Cyclopropenium salts cyclopropene derivative

It followed from Hiickel s rule that the cyclopropenium cation, derived from cyclopropene by loss of a hydride anion, should be a stabilised system, having 2 ir-electrons delocalised over three ring atoms. Realisation of this led to atteiipts to prepare examples of cyclopropenium salts and this was achieved as follows in 1957 [1] ... 

Attention was drawn [2] to the contrast between triphenylcyclo-propenium bromide which exists as an ionic salt and triphenylmethyl bromide in which the bromine atom is covalently bound. Yet strain factors, and the increased electronegativity of the ring carbon atoms in the cyclopropene derivative, due to the different hybridisation in the three-membered ring, should favour ionic dissociation of the triphenylmethyl compound rather than the triphenylcyclo-propenium derivative. The stable existence of this cyclopropenium cation was thus evidence for the extra stability conferred on this ion by its delocalised system of two 7r-electrons. 

Cyclopentazulenes, 409, 411 Cyclopentindenes, 406 Cyclopropene derivatives, 148 Cyclopropeneimines, 173, 178 Cyclopropene, methylene, 183 Cy clopropenethiones, 178 Cyclopropenide anions, 187 Cyclopropenium salts, 150 preparation, 150 properties, 152 reactions, 154 with nucleophiles, 156 photochemical, 159 reduction, 154 spectra, 153 stability, 152 structure, 153 Cyclopropenium salts, alkoxy, 159 Cyclopropenium salts, alkylthio, 159 Cyclopropenium salts, amino, 159... 

Following sections of this chapter discuss in turn cyclopropenium salts, cyclopropenones and other cyclopropene derivatives such as methylenecyclopropenes, also called triafulvenes. 

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