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Cyclopropylethylenes have been prepared by Wittig reactions, by coupling of dicyclopropylcarbene, and by dehydration of a suitably substituted ethanol. The scope and limitations of these preparations are discussed, particularly with regard to the synthetic application of the Wittig reaction to tetrasubstituted ethylenes.

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A number of ethylenes substituted by cyclopropyl group(s) (la-lk) are found to undergo thermal [2 + 21 cycloaddition with tetracyanoethylene under mild conditions. Among the olefins studied, 1, l-dicyclopropylethylene ( l a ) was the most reactive while 1,2-dicyclopropylethylene (Id and l e ) were theleast reactive; namely, geminal cyclopropyls greatly enhanced the reactivity of ethylene. It was noted that the rate of reaction is influenced by solvent polarity; in other words, the reaction proceeds rapidly in a polar solvent: k~ (acetonitrile)/ (ethyl acetate) = lo3.It was surmised that the present cycloaddition should be a donor-acceptor cycloaddition of tetracyanoethylene with cyclopropylethylene, which is highly electron rich.Further, the cycloaddition was found to be more than 90% stereospecific. Tetracyanoethylene (TCNE) has been known not only as a very reactive dienophile3 but also as an act'ivated olefin which is capable of undergoing [ 2 + 21 cycloaddition4 with some conjugated olefins and with electron-rich olefins.2J$6 Interestingly, it reacts also with cycl~propylethylenes~ in a [2 + 21 manner under mild c0nditions.l Recently, Effenberger and PodszunR and Barton and Rogido9 also demonstrated the high reactivities of 1,l-dicyclopropylethylene and 2-cyclopropylpropene in a [2 + 21 cycloaddition wit'h isocyanates. Results Structure and Reactivity of Olefins. -Various mono-, di-, and tricyclopropylelhylenes (la-lk) produce [2 + 21 cycloadducts in their reaction with TCNE (Table I). Among olefins, t'he most reactive ethylene is 1,1dicyclopropylethylene (la). The rate of reaction was so rapid that the color developed and disappea,red at once, and immediate evaporation of the solvent gave the adduct 2a in a quant'itat'ive yield. The reaction was somewhat slowcr with 1 , 1-dicyclopropylpropene (lb) and much slower with 1,l-dicyclopropyl-3-methyl-1butene (IC). The reactivity sequence observed here,