Cycloaddition reactions tetracyanoethylene

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

The azomethine imines exhibit the typical cycloaddition behavior expected of 1,3-dipolar species [fSJ] Numerous [3+2] cycloaddition reactions have been performed [201 204] Tetracyanoethylene adds to azomethine imines across the nitnle function instead of the C=C double bond This reaction is a rare example of this type of periselectivity [208] (equation 47)... 

We have also used poly(propynoic acid) in our studies of the photochemical interaction of PCSs with dienophiles, such as maleic anhydride, tetracyanoethylene, and styrene. This photochemical reaction of Diels-Alder type is accompanied by the breakdown of the conjugation system and the formation of slightly colored adducts266. Together with the cycloaddition reaction, photodegradation of PPA and its adducts takes place. A cycloaddition reaction is always preceded by the formation of a donor-acceptor complex of a PCS with a dienophile. 

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]. 

Studies focused on the ability of ALvinyl carbodiimides to undergo cycloaddition reactions have been carried out in recent years (Scheme 56). Thus, 2-azadiene derivatives 245 reacted with tetracyanoethylene to yield dihydropyridines 246 (86CL135), whereas treatment of 245 with... 

The cycloaddition reactions of 368 (X = CO) with 7V-phenylmaleimide, p-benzoquinone, dimethyl acetylenedicarboxylate, and tetracyanoethylene afforded [4 + 2]-cycloadducts with endo-stereochemistry ethyl acrylate gave two isomers (probably endo and exo). l,4-Dihydro-l,4-oxidonaph-thalene (19) yielded the endo-exo isomer. The Diels-Alder reaction of 368 (X = SO2) with ethyl acrylate gave a mixture of isomers, from which the higher melting product could be isolated in pure form. ... 

Dihydrothiophenes are cyclic vinyl sulfides. They have been shown to undergo interesting, and sometimes unexpected, cycloaddition reactions. With tetracyanoethylene... 

It is easy enough to see how to extend the labelling of cycloaddition reactions to those involving larger conjugated systems. As just one example, the cycloaddition of heptafulvalene to tetracyanoethylene has overlap developing on opposite sides of the 14-electron component, which is therefore a (4q + 2)a... 

Numerous [3 + 2] cycloaddition reactions have been performed with bis(trifluoromethyl)-substituted azomethine imines (79CB2609,79LAI33). Noteworthy is the [3 + 2] cycloaddition reaction with tetracyanoethylene, which adds across one of the nitrile functions instead of adding across the CC double bond. This is one of the rare examples of this type of periselectivity found in the case of tetracyanoethylene in [3 + 2] cycloaddition processes (76LA30). 

Tetracyanoethylene (TCNE) is widely used as an excellent dipolarophile in cycloaddition reactions, also including syntheses of 1,3-dithiolane derivatives. Thus, the reaction of the allylic dithioesters 583 with TCNE produced the 1,3-dithiolane cation radical 584, which next reacted with TCNE yielding 3,3,4,4-tetracyano-5-phenyl-6,8-dithia-bicyclo[3.2.1]octane 585 (Scheme 83) <1995T11503>. 

Cyclopropanone ketals undergo a unique [2s + 2a] cycloaddition reaction with tetracyanoethylene (TCNE) ". In the case of cis- and trans-dimethyl 0,S ketals 151... 

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