Cyclopropenide anion

Cyclopropenide anion, structure, 56, 368 Cyclopropylamine, N-i-butoxycarbony 1-1 -ethoxy, 58, 179... 

Reaction of cyclopropane with sodium or potassium amides in liquid ammonia results in the loss of a vinylic proton rather than a methylene proton [198,199] similarly methyl cyclopropene-1--carboxylate exchanges its vinylic but not its methylene proton in base [200]. The structure of the cyclopropenide anion is clearly shown by its H-n.m.r. spectrum. In the case of the sodium salt this is as follows 56.73 (t,lH), -0.45 (d,2H), J = 1.94 Hz [199]. These compounds dimerise on steuiding with alkyl halides they give mono- and di-alkyl cyclopropenes [199] ... 

Cyclopropenide anions thus appear to be reluctant to be formed and this could be associated with the destabilisation foreseen for this 4 TT-electron system. Furthermore when they are formed their structure is such as to prevent formation of a cyclic 4 -rr-electron system. 

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... 

A small amount of triphenylcyclopropene may be formed as a by-product [41,43], especially when guanidinium perchlorate was used as supporting electrolyte [43]. Electrolytic reduction proceeds vta a radical to a cyclopropenide anion which is captured very effectively by a cyclopropenium cation to provide the biscyclopropenyl [43] ... 

There has been much interest in the cyclopropenide anion because of its proposed destabilisation by having 4 iT-electrons, also described as anti aromaticity [190] useful reviews are available [191]. 

Thus both the cyclopropenide anion and the cyclopropyl radical appear to be energetically disfavoured species. 

In this vein, a variety of cyclic systems related to benzene (CgFIg), although not arenes (as arenes are derivatives of benzene), have been examined. Consider cyclo-propene (C3H4) and the ions derived from it as shown in Figure 6.25. If a hydride ion (FI) is removed, the cyclopropenyl cation (C3FI3+), a planar array with 4n -1- 2 electrons (n = 0) results, while if a proton (H+) is removed and the electron pair left behind, the cyclopropenide anion, a planar array with 4n (n = 1) electrons can be... 

Figure 6.25. A depiction of the cyclopropenyl cation and the cyclopropenide anion. The former, with 4 -I- 2 electrons ( = 0) is particularly stabilized and is considered aromatic while the latter, with 4n electron (n = 1) is considered antiaromatic.

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