Tetracyanoethylene reaction with enol ethers

The stereochemistry of these reactions depends on the lifetime of the dipolar intermediate, which, in turn, is influenced by the polarity of the solvent. In the reactions of enol ethers with tetracyanoethylene, the stereochemistry of the enol ether is retained in nonpolar solvents. In polar solvents, cycloaddition is nonstereospecific, as a result of a longer lifetime for the zwitterionic intermediate.177... 

The first reference to 2-trimethylsilyloxy-l,3-butadiene (1) was a report2 of its reaction with tetracyanoethylene by Cazeau and Frainnet without mention of any experimental details. Later, Conia3 reported its synthesis in 50% yield with only a reference made to the usual House procedure4 for silyl enol ethers. The diene 1 has also been prepared using lithium diisopropylamide as base and chlorotrimethylsilane in tetrahydrofuran-ether (1 1) in yields up to 65%, but on a smaller scale.s... 

The nature of the intermediates in the addition of tetracyanoethylene to electron-rich olefins such as enol ethers continues to arouse interest. Huisgen has studied the concentration dependence of the rate constant for the reaction of TCNE with dihydropyran in a variety of solvents." The logarithms of the second-order rate constants in the various solvents show a linear relationship with solvent polarity. The lack of deviation from linearity shows that the charge-transfer complex concentration does not affect the rate. Le Noble and Mukhtar reported a volume of... 

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 SR (enol and thioenol ethers), cyclopropyl, or certain aryl... 

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