Thermolyses of O-phenyl oxime ethers. A new source of iminyl radicals and a new source of aryloxyl radicals

JA Blake, DA Pratt, S Lin, John Christopher Walton, P Mulder, KU Ingold

Research output: Contribution to journalArticlepeer-review

49 Citations (Scopus)

Abstract

Six O-phenyl ketoxime ethers, RR'C=NOPh 1-6, with RR' = diaryl, dialkyl, and arylalkyl, together with N-phenoxybenzimidic acid phenyl ether, PhO(Ph)C=NOPh, 7, have been shown to thermolyze at moderate temperatures with "clean" N-O bond homolyses to yield iminyl and phenoxyl radicals, RR'C=N-. and PhO.. Since 1-6 can be synthesized at room temperature, these compounds provide a new and potentially useful source of iminyls for syntheses. The iminyl from 7 undergoes a competition between beta-scission, to PhCN and PhO., and cyclization to an oxazole. Rate constants, 10(6) k/s(-1), at 90 degreesC for 1-6 range from 4.2 (RR' = 9-fluorenyl) to 180 (RR' = 9-bicyclononanyl), and that for 7 is 0.61. The estimated activation enthalpies for N-O bond scission are in satisfactory agreement with the results of DFT calculations of N-O bond dissociation enthalpies, BDEs, and represent the first thermochemical data for any reaction yielding iminyl radicals. The small range in k (N-O homolyses) is consistent with the known sigma structure of these radicals, and the variations in k and N-O BDEs with changes in RR' are rationalized in terms of iminyl radical stabilization by hyperconjugation: RR'C=N-. <----> (RR)-R-.'CdropN. Calculated N-H BDEs in the corresponding RR'C=NH are also presented.

Original languageEnglish
Pages (from-to)3112-3120
Number of pages9
JournalThe Journal of Organic Chemistry
Volume69
DOIs
Publication statusPublished - 30 Apr 2004

Keywords

  • BOND-DISSOCIATION ENTHALPIES
  • THERMAL-DECOMPOSITION
  • PHENOXYL RADICALS
  • PHENOLIC ANTIOXIDANTS
  • SUBSTITUTED PHENOLS
  • IMINOXY RADICALS
  • H BONDS
  • ENERGIES
  • REARRANGEMENT
  • REACTIVITY

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