Frans-Dicyanoethylene

The cycloadditions in entries 1-3 are still believed to occur via a diradical stepwise pathway, as confirmed by obtaining a thermodynamic 78 22 trans/cis mixture of dispirooctanes 536 from frans-dicyanoethylene (533) (entry 3) [13b, 143], The cycloaddition to tetracyanoethylene (131) in the absence of oxygen gives only low yields of the [2 + 2] adduct, due to the simultaneous formation of products 542 and 543 (Scheme 74) [13b]. Still, the formation of the cyclobutanes 537 and 542 is noteworthy, since the reactions of TCNE with phenyl substituted MCPs exclusively afford methylenecyclopentane derivatives [37,144], The reaction is thought to occur via dipolar intermediates 539-541 formed after an initial SET process (Scheme 74) [13b]. The occurrence of intermediates 540 and 541 has been confirmed by trapping experiments [13b]. 

Thermolysis of indenone oxides (151) is an equally useful route to the betaines (150). Dimethyl acetylenedicarboxylate and compound 151 (R = R = Ph) at 175°C give adduct 153, and cyclohexanone at 150 C gives adduct 154. - Similarly, 1,3-dipolar adducts (e.g., 155) have been obtained using a wide variety of olefins—including cis- and [rans-, 2-dichloroethylene, dimethyl maleate, dimethyl fumarate, maleic anhydride, cis- and tran -stilbene, fran -dibenzoylethylene, tra .y-l,2-dicyanoethylene, A -phenylmaleimide, vinylene carbonate, acenaphthylene, and norbor-nadiene. With cis olefins the endo adduct (155) is usually the predominant isomer. Diphenylcyclopropenone gives compound 156 by spontaneous elimination of carbon monoxide from the initial adduct (157). Adduct 156... 

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