Methyl cation formation

Successive introduction of two methyl groups at ring carbon increases the hydrolysis rate by a factor of 10 in each step, indicating cation formation in the transition state as in acetal hydrolysis. Equilibrium protonation before hydrolysis becomes evident from an increasing rate of hydrolysis with a decreasing pH value (Table 3). Below pH 3 no further increase of rate is observed, so that protonation is assumed to be complete. 

The hydrochloride of (3) holds water rather tenaciously, and the infrared spectrum indicates that the water is covalently bound. Mild oxidation of the cation (3) gives 4-hydroxyquinazoline in high yield and ring-chain tautomerism is excluded on the grounds that quinazo-line does not give a positive aldehyde test in acid solution, 2-Methyl-quinazoline also has an anomalous cationic spectrum and a high basic strength (see Table I), but 2,4-dimethylquinazoline is normal in both these respects, which supports the view that abnormal cation formation entails attack on an unsubstituted 4-position. ... 

The photolysis of arenediazonium salts has been widely used for intramolecular cyclizations in the synthesis of 1-phenylethylisoquinoline alkaloids by Kametani and Fukumoto (review 1972). An example is the photolysis of the diazonium ion 10.73, which resulted in the formation of O-benzylandrocymbine (10.74) (Kametani et al., 1971). The mechanism of this cyclization is obviously quite complex, since the carbon (as cation or radical ) to which the diazonio group is attached in 10.73 does not react with the aromatic CH group, but with the tertiary carbon (dot in 10.73), forming a quinone-like ring (10.74). In our opinion the methyl cation released is likely to react with the counter-ion X- or the solvent. 

The formation of a further single bond between sulfur and carbon, as in the trimethylsulfonium cation, may be pictured as involving a 3sp3 unshared pair orbital on sulfur and an empty 2sp3 orbital on carbon in a methyl cation. Thus the three a bonds and the remaining unshared pair (in a 3sp3 orbital) in a trialkylsulfonium ion are distributed approximately tetrahedrally, i.e. the ion is pyramidal, with the sulfur atom at the apex (2). 

This rearrangement, which accounts for the scrambling, is completely stereospecific.The rearrangements probably take place through a nonplanar cyclobutyl cation intermediate or transition state. The formation of cyclobutyl and homoallylic products from a cyclopropyl-methyl cation is also completely stereospecific. These products may arise by direct attack of the nucleophile on 58 or on the cyclobutyl cation intermediate. A planar cyclobutyl cation is ruled out in both cases because it would be symmetrical and the stereospecificity would be lost. 

Wender and coworkers conclude that cobalt-catalyzed benzyl alcohol homologation involves the intermediate formation of car-bonium ions (8). However, since the methyl cation (CH3+) is unstable and difficult to form (9), it is more likely that methanol homologation to ethanol proceeds via nucleophilic attack on a protonated methyl alcohol molecule. Protonated dimethyl ether and methyl acetate forms have been invoked also by Braca (10), along with the subsequent formation of methyl-ruthenium moieties, to describe ruthenium catalyzed homologation to ethyl acetate. 

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.

These bimolecular reactions have provided accurate proton affinities (PAs) for many amines165,166. In addition, cation affinities are accessible, usually by combining the enthalpy of formation (AH[) of cationic species derived from PA measurements with similar data for the bare cation. Thus, the knowledge that the PA of CH3NH2 is 896166 kJmol-1 sets A//f(CH3NH3 + ) = 611 kJmor1. Since A//f(CH3+) = 1092 kJmol-1 and A//f(NH3) = —46 kJmol-1 9, the methyl cation affinity of NH3 may be deduced to be 1092 — 46 — 611 = 435 kJ mol-1. 

Even methane, the least reactive alkane, was shown to undergo carboxylation under superacidic conditions.115,176 The formation of carboxylated products (acetic acid and methyl acetate) from methane was first observed by Hogeveen and coworkers by trapping methyl cation formed in SbF5 (60°C, 50 atm CO pressure) followed by quenching with H20 or MeOH. The intermediate methylcarboxonium ion (CH3CO+) and CH3CH2CO+ formed in a similar reaction of ethane were identified by NMR spectroscopy.176,177... 

Similarly, the ionisation of isobutane will more often lead to the formation of an isopropyl cation by elimination of a methyl radical because the isopropyl cation is thermodynamically more stable than the methyl cation. 

It is not necessary to assume a complete cleavage of methonium ion [CH5]+38 to a free, energetically unfavorable methyl cation. The carbon-carbon bond formation can indeed be visualized as the C—H bond of methane reacting with the developing methyl cation [Eq. (5.68)]. 

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