Dynamic and substituent effects in the ESR spectra of triplet biradical intermediates in lithiation reactions of 2-alkylbenzotriazoles: A cautionary note

Investigations of intermediates generated in the course of lithiation of four 2-alkylbenzotriazoles revealed largescale temperature-dependent changes in their ESR spectra. A cautionary observation was that the ESR spectra obtained under synthesis conditions exhibited lineshapes that were assignable to a monoradical, while additional, lower temperature measurements clearly showed the species to be a biradical. The electron–electron dipolar couplings yield an average distance between the two unpaired electrons in the biradical, Rav, of about 5.3 ± 0.16 A. These Rav values are consistent with the unpaired electrons being localized on the nitrogen atoms of the two benzotriazole moieties in the biradical fragment. This conclusion is supported by oxygen trapping (nitroxide radical formation) studies. The biradicals are shown to have triplets as the ground state with a single-triplet separation of about 26±4 kJ mol−1.