Radical versus\nNon-Radical Reactivity in <i>ortho</i>- and <i>para</i>-Quinonedimethides and Implications for\nCycloaddition Mechanisms

The latent singlet diradical character of the parent <i>ortho</i>-quinonedimethide (<i>o</i>-QDM), as revealed\nby valence\nbond calculations, is demonstrated experimentally by trapping with\nthe kinetically stable free radical TEMPO at room temperature. In\nthe absence of TEMPO, the main pathway for decomposition at ambient\ntemperature is not (as previously proposed in the literature) a radical\nreaction but instead a concerted Diels–Alder dimerization,\nwhich through ωB97X-D/aug-cc-pVTZ/SMD//M06-2X-D3/6-31+G(d,p)/SMD\ncalculations is shown to proceed through an ambimodal bispericyclic\ntransition state. The predominantly non-radical reactivity of <i>o</i>-QDM at room temperature differs from that of its isomeric <i>para</i>-quinonedimethide (<i>p</i>-QDM) congener,\nwhich self-reacts <i>exclusively</i> through radical pathways.\nThese findings suggest the potential for tunable concerted/stepwise\ncycloadditions.