Carbocation from heterocycles

Deprotonation of the a-position of a carbenium center is one of the most typical properties of carbocations. All the alkyl-substituted heterocyclic ions possess an appreciable CH-acidity [82AHC(S)1] the 3-azapyrylium salts are no exceptions to this rule. The formation of anhydro bases, i.e., methylene-1,3-oxazines (e.g., 69), from methyl-3-azapyrylium salts is well known (72S333). 

Cationic mechanisms are much more characteristic of the polymerization of oxygen heterocycles, both ethers and acetals. A wide variety of catalysts has been used, including protonic acids, such Lewis acids as boron trifluoride, phosphorus pentafluoride, stannic chloride, antimony pentachloride, titanium tetrachloride, zinc chloride, and ferric chloride, and salts of carbocations or tri-alkyloxonium ions having anions derived from Lewis acids. Some complex, coordination catalysts that appear to operate by a mechanism... 

The second type is the construction of a heterocyclic system from units containing perfluoroalkyl groups or their fragments. Each type has its own advantages and weaknesses. Thus, reactive species used in processes of the first type are perfluoroalkyl radicals and carbocations, and the processes are conducted by elaborate methods (thermolysis, photolysis, electrolysis, one-electron oxidation, etc.), whereas reactions of the second type use condensation of molecules with suitable groups and nucleophilic reactions of perfluoroolefins. 

Also with heterocyclic monomers, simple cations are attached to the atom with the highest electron density in the valence sphere, i. e. to the heteroatoms. The reactions of more complicated carbocations are evidently of greater complexity. The triphenylmethyl cation separates a hydride ion from tetrahydrofuran, producing triphenylmethane and an oxonium ion... 

The mechanism initially proposed for the Rubottom oxidation involved epoxidation of the enolsilane to afford intermediate silyloxyoxirane 4. It was suggested that this intermediate undergoes acid-mediated cleavage to afford stabilized carbocation 5, which is transformed to the a-silyloxy ketone 6 via 1,4-silicon migration. Hydrolysis of 6 by aqueous acid in a subsequent step generates the a-hydroxy ketone 7.lb 15 Attempts to provide support for this mechanism via isolation of intermediate silyloxyoxiranes derived from simple ketones proved difficult due to the lability of these compounds. However, Brook demonstrated that the related heterocyclic silyloxyoxirane 8 was isolable and was transformed to ketone 9 upon treatment with /j-TsOH. 

Data obtained from the intramolecular rearrangements of heterocyclic compounds support the probability of 1,2-shifts in respective carbocations (2) ... 

The mechanism of the Nicholas reaction is best described as an SnI process. Protonation of the alcohol in 4 followed by loss of water from cation 8 yields cobalt-stabilized carbocation 5. Friedel-Crafts reaction of this electrophile with anisole provides resonance-stabilized carbocation 9 which, upon removal of a proton, furnishes the substitution product 6. In addition to electron rich aromatics like anisole, a variety of neutral carbo- and heterocyclic nucleophiles react successfully with the carbocation... 

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