Cyclopropenes, acetylenic character

As cyclopropanes have olefin-like properties, the presence of a double bond in the three-membered ring adds some acetylenic character to the methine protons. X-ray crystallographic and gas-phase structural studies on cyclopropenes have been reviewed by Allen11 and recently by Boese5. 

Relevant examples of 1,2-dialkylcycloalkenes with which to test our conjecture about reaction 12 are disappointingly few. The simplest example of this class of compounds is 1,2-dimethylcyclopropene (24, n = 3, R1 = R2 = Me), and indeed, the requisite thermochemical data are available. Using the derived enthalpy of formation of 1,2-dimethylcyclopropene from Reference 56, we find this reaction to be 32 kJmol-1 endothermic. A posteriori, we are not surprised that this reaction is endothermic. After all, if it is part of the folklore of cyclopropanes that they are said to have olefinic character, then cyclopropenes are also said to have acetylenic character. Indeed, the related transalkylation reaction involving acetylenes... 

The s character of the vinylic C-H bond of cyclopropene is midway between that of sp and sp hybridized bonds and some similarity in behaviour to the acetylenic C—H bond is noted, particularly with regard to the acidity of the proton. Molecular orbital calculations at the ab initio level support an enhanced acidity of C(l)-H over C(3)-H in cyclopropene and correctly predict the preference for a 1-lithio derivative. A bis-lithio derivative is expected to have both lithium atoms bridging the C(l)-C(2) a bond. 

The cyclopropene ring system has special interest because of its highly strained double bond. The vinyl C-H bond has additional s-character that is in between that of a vinyl C-H and an acetylenic C-H. 1-Lithiocyclopropene (5) was calculated to be approximately 27 kcal mol" more stable than 3-lithiocyclopropene, but still about 26 and 35 kcal mol higher in energy than allenyllithium and propynylithium, respectively. ... 

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