Vinylcyclobutane cation radical

For a review of ring expansion of vinylcyclobutane cation radicals, see Bauld, N.L. Tetrahedron, 1989, 45, 5307. 

Diels-alder adducts at 0°C. This cation radical-vinylcyclobutane rearrangement is non-stereospecific, thus accounting for the formation of a cis-trans mixture of Diels-Alder adducts. Kinetic studies revealed (Scheme 8) that the ionization of these ethers involves an inner-sphere electron-transfer mechanism involving strong covalent (electrophilic) attachment to the substrate via oxygen (oxonium ion) or carbon (carbocation). 

Styrene type [59a], As will be noted further on, the tendency of these initially formed vinylcyclobutane adducts to quickly rearrange via a cation radical mechanism to the cyclohexene (i.e., Diels-Alder) adducts is also more pronounced. In the case of a very highly electron rich substrate such as A-methyl-A-vinylacetamide, even the reaction with the rigidly s-cw-l,3-cyclohexadiene is CB periselective (Scheme 28) [61]. 

The Cation Radical Vinylcyclobutane (VCB) Rearrangement Scope and occurrence... 

Efficient and facile rearrangements analogous to the cation radical vinylcyclobutane rearrangement have also be identified in the vinylcyclopropane (VCP) series (Scheme 49). 

In general, radical cations of alkenes or cyclopropanes produce nonconjugated radicals, while those of dienes give rise to allyl radicals, and those of vinylcyclopropane or vinylcyclobutane systems generate either allylic or nonconjugated radicals with an additional element of unsaturation. In contrast to the most thoroughly characterized nucleophilic substitution of appropriate neutral molecules. 

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