Carbenium ions, deprotonation

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

In trifluoroacetic acid [0.4 M TBABF4 (tetrabutyl ammonium tetrafluoroborate)] unbranched alkanes are oxidized in fair to good yields to the corresponding triflu-oroacetates (Table 2) [16]. As mechanism, a 2e-oxidation and deprotonation to an intermediate carbenium ion, that undergoes solvolysis is proposed. The isomer distribution points to a fairly unselective CH oxidation at the methylene groups. Branched hydrocarbons are preferentially oxidized at the tertiary CH bond [17]. 

This bimolecular mechanism also applies to cycloalkanes which can be activated by intermolecular hydride transfer to small carbenium ions to form cyclohexyl cations prior to cracking. Alternately, the cyclohexyl cations can deprotonate and form cyclohexene. With two similar intermolecular hydride transfers an aromatic can also form [46]. 

Systems. As already observed for methane (vide supra), with decreasing acidity it becomes more and more difficult to protonate reversibly C—H bonds. Nevertheless, when alkanes with more than two carbon atoms are used as starting material, carbenium ions are generated by competitive protolytic and oxidative processes. Depending on the strength of the superacid system, proton exchange can take place by two competitive reactions (i) directly via reversible protonation and (ii) via deprotonation of the carbenium ion and reprotonation of the alkene. 

S-Ketoesters, /3-ketophospbonates, and /3-ketosulfones have been used to alkylate ferrocene to afford the corresponding /3-ferrocenyl-a,/3-unsaturated derivatives in excess triflic acid262 263 [Eq. (5.98)]. The transformations are highly stereoselective, giving exclusively the (El-isomers this was explained by the exo-deprotonation of carbenium ion 71a of more stable conformation. Acetals of formylphosphonates and formylsulfones react in a similar manner. 

When a carbon atom bearing a positive charge is bound with the methyl group, the reaction path consists of a methyl proton loss. The deprotonation generates a benzylic radical that is rapidly oxidized to a benzylic carbenium ion that reacts with a nucleophile according to Scheme 3-64. In accordance with Scheme 3-64, BP-6-methyl is characterized as an active carcinogen (Cavalieri Rogan 1995). 

The conversion is thought to involve formation of the carboxonium ion (77) by protonation of the carbonyl oxygen, and subsequent protonation then occurs at the C-H bond. The resulting carboxonium-carbonium dication (78) possesses the maximum possible charge-charge separation for this bicyclic framework. Subsequently, an intermediate carboxonium-carbenium dication (79) is produced, which isomerizes to the tertiary -carbenium ion, and deprotonation provides the product enone (80). Similar distonic superelectrophiles are proposed in other rearrangements of terpenes in superacid.28... 

The fert-butyloxycarbonyl (BOC) group is cleaved using TFA in an aprotic solvent like CH2C12. The cleavage proceeds via protonation of the carbonyl group of carbamate 16, subsequent elimination of carbenium ion 58 and liberation of unstable free carbaminic acid 60 which breaks down yielding the unprotected indole 61 with loss of C02. Cation 58 is deprotonated giving 2-methyl propene 59. 

A carbonyl group can be regarded as a very stabilized carbenium ion, and so the same sort of treatment should apply to deprotonation of carbenium ions to give alkenes (or protonation of alkenes to give carbenium ions). This is currently being actively pursued. The goal is to have a uniform mechanistic model for all proton transfers from a carbon next to an atom with a formal positive charge. 

The formation of the ring-opened products by the electrooxidation may be explained by one-electron oxidation to the cation radical followed by C—C bond cleavage forming a tertiary carbenium ion and an allyl radical. Deprotonation, further one-electron oxidation, and attack of the nucleophile yield 18 and 19. 

When the carbenium ion intermediate of an El elimination can be deprotonated to give two regioisomeric alkenes, generally both of them are produced. If these alkene isomers are Saytzeff and Hofmann products, the first one is produced preferentially because of product development control. This is illustrated in Figure 4.34 by the El elimination from ferf-amyl... 

Fig. 4.34. Saytzeff preference of El eliminations from tert-amyl halides and energy profile. The SaytzeffiHofmann preference amounts to 82 18 regardless of which halide ion accompanies the carbenium ion. This observation is explained most simply by the assumption that this halide ion is not involved in the deprotonation step forming the C=C double bond.

The elimination in Figure 4.36 supports the idea that the alkenes initially formed from tertiary alcohols under El conditions can be reprotonated. The Saytzeff and the Hofmann products shown there can be protonated to provide the tertiary carbenium ion through which they were formed and also to a different tertiary carbenium ion. The consequence of this is that in the major product obtained after the final deprotonation, the C=C double bond is no longer located at the C atom that carried the OH group in the starting alcohol, but is moved one center away. 

Cation B is first deprotonated to give the hydroxycamphene derivative C. C reacts with an electrophile of unknown structure that is generated from sulfuric acid under these conditions. In the discussion of the sulfonylation of aromatic compounds (Figure 5.17), we mentioned protonated sulfuric acid H3S04 and its dehydrated derivative HS03 as potential electrophiles, which might assume the same role here. In any case, the reaction results in the formation of carbenium ion E. 

The carbenium ion formed last is deprotonated in the last step of this biosynthesis, thus furnishing the central C=C double bond of the final product lanosterol (G). 

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