Triphenylcyclopropenium cation

The stable triphenylcyclopropenium cation (81) undergoes an electron-transfer reaction when photolyzed in acidic medium (van Tamelen et al., 1968, 1971). Irradiation of 81 for 4 hours in 10% aqueous sulfuric acid resulted in a 49% yield of hexaphenylbenzene (82). The reaction is presumed to proceed by initial charge transfer to produce the cyclopropenyl radical 83, which then couples to give 84. This compound in... 

The triphenylcyclopropenium cation (2), first prepared by Breslow,14 is an unusually intriguing species a 2tt-electron aromatic species constituted by a three-membered carbon ring. In every respect the triphenylcyclopropenium cation displays traditional aromatic behavior, the same as benzene. 

The p-phenylene-bis(diphenylcyclopropenium) dication, 39 (R = H), was found to have a />Kr+ value of -1.1, that is, the dication is less stable than the triphenylcyclopropenium cation (pkR+ = 2.8) by 1.7 pK units, again owing to charge-charge repulsion in this dication. 

Reduction of the triphenylcyclopropenium cation with zinc, magnesium and chromium salts, or by electrolysis affords hexaphenyl 3,3 -bicyclopropenyl via the triphenylcyclopro-penyl radical With lithium aluminium hydride the triarylcyclopropenium ions are reduced to the corresponding triarylcyclopropene. 

Other studies on systems claimed to be aromatic include calculations on di- and triphenylcyclopropenium cations, in which modifications of the conventional Johnson-Bovey-Waugh-Fessenden model... 

Reduation. The triphenylcyclopropenium cation is reduced in good yield to triphenylcyclopropene by means of lithium aluminium hydride [34,35]. Other triaryl derivatives have been reduced similarly [34]. On the other hand reduction of diphenylcyclopropenium salts by hydrides leads to the formation of 1,2,4,5-tetraphenylbenzene [35]. 

In contrast the triphenylcyclopropenium cation does not readily add to cyclopropenes, explaining why in this case triphenylcyclopropene is formed when the cyclopropenium salt is reduced by hydrides [35]. 

An interesting chemical proof of the symmetry of this cation was provided by preparing it from C-labelled benzal bromide and then ring-opening the derived triphenylcyclopropenium cation [33] ... 

The l,2,3-tri-/-butylcyclopropenium cation is so stable that the perchlorate salt can be recrystallized from water. An X-ray study of triphenylcyclopropenium perchlorate has verified the existence of the carbocation as a discrete species. Quantitative estimation of the stability of the unsubstituted ion can be made in terms of its pXn+ value of —7.4, which is intermediate between those of such highly stabilized ions as triphenylmethyl... 

According to this procedure, with proper choice of reaction conditions, a sizeable amount of diphenylacetylene can be converted quantitatively to triphenylcyclopropenium bromide in a few hours. The reaction is of wide generality and can be applied to p-anisylphenylacetylene and to di-p-anisylacetylene, or with p-anlsal chloride instead of a,a-dichlorotoluene. to prepare p-methoxy derivatives of the title compound. Since the initial preparation of this derivative of the cyclopropenyl cation by a less efficient procedure, many aryl-, alkyl- and heteroatom-substituted derivatives of this simplest cyclic aromatic system have been synthesized, including... 

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