A variety of persistent radicals of the cyclohexadienyl type have been generated by the addition of certain carbon, silicon, oxygen, and phosphorus centered radicals (e.g., CF3, Cl3Si·, Me3CO·. (CH3CH2O)2P=O) to the following sterically hindered aromatic compounds: 1,3,5-tri-tert-butylbenzene, 2,4,6-lri-terf-butylpyridine, 2,4,6-tri-tert-butyS-λ3-phosphorm, 1,3-di-tert-butylbenzene, and 2,0-di-tert-butylpyridine. The structures of the radicals, which have been deduced from their epr spectra, should assist in the identification of more transient cyclohexadienyls. It is shown that persistence is a consequence of steric protection of the reactive sites in these radicals. Kinetics for decay of some of the radicals are reported.
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Griller et al. (1975) studied this question.