C–ON bond homolysis of alkoxyamines triggered by paramagnetic copper(ii) salts

Abstract

The metal complexation reactions of bis(hexafluoroacetylacetonato)copper(II) (Cu(hfac) 2 ) with alkoxyamines (diethyl(2,2-dimethyl-1-(tert-butyl-(1-( pyridine-4-yl)ethoxy)amino)propyl)phosphonate and diethyl (2,2-dimethyl-1-(tert-butyl-(1-( pyridine-2-yl)ethoxy)amino)propyl)phosphonate) were studied.According to X-ray analysis, the molecular and crystal structures of 1 : 1 complexes depend on the configuration of the free alkoxyamines, that is dimeric (RSSR) and chain-polymeric (RR/SS) structures for para-pyridylsubstituted alkoxyamines, and cyclic unimeric (RS/SR) structure for ortho-pyridyl derivative. The complex(2 : 1 ratio Cu(hfac) 2 /alkoxyamine) for ortho-pyridyl-substituted alkoxyamine is not resolved. Upon warming, ortho complexes decomposed into free alkoxyamines and only a weak activation was observed. Upon warming, para complexes decomposed into their corresponding unimers, and then, a 21-fold increase in the rate constant of the C-ON bond homolysis was observed compared to the corresponding free alkoxyamines. Tuning of the homolysis rate constant of the C-ON bond via addition of pyridine is also reported.Scheme 1 C-ON bond homolysis in alkoxyamine. † Electronic supplementary information (ESI) available: Preparation, characterization, and Table S1 with XRD data of complexes 7 and 8. Fig. S1 ( 1 H NMR) and S2 ( 31 P NMR) of complexes 7 and 8. CCDC 1483562-1483565. For ESI and crystallographic data in CIF or other electronic format see View Article Online ‡ ‡ Here, k d is considered as the apparent rate constant of the C-ON bond homolysis. § § The equilibrium implying the scavenger and pyridine is disregarded. ¶ ¶ For eqn (6), a first order growth might be observed provided k 3 ·k 5 ·[P] is larger than k −1 ·(k −3 + k 5 ).

How intramolecular hydrogen bonding (IHB) controls the C–ON bond homolysis in alkoxyamines

“…9) are very different for RR/SS-4 and RR/SS-5 in benzene-d 6 , while they are similar in DMSO-d 6 . As similar observations were also reported upon protonation (or other types of activation) of the pyridyl moiety in alkoxyamines, [20][21][22] this change in the signal is ascribed to the occurrence of IHB between the hydroxyl group and the pyridyl moiety (interN in Fig. 8).…”

“…For several years, our group has been promoting the chemical alkoxyamine activation using protonation, 20 oxidation, 20 alkylation, 20 or metal cation coordination. 21,22 However, in our view, activation processes based on physicochemical events cannot be disregarded. Indeed, very recently, 23 a dramatic solvent effect on the C-ON bond homolysis rate constant k d (Scheme 1) has been reported for 1 (Fig.…”

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

“…10,11 Furthermore, formation of complexes of the Zn(hfac) 2 electron acceptor matrix with alkoxyamines has a positive effect on NMP because it enhances initiation efficiency. 12 As we have demonstrated earlier, [7][8][9] alkoxyamine-metal complexes undergo equilibrium dissociation in solution. One can shi this equilibrium by adding a complexation agent for zinc ions, e.g., pyridine (Py) or bi-pyridine.…”

“…2 The rate constant of C-ON bond homolysis k d is important for these applications. 3 Recently, several methods for tuning k d without alteration of alkoxyamine structure have become known, e.g., protonation of functional groups [4][5][6] and formation of complexes with metal ions Zn 2+ , 7 Cu 2+ , 8 or Tb 3+ . 9 Indeed, complexation is a valuable approach to inuence the reactivity of alkoxyamines because it paves the way to metal-polymer complexes and to the tuning of the reactivity of theranostic agents upon reaction with metal-containing proteins in vivo.…”

Smart Control of Nitroxide-Mediated Polymerization Initiators’ Reactivity by pH, Complexation with Metals, and Chemical Transformations

“…Aside from protonation, formation of a metal–alkoxyamine complex is another way to alter the C–ON bond reactivity: by changing the polarity of the bond. A recent series of articles describes the synthesis of such complexes, research into the influence of complexation on C–ON bond homolysis, and application to the NMP of styrene and n -butyl acrylate [ 44 , 45 , 46 ].…”