Radical versus Non-Radical Reactivity in <i>ortho</i>- and <i>para</i>-Quinonedimethides and Implications for Cycloaddition Mechanisms

The latent singlet diradical character of the parent <i>ortho</i>-quinonedimethide (<i>o</i>-QDM), as revealed by valence bond calculations, is demonstrated experimentally by trapping with the kinetically stable free radical TEMPO at room temperature. In the absence of TEMPO, the main pathway for decomposition at ambient temperature is not (as previously proposed in the literature) a radical reaction but instead a concerted Diels-Alder dimerization, which through ωB97X-D/aug-cc-pVTZ/SMD//M06-2X-D3/6-31+G(d,p)/SMD calculations is shown to proceed through an ambimodal bispericyclic transition 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, which self-reacts <i>exclusively</i> through radical pathways. These findings suggest the potential for tunable concerted/stepwise cycloadditions.