Tris methyl cation

C-NMR spectroscopic studies on a-substituted tris(ethynyl)methyl cations 49 prepared from alcohols 50 (equation 18) provided evidence for the participation of resonance structures with allenyl cationic character38. The parent tris(ethenyl)methyl cation (49, R = H) cannot be generated under stable carbocation conditions (SbFs/FSOsH) presumably due to the highly reactive unsubstituted termini of the three ethynyl groups and the resulting low kinetic stability. The chemical shift data (Table 1) give evidence that in all cases Ca and CY are deshielded more than Cg (relative to their precursor alcohols). 

TABLE 1. 13C-NMR Chemical Shifts of Tris(ethynyl)methyl Cations (49a-d) and Their Precursor Alcohols 50a-d°... 

We have reported the synthesis of a series of azulene-substituted methyl cations, i.e., tri(l-azulenyl)methyl, di(l-azulenyl)phenylmethyl, and (1-azul-enyl)diphenylmethyl cations (2a+, 3a+, and 4a+) (Figure 6) (6). In order to examine substituent effects on the azulene rings and to enhance their stabilities, a series of the cations (2b-d+, 3b-d, and 4b-d+) bearing tert-buty groups on each azulene ring were also synthesized (7). 

Azulene-substituted methyl cations were prepared as illustrated in Figure 7. The hydro derivatives (7a-d) became good precursors for the methyl cations. These derivatives were readily obtained by the reaction of azulenes 6a-d with 1-formyl compounds 5a-d under acidic conditions. The synthesis of the tri(l-... 

The pKR+ value for the parent tri(l-azulenyl)methyl cation (2a+) is 11.3. Hydrocarbon-based carbocations, which comprise only of carbon and hydrogen, are generally very reactive species. Some extremely stable hydrocarbon carbocations, which exist even under basic conditions, were reported in the literature (5). However, most of these examples are cyclic cations, such as cyclopropenylium or tropylium ions (Figure 8). The tropylium ion 8+ annelated to three bicyclo[2.2.2]octane units is one of the most stable hydrocarbon-based carbocation ever reported (9). 

Table I summarizes the pKR+ values and redox potentials for the tri(l-azulenyl)methyl cations. The oxidation exhibited a barely separate two-step, two-electron oxidation wave. This wave is ascribed to the oxidation of two azulene rings to generate a tricationic species. The reduction showed a one-electron wave, which is ascribed to the formation of a neutral radical.

Cation 24+ showed strong absorption in visible region, as did the parent tri(l-azulenyl)methyl cation (2a4). The longest wavelength absorption showed an appreciable bathochromic shift as compared to that of 2a+. As expected, the CV exhibited two reversible reduction waves (-0.56 V and -0.73 V), which correspond to two single-electron transfers. The less negative second reduction potential, as compared to parent tri(l-azulenyl)methyl cation (2a+), corresponds to an increase in electron affinity of the cation due to the 6-azulenyl substituent. 

Under the same conditions, the methyl ester is also converted to the tri-methylbenzoyl cation. The i-value in this case is 5, since the molecule of methanol lost is converted to methyl sulphate18, viz. 

The highly crowded tris( 1 -naphthyl)methyl cation 150 and tris(2-naphthyl)methyl cation 151 were prepared and used to abstract hydride ion from cycloheptatriene to generate tropylium ion.363 Hydride abstraction, however, could be performed only with the less crowded cation 150. 

Stable fluorinated bis(alkoxy)methyl cations 324 and tris(alkoxy)methyl ion 325 were prepared reacting the corresponding difluoroformals and fluoroorthoester with... 

The kinetics of the reaction of various silyl hydrides with tris(2,6-dimethoxyphenyl)methyl cation and some other stable carbocations in acetic acid have been systematically investigated by Carey and Wang-Hsu (92). The results were interpreted in terms of a four-center transition state involving a trigonal bipyramid at silicon with nucleophilic participation of the solvent. 

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Fig. 25 The Y-T plot of gas phase stabilities of substituted a-methyl-a-tri-fluoromethylbenzyl cations [SC ] r = 1.39. For interpretation of symbols, see Fig. 1. Data taken from Mishima et al. (1996b).

According to this scheme, the reaction between t-BuQ and MejAl generates t-butyl cation (tBu ) [Eq. (8)], which t-butylates 2,4,4-tri-methyl-l-pentene (Cg ) [Eq. (9)]... 

A novel acyclic SOjH-functional Brdnsted acidic halogen-free TSIL that bears a butane sulfonic acid group in an acyclic tri-methyl-ammonium cation has been synthesized (Fig. 12.65) [40] and used as the catalyst for one-pot three-component Mannich reaction (Fig. 12.66). The procedure was made up of two-step atom economic reaction. The zwitterionic-type precursor (trimethylammonium butane sulfonate) was prepared through a one-step direct sulfonation reaction of trimeth-ylamine and 1,4-butanesulfone. The zwitterion acidification was accomplished by... 

The 1,2,3-tri-f-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 ion. ° Quantitative estimation of the stability of the unsubstituted cyclopropenium ion can be made in terms of its pXjj+ value of —7.4, which is intermediate between such highly stabilized ions as triphenylmethyl cation and the M-(4-methoxyphenyl)methyl cation. (See Section 4.4.1 for the definition of pXr+). An HF/6-31G MO calculation on the following isodesmic reaction ... 

An extensive analysis of couplings performed by Ito et al. for stable tris-, bis- and mono[9-(azuleno[l,2-h]thienyl)]methyl cations provided a valuable insight into the bonding situation of the seven-membered rings in these compounds. Vicinal proton-proton couplings have been used by Borodkin et al " to study a series of nitrosonium cations. 

From the experimental results obtained three liquid-liquid systems have been selected to be included here aniline/water, ethylacetate/water and ethylacetoacetate/water. The selected surfactants are SDS (sodium dodecyl sulphate), which is anionic, DTAB (dodecyl tri-methyl ammonium bromide), which is cationic, and Atlas G1300 (polyoxyethylene triglyceride ester) which is non-ionic. 

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