that isand bedecompose safe to handle and decompose it is needed as a polymerization describes concept so-calledthe smart alkoxyamines, change their reactivity after an initiator.the. In electron-withdrawing terms of the influence and on reactivity of the C–ON bond Because it is polar δ located on the in electron-donating properties of substituents arewith of special importance oxygen electronicon effects that reduce of the Because bond should favor homolysis Subsection, we review possible scenarios relatedof the to C–ON reversible switching of groups into the alkyl and nitroxyl part of an alkoxyamine on the polarity bond and electron-donating properties of substituents described in the literature. Chemical activationof did not attract interest during the first decade despite the unexpected
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