Ionic compounds carbocations

The cycloheptatrienyl carbocation, also known as the tropylium cation, has six pi electrons. It is also aromatic and is quite stable. In fact, 7-bromo-l,3,5-cycloheptatriene actually exists as an ionic compound. 

Before we analyze the results obtained for a large variety of compounds, we recall the main formulas we have previously deduced for calculating the stabilization energy of neutral species, molecules and free radicals, and of ionic species, carbocations and carbanions. Furthermore,... 

In contrast to triphenylmethyl chloride, which has the properties of a covalent compound, triphenylmethyl perchlorate behaves as an ionic compound. The presence of triphenylmethyl cations in this solid has been confirmed by an X-ray crystal structure determination.The central carbon is planar, but the three phenyl rings are at an angle of 54 to the plane of the trigonal carbon so that the overall cation has a propeller-like shape. The temperature-dependent NMR spectrum of the carbocation also indicates that it has this structure in solution.The twisting of the aromatic rings with respect to each other is evidently the result of van der Waals repulsions between the ortho hydrogens. 

In the course of the salt synthesis, it was found that a hydrocarbon [3-2], which was formed by an unfavourable cation-anion combination reaction, dissociates into the original carbocation and carbanion in a polar aprotic solvent (Okamoto et ai, 1985) (1). This was the first example of ionic dissociation of the carbon-carbon a bond in genuine hydrocarbons, although a few cases of heterolytic dissociation of carbon-carbon tr bonds had been reported by Arnett (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985) for compounds bearing cyano and nitro groups, e.g. [4-6] and [5-6] as in (2). 

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

Unlike such unstable intermediates, the first, rare example of reversible dissociation of a carbon-carbon a bond into a stable carbocation and carbanion was reported for a nitro-dicyano compound (20) prepared from trimethyl- and triphenyl-cyclopropenylium tetrafluoroborate ([4" ]BF4 and [5 JBFJ) with the potassium salt of p-substituted-phenylmalononitrile anions (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985). Other ionically dissociative malononitrile derivatives have been prepared from such carbocations as the tropylium [S ] (Arnett and Troughton, 1983) and the tris(p-methoxyphenyl)methylium [93 j (Arnett and Troughton, 1983) ions. 

In view of the observations of the ionic dissociation of nitro-cyano compounds, it is hardly surprising that even a hydrocarbon could dissociate ionically into a stable carbocation and carbanion, provided that the medium is polar enough to prevent the recombination reaction and to ensure equilibration. 

The use of deuterated organosilicon hydrides in conjunction with proton acids permits the synthesis of site-specific deuterium-labeled compounds.59 126 221 Under such conditions, the deuterium atom in the final product is located at the charge center of the ultimate carbocation intermediate (Eq. 62). With the proper choice of a deuterated acid and organosilicon hydride, it may be possible to use ionic hydrogenation in a versatile manner to give products with a single deuterium at either carbon of the original double bond, or with deuterium atoms at both carbon centers.127... 

The tendency to react according to (6) or (7) depends on the stability of the (incipient) carbocation [16, 17] and on the oxidizing power (redox potential) of the nitro compound [12, 18]. It also depends on solvent, more polar solvents favoring the ionic path (Eq. 7) [18]. 

The synthesis of C-glycosyl compounds, commonly known as C-glycosides, in ionic reactions relies on the electrophilicity of the anomeric center and, therefore, involves the attack of an appropriate C-nucleophile. An umpolung method has been developed, and is described in the previous chapter. But instead of going from a carbocation to a carbanion, one can also consider homolytic or radical reactions to reverse the philicity (Scheme 1). 

Such a dethroning of carbocations and carbanions can be carried another step further. Certain purely covalent compounds possessing weakly polarized bonds can serve as ionic reagents if their electronic system is highly polarizable. In such molecules the approach of a charged particle, or even a dipole, induces a significant displacement of electrons. The electron displacement is to such an extent that the reaction intermediates may become almost fully ionized, as if an ionic reagent had been actually used. A typical example is represented by the previously mentioned reactions of electrophilic substitution in aromatic series, where a neutral molecule of an aromatic hydrocarbon, ArH, behaves as an efficient equivalent to a carbanion, Ar . 

Unlike Lewis-acid-catalyzed rearrangements of organic compounds, ionic intermediates are not important in the mechanism. No evidence has been found to support the existence of silylenium ions, RgSi, and careful studies have ruled out carbocations as well. 

The reactions illustrated in equations (62-64) are each catalyzed by rhodium acetate. Diazo compounds, especially diazoalkanes, can also react by a simple ionic mechanism. The esterification of a carboxylic acid with diazomethane is a familiar example. The ionic pathway is especially likely when the intermediate carbocation would be stabilized. Thus, diazine (158) couples smoothly with phenols, presumably by thermal rearrangement to the corresponding diazo sugar, followed by acid-catalyzed N2 loss... 

The rate-determining step in ionic hydrogenation is the protonation of the C=C bond . The unsaturated substrate must be capable of forming a stable carbocation by protonation with CF3CO2H which strongly limits the application of this reaction. Unsaturated compounds which are branched at the alkenic carbon atom can be easily reduced -, but unbranched compounds are not reduced under conditions of ionic hydrogenation reaction - -. 

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