1.3- Diradicals, rearrangement

In the presence of proper substituents the primarily formed diradical rearranges to a more stable one, as by a ring opening reaction. In (2.7) medium ring lactones are synthesized by such a sequence 2U). 

Vinylcyclopropane-Cyclopentene, Bergman, Di-jr-methane, and Related Diradical Rearrangements... 

When thioketenes are generated by thermolysis or photolysis of 1,2,3-thiadiazoles, the initially formed 1,3-diradical undergoes a [3-1-3] cycloaddition to forma 1,4-dithiine derivative 9, which can extrude sulfur to produce a thiophene derivative 10. When the initially formed diradical rearranges to the thioketene it can react via a [3-1-2] cycloaddition across the 1,3-diradical to give the 1,3-dithiol derivative 11. 

A new type of rearrangement has been reported for certain l-(o-nitrophenyl)pyrazoles (169), giving cis- and trans-benzotriazole 1-oxides (170 Scheme 12) (73TL891). The reaction was rationalized in terms of an intermediate azo compound (171 formed in turn either from the diradical species (172) or from the intramolecular 1,4-adduct (173). Subsequently... 

It has been proposed that oxygen adds to the excited keto group [- (112)]. The rearrangement of the resulting hydroxyhydroperoxy diradical (112) could then proceed by intramolecular hydrogen abstraction involving a six-membered cyclic transition state, followed by fission of the former C —CO bond to form the unsaturated peracid (113) as the precursor of the final product. Such a reaction sequence demands a hydrogen atom in the J -position sterically accessible to the intermediate hydroperoxy radical. 

Nonfluonnated allenes also readily react with fluoroalkenes to give diverse fluonnated alkylidenecyclobutanes [727, 12S, 129, 130] (equations 55 and 56), except for tetramelhylallene, which rearranges to 2,4-dimethyl 1,3-pentadiene under the reaction conditions prior to cycloaddition (equation 57) Systematic studies of l,l-dichloro-2,2-difluoroethylene additions to alkyl-substituted allenes establish a two-step, diradical process for alkylidenecyclobutane formation [131, 132, 133]... 

Upon heating the enediyne la rearranges reversibly to the 1,4-benzenediyl diradical 2a, which in its turn can rearrange to the enediyne lb or—in the presence of a hydrogen donor (e.g. cyclohexa-1,4-diene)—react to the aromatic compound 3a. 

For the mechanistic course of that reaction two pathways are discussed " a concerted [l,3]-sigmatropic rearrangement, and a pathway via an intermediate diradical species. Experimental findings suggest that both pathways are possible. The actual pathway followed strongly depends on substrate structure the diradical pathway appears to be the more important. 

The dominant pattern for the thermal fragmentation of thietane dioxides involves extrusion of sulfur dioxide leading to a 1,3-diradical (i.e. 242) which closes to final products, mainly cyclopropanes, accompanied by rearrangement products resulting from hydrogen migration within the diradical191,1930 230,256-258 (equation 92). 

No firm decision, between an anion diradical mechanism and a concerted S ->0 1,2-anionic shift, could be made from the available evidence106. Interestingly, the use of a stronger base such as ethylmagnesium bromide results in rearrangement to trans-1,2-diphenylcyclopropanesulfinic acid in highly stereoselective manner (equation 36)107. 

It has been suggested that another type of diradical two-step mechanism may be preferred by some substrates. Indeed, a nonconcerted Cope rearrangement has been reported. In this pathway, the 1,6 bond is formed before the 3,4 bond breaks ... 

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