Cyclopropenyl ions

The fragmentation of the pyrrolyl cation to give the cyclopropenyl ion accounts for the peak at mass 39 (cf. Scheme 1). 

The presence of the methyl groups causes a shift of the cyclopropenyl ion [1] of unsubstituted furan by 14 mass units (MU), to mass 53, because in this case a methylcyclopropenyl cation is produced. 

In a-substituted methyl pyrroles the a position of the substituent is indicated in the same manner as in furans and thiophenes by the fragment of mass 53 corresponding to the methylcyclopropenyl ion accompanying that of mass 39, the cyclopropenyl ion [1]. All alkyl-pyrroles have a strong tendency to form pyridinium ions by ring enlargement (48)->[49] ... 

The cyclopropenyl ions , as well as other compounds which incorporate this structural feature by virtue of resonance with an aromatic dipole, occupy an important position in this family. In this chapter our aim is to cover the significant aspects of this group of theoretically important compounds. 

This discovery represented an important advance in the synthesis of cyclopropenyl ions, since 18 can serve as a point of departure in the synthesis of a large variety of new cyclopropenyl salts by reaction with nucleophiles. This is discussed in the subsequent section of this chapter. The conversion of cyclopropenones to cyclopropenyl salts is similarly reserved for a later section. 

The chemistry of cyclopropenyl ions is dominated by reactions which lead to cyclopropenes, usually either by reduction or by their reactions with nucleophiles. For example, triphenylcyclopropenyl cations, as well as some other derivatives, can be reduced in good yield to the corresponding cyclopropene using lithium aluminum hydride (equation 11)27,28 however, diphenylcyclopropene which is produced in the analogous reduction of... 

It is not surprising that cyclopropenyl ions react with a large variety of nucleophiles. The reversible equilibrium with ethers in alcohols has been discussed earlier. 

Since halogen attached to a cyclopropenyl ion can be displaced by a large number of nucleophiles, the trichlorocyclopropenyl cation 18 serves as a precursor, par excellence, for heterosubstituted cyclopropenyl cations, or cyclopropenes. A few of these are outlined in equation 23 without comment. The reader is referred to the review by Yoshida for a more detailed account of this area. 

Although many of the substituted cyclopropenyl ions are marvels of stability, salts of the parent ion darken rather rapidly on heating and exposure to atmospheric moisture causes rapid decomposition. The hexachloroantimonate salt is stable for a long period at — 20° C and for a few hours at room temperature. As might be expected, the cyclopropenyl salts are soluble in polar solvents such as acetonitrile and dimethylformamide, but are insoluble in non-polar solvents. They, of course, react with protic solvents as described earlier. In aqueous solution the equilibrium of equation 24 is established. The pH required to... 

The special effect of cyclopropyl is noteworthy since the tricyclopropyl-cyclopropenyl cation, the most stable of all the hydrocarbon cyclopropenyl cations, is nearly as stable as cations stabilized by complexation with organometallic reagents. The triferrocenyl and tri(3-guaiazulenyl)cyclopropenyl ions can be cited as examples since these species have P r+ values greater than 10. Other metal complexed cyclopropenyl species have also been reported " " ... 

Spectral properties of the cyclopropenyl ions are all consistent with a fully delocalized structure The cyclopropenyl ion itself exhibits a strongly deshielded singlet at S 11.0 resulting from both hybridization effects and the positive charge. The large C-H coupling constant of 235 Hz corresponds to sp hybridization at the carbon. The infrared spectrum of both the hexachloroantimonate and the tetrachloroaluminate show four bands at 3105,1276,908 and 736 cm " for the cation. The observation of four bands in the infrared spectrum is expected for a symmetrical, delocalized structure of the cation with Djh symmetry. 

Among the imobserved species is the highly symmetrical, very stable ion CsHt-The cyclopropenyl ion is regarded as the first aromatic hydrocarbon. As a consequence of its center of symmetry, it does not have a permanent electric dipole... 

As seen from these data the DE of the aromatic cyclopropenyl cation is far larger than that of the acyclic allyl cation because for the cyclopropenyl ion with one split a-bond the 1,3-resonance interaction (pi) is still significant. Such particles are called homoaromatic. The a-skeleton of an aromatic particle can be broken at one, two or three sides, and such ions are called, respectively, mono-, bis- and tris-homoaromatic. The prefixes mono- , bis and tris -stand here for the number of the sides where the a-bond is broken or stretched, but not for the number of CH groups introduced in place of one a-bond. 

The tropylium ion and the cyclopropenyl ions included in Table 5.1 are examples of cations stabilized by being part of a delocalized aromatic system. These ions are aromatic according to Hiickel s rule, with the cyclopropenium ion having two w electrons and the tropylium ion six. Both ring systems are planar and possess cyclic conjugation, as is required for aromaticity. 

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