Tetracyanoethylenes, reactions with enamine

Alkenes with electron-withdrawing groups may form cyclobutanes with alkenes containing electron-donating groups. The enamine reactions, mentioned above, are examples of this, but it has also been accomplished with tetracyanoethylene and similar molecules, which give substituted cyclobutanes when treated with alkenes of the form C=C—A, where A may be... 

Tetracyanoethylene is unusual in that it reacts with cyclohexanone enamines to give a five-membered ring instead of a cyclobutane adduct. Reaction occurs at the y-position of the enamine and an initial one-electron transfer between the two reactants is... 

The opposite type of reaction has also been reported, viz. one in which the heterocyclic molecule reacts via an electron-rich double bond with electron-poor olefins, in particular with tetracyanoethylene. Tanny and Fowler43 found that 2-azabicyclo[3.1.0]hex-3-enes reacted with tetracyanoethylene via a (2 + 2)-cycloaddition of the enamine double bond to give 13. Other electron-deficient reactants, such as JV-phenyl-maleimide, reacted differently, yielding an 8-azabicyclo[3.2.1]oct-2-ene (16). This type of reaction possibly occurs via a concerted [ 2 +ff2 +n2]-cycloaddition.43 At room temperature tetracyanoethylene also readily formed (2 + 2)-cycloadducts with heterocycles that contained a vinyl ether group for instance, 3,4-dihydro-2ff-pyran, 2,3-dihydrofuran, and 2,2-dimethyi-l,3-dioxole afforded the adducts 17-19 in yields of... 

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