Thermolyses of O-Phenyl Oxime Ethers. A New Source of Iminyl Radicals and a New Source of Aryloxyl Radicals | doi.page

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Thermolyses of O-Phenyl Oxime Ethers. A New Source of Iminyl Radicals and a New Source of Aryloxyl Radicals

The Journal of Organic Chemistry2004Vol. 69(9), pp. 3112–3120

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Jessie A. Blake, Derek A. Pratt, Shuqiong Lin, John C. Walton, Peter Mulder, K. U. Ingold

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 degrees C 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(*) R(*)R'C[triple bond]N. Calculated N-H BDEs in the corresponding RR'C=NH are also presented.

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Abstract

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