Three- and Four-membered Ring Systems

Symmetry-allowed extrusion of the cyclopropenyl cation is not observed on loss of nitrogen from (2). Instead the ether (3) is formed in methanol, possibly via the pyramidally bridged cation (4).  

Yoshida and his co-workers have continued to contribute greatly to the field of cyclopropenium ion chemistry. In addition to describing a large number of cyclopropenium ions, cyclopropen-ones, and -thiones in the patent literature they have produced the first triafulvalene dication (5). This was obtained, by treatment of (6 X = H) with butyl-lithium and then allowing the resulting carbenoid (6 X = Li) to react with (6 X = C1). The transformation illustrates 

Hydride abstraction from (7) with trityl perchlorate does not give the dication (8), but rather the monocation (9) resulting from deprotonation of (8). C n.m.r. spectroscopy reveals that the central carbon possesses a considerable amount of electron density, indicating that the dipolar form (9a) makes a significant contribution. There is evidence that formation of dicyclopropene from cyclopropenyl cations under conditions of two-electron reduction proceeds through coupling of cyclopropenyl radicals produced by electron transfer from anion to cation. 

The cyclopropenyl ether (10 R = Et), with FeCla, in refluxing ethanol, gives triphenylcyclopropene and indenone (11). The former results from reduction of the triphenylcyclopropenyl cation by ethanol the latter is thought to involve oxidation of the hydroxy-cyclopropene (10 R = H) to the radical (12) (or further to the cation), which cyclises.  

The cation (13), prepared from ll-methyl-ll-bromotricyclo[4.4.1.0 ]-undecane in SbF5-S02ClF at — 120°C, is neither a symmetrical cyclopropyl cation nor an allylic cation, but is best considered as a bent cyclopropyl cation showing significant 2ir-homoaromatic nature. An excellent review, of the concept and experimental evidence for homoaromaticity includes sections on mono-, bis-, and tris-homocyclopropenium cations.  

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Three- and Four-Membered Ring Systems Three-Membered Ring Systems... 

Electrodecarboxylation followed by C-C bond cleavage has been investigated for three- and four-membered ring systems. The dichlorocyclopropane carboxylic acid (CXLI) undergoes decarboxylation in either acetic acid or methanol to give the ring-opened acetate (CXLII) or methyl ether (CXLIII) in good yields [184] ... 

Within certain structural requirements alkyl halides may cause intramolecular alkylation to give three- and four-membered ring systems. For example, methyl bicyclo[1.1.0]butane-l-carboxylate is obtained in 77% yield from methyl 3-bromocyclobutanecarboxylate and sodium hydride 388... 

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