Unsaturated Derivatives of Cyclopropane

The superb general review by Closs must still be regarded as the essential primer in cyclopropene chemistry despite its 1966 publication date. The newest review, that by Halton and Ban well in this volume (Chapter 21), clearly illustrates the enormous growth in this field in the past two decades. 

In connection with acidity, the introduction of a carbon-carbon double bond into the cyclopropane ring has two obvious effects (a) the overall ring strain (54.5 kcal mol ) is 

There are two additional features of the hydrocarbon, cyclopropene, which have not been considered and which may contribute to its reduced kinetic and thermodynamic acidity. First of all, the reversed sign (polarity) of the dipole moment of cyclopropene compared to other alkenes, including cyclobutene, and its larger than usual magnitude (0.455 D) , is indicative of significant electron shift from the double bond to the methylene group . This has been shown to be a result of electron transfer from the n c-orbital to the 7 component orbital . Related to this polarity reversal is the observation 

3-cyano- and 3-benzoyl-substituted cyclopropenes 21a and 21b under basic conditions proceeds with little or no retention of configuration, suggesting that the degree of 

Efforts directed towards the synthesis of stable, perhaps isolable, cyclopropenyl anions having three identical anion stabilizing groups, such as cyano or benzoyl, have been attempted without much success. Tricyanocyclopropene (22a) could not be prepared except as a transient intermediate which could be trapped with diphenylisobenzofuran The corresponding triketone (22b) was obtainable, but attempted generation of the corresponding anion was thwarted by conjugate (Michael) addition of the base to the double bond. The cyclopropenyl anion remains an illusive species  

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An attempt has been made to extend the discussion to the unsaturated derivatives of cyclopropane, i.e cyclopropene and methylenecyclopropane however, the treatment is not extensive either due to a paucity of pertinent chemistry or to coverage elsewhere in this volume (Cyclopropenes, Chapter 21 Cyclopropenyl derivatives. Chapter 24). The reader will also note that the discussion on basicity of cyclopropanes is considerably more extensive due to the wealth of new chemistry and conceptualizations in the past dozen or so years. 

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