Super radical stabilizers

A comprehensive series of substituted 1,1-dimethyl-2-methylenecyclopropanes have been thermally rearranged. These rearrangements proceed via singlet biradical intermediates that can be stabilized by substituents. Rates are greatly enhanced by certain groups that are termed super radical stabilizers. Substituents included 4-pyridyl N-oxide, 2-(1,6-methano[10] annulenyl), and a number of anion-substituted phenyl groups. Simple valence bond theory, as well as more sophisticated computational studies, gives insights into modes of radical stabilization.