Electron-Transfer-Induced Dimerization of Spiro[2.4]heptadiene. A Facile Radical Cation Diels-Alder Reaction

The oxidative dimerization of diolefins resulting in cycloadducts of the [4 + 2] type (cation radical Diels-Alder reaction) has been the focus of much interest recently. ^1-8 However, only limited experimental evidence is available concerning the key step of the dimerization, i.e., the addition of the radical cation to the parent olefin. Is this addition stepwise or concerted? Does the radical cation serve as the “diene” component ([3 + 2] cycloaddition) or as dienophile ([4 + 1] cycloaddition)? The observed retention of dienophile stereochemistry supports a concerted mechanism,² whereas the existence of a singly linked doubly allylic radical cation derived from endo-dicyclopentadiene⁹ might be viewed as evidence for a stepwise mechanism. No experimental evidence differentiates the [3 + 2] from the [4+1] cycloaddition type, but orbital symmetry arguments favor the latter type.^4,5 In this paper we describe a system which provides evidence for a nonconcerted component in the cycloaddition/cycloreversion.