Multistep, or Non-concerted Pericyclic Processes

The gas-phase thermolysis of (313) at 425 °C gave dimethylenecyclobutene and cyclopentadiene by Diels-Alder retrogression, and only a trace of the a-cleavage product (314). In contrast, benzocyclobutene is not formed from either of the isomers 

A charge-transfer complex formed upon addition of tetracyanoethylene to 7,7-dicyclopropylfulvene decomposes at 0°C to give (317) quantitatively. At slightly higher temperatures (317) is isomerized to (318) which in turn yields (320) by way of the non-isolable intermediate (319). The transformation (319) (320) probably involves a suprafacial [1,5] hydrogen shift, whereas the solvent-dependence of the rates for (317) (318) and (318) - (320) indicates non-concerted pathways involving 

The dichloropropellene (324) underwent exothermic dimerization at room temperature in the neat state or when dissolved in polar solvents, the R,S-dimer (326) being isolated in 84 % yield. The intermediate formation of the bridgehead olefins S-(325) and i -(325) is indicated by the isolation of a [4 -I- 2] adduct when furan was added as a trapping agent. On statistical grounds, assuming 5-(325) and jR-(325) to be 

The thermal rearrangenents of all of the possible [4 + 2] and [2 + 2] adducts of cyclopentadiene with ds- or trans-1,2-dichloroethylene have been studied, and estimates made of the relative importance of the various competing mechanisms in their intercon version.  

It is reported that unless special precautions are taken for the vacuum distillation of crude -ionylideneacetaldehyde extensive rearrangement is observed and the tricyclic ketone (327) may be isolated in about 50% yield. The formation of (327) can be rationalized by a scheme involving successive E-Z isomerization, [1, 5] shift of aldehyde hydrogen, and intramolecular cycloaddition of the unsaturated ketene so formed to the cyclohexene part of the molecule. A second product is formed in the course of the reaction and the compound (328) is a tentative suggestion for its structure since a second mode for the ketene-cyclohexene addition is possible. 

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