Experimental evidence for competitive N-O and O-C bond homolysis in gas-phase alkoxyamines

The extensive use of alkoxyamines in controlled radical polymerisation and polymer stabilisation is based on rapid cycling between the alkoxyamine ((RRNO)-R-1-N-2-R-3) and a stable nitroxyl radical ((RRNO center dot)-R-1-N-2) via homolysis of the labile O-C bond. Competing homolysis of the alkoxyamine N-O bond has been predicted to occur for some substituents leading to production of aminyl and alkoxyl radicals.This intrinsic competition between the O-C and N-O bond homolysis processes has to this point been difficult to probe experimentally. Herein we examine the effect of local molecular structure on the competition between N-O and O-C bond cleavage in the gas phase by variable energy tandem mass spectrometry in a triple quadrupole mass spectrometer. A suite of cyclic alkoxyamines with remote carboxylic acid moieties (HOOC-(RRNO)-R-1-N-2-R-3) were synthesised and subjected to negative ion electrospray ionisation to yield [M-H](-) anions where the charge is remote from the alkoxyamine moiety. Collision-induced dissociation of these anions yield product ions resulting, almost exclusively, from homolysis of O-C and/or N-O bonds. The relative efficacy of N-O and O-C bond homolysis was examined for alkoxyamines incorporating different R-3 substituents by varying the potential difference applied to the collision cell, and comparing dissociation thresholds of each product ion channel. For most R-3 substituents, product ions from homolysis of the O-C bond are observed and productions resulting from cleavage of the N-O bond are minor or absent. A limited number of examples were encountered however, where N-O homolysis is a competitive dissociation pathway because the O-C bond is stabilised by adjacent heteroatom(s) (e.g. R-3= CH2F). The dissociation threshold energies were compared for different alkoxyamine substituents (R-3) and the relative ordering of these experimentally determined energies is shown to correlate with the bond dissociation free energies, calculated by ab irzitio methods. Understanding the structure-dependent relationship between these rival processes will assist in the design and selection of alkoxyamine motifs that selectively promote the desirable O-C homolysis pathway. (C) 2014 Elsevier B.V. All rights reserved.