Cationic intermediates protonation

Acidic hydrolysis of 14 occurs via protonation of the nitrogen followed by attack of water on the resulting cationic intermediate. Proton transfer followed by ring-opening affords cation 15, which is trapped by a second equivalent of water. Another proton transfer followed by loss of the amino group affords protonated carboxylic acid 16, which loses to provide the carboxylic acid product. 

They suggested that dimethyl ether (or methanol) gave CH2 as the primary step, with C2H4 being formed via concerted methylene insertion reactions. However, the necessity for strong acidity in the catalyst implies cationic intermediates. Protonated methylene, the methyl carbonium ion CHa", is improb-able, especially in the presence of water. 

The nitronium ion (a strong electrophile) is attacked by the benzene ring (a weak nucleophile) in Step 1 to give a resonance-stabilized cation intermediate. Proton transfer from this intermediate to either H O or HSO4 in Step 2 regenerates the aromatic ring and gives nitrobenzene. 

Figure 12 3 adapts the general mechanism of electrophilic aromatic substitution to the nitration of benzene The first step is rate determining m it benzene reacts with nitro mum ion to give the cyclohexadienyl cation intermediate In the second step the aro maticity of the ring is restored by loss of a proton from the cyclohexadienyl cation... 

Figure 12 7 illustrates attack on the benzene ring by tert butyl cation (step 1) and subsequent formation of tert butylbenzene by loss of a proton from the cyclohexadienyl cation intermediate (step 2)... 

Step 2 Loss of a proton from the cyclohexadienyl cation intermediate yields tert butylbenzene C(CH3)3... 

The cationic intermediate is simply the protonated form (conjugate acid) of the a halo ketone Deprotonation of the cationic intermediate gives the products... 

In cationic polymerization the active species is the ion which is formed by the addition of a proton from the initiator system to a monomer. For vinyl monomers the type of substituents which promote this type of polymerization are those which are electron supplying, like alkyl, 1,1-dialkyl, aryl, and alkoxy. Isobutylene and a-methyl styrene are examples of monomers which have been polymerized via cationic intermediates. 

In chlorination, loss of a proton can be a competitive reaction of the cationic intermediate. This process leads to formation of products resulting from net substitution with double-bond migration ... 

The electrophile 4 adds to the aromatic ring to give a cationic intermediate 5. Loss of a proton from 5 and concomitant rearomatization completes the substitution step. Subsequent hydrolysis of the iminium species 2 yields the formylated aromatic product 3. Instead of the highly toxic hydrogen cyanide, zinc cyanide can be used. The hydrogen cyanide is then generated in situ upon reaction with the hydrogen chloride. The zinc chloride, which is thereby formed, then acts as Lewis acid catalyst. 

Keto-enol tautomerism of carbon) ] compounds is catalyzed by both acids and bases. Acid catalysis occurs by protonation of the carbonyl oxygen atom to give an intermediate cation that Joses H+ from its a carbon to yield a neutral enol (Figure 22.1). This proton loss from the cation intermediate is similar to what occurs during an El reaction when a carbocation loses H+ to form an alkene (Section 11.10). 

It might be pertinent to consider the basis of the extremely facile isomerization in anhydrous HF or pyridinium poly(hydrogen fluoride). HF is an extremely strong proton donor, but it is also a potent fluorinating agent. It is highly probable that the postulated cationic intermediates in these isomerizations are fluori-nated and serve as reactive intermediates in the same way as the fructofuranosyl... 

An Sn 1 -like reaction, on the other hand, is much more favorable, because the glycosyl cation intermediate is stabilized by charge distribution between C-1 and the ring-oxygen atom. The unfavorable formation of an ion pair on bond cleavage can be avoided by protonation of the glycosidic oxygen atom and thus the requirement for acid catalysis (see Scheme 1). 

The linear appearance of the plot shows that this reaction obeys a first-order rate law. Additional mechanistic studies suggest that alkene formation proceeds in a two-step sequence. In the first step, which is rate-determining, the C — Br bond breaks to generate a bromide anion and an unstable cationic intermediate, hi the second step, the intermediate transfers a proton to a water molecule, forming the alkene and H3 ... 

For amide hydrolysis in acid, proton transfer to give a cationic intermediate is easy, and breakdown to products is favored over reversion to starting material process b is hopelessly bad, but process b is better than a. 

Oae found that for both base- and acid-catalyzed hydrolysis of phenyl benzenesul-fonate, there was no incorporation of 0 from solvent into the sulfonate ester after partial hydrolysis. This was interpreted as ruling out a stepwise mechanism, but in fact it could be stepwise with slow pseudorotation. In fact this nonexchange can be explained by Westheimer s rules for pseudorotation, assuming the same rules apply to pentacoordinate sulfur. For the acid-catalyzed reaction, the likely intermediate would be 8 for which pseudorotation would be disfavored because it would put a carbon at an apical position. Further protonation to the cationic intermediate is unlikely even in lOM HCl (the medium for Oae s experiments) because of the high acidity of this species a Branch and Calvin calculation (See Appendix), supplemented by allowance for the effect of the phenyl groups (taken as the difference in between sulfuric acid and benzenesulfonic acid ), leads to a pA, of -7 for the first pisTa of this cation about -2 for the second p/sTa. and about 3 for the third Thus, protonation by aqueous HCl to give the neutral intermediate is likely but further protonation to give cation 9 would be very unlikely. 

Chlorination can be accompanied by other reactions that are indicative of carbocation intermediates. Branched alkenes can give products that are the result of elimination of a proton from a cationic intermediate.35... 

We have, however, already seen an alternative acid hydrolysis pathway (Aac1, p. 242) in which a water molecule is not involved in the slow, rate-limiting step. In addition, this step is one in which considerable +ve charge is developed at the reaction centre as the protonated ester (35a) is converted into the acyl cation intermediate (36a) a necessary requirement for a reaction with a large -ve (-3-25) p value ... 

The homopolymerization ofl consists of a room-temperature reaction of the monomer dissolved in nitrobenzene in the presence of anhydrous ferric chloride. Polymerizations were carried out under a stream of dry nitrogen. As depicted in Scheme 2, the homopolymerization of 1 to form 6FNE takes place by means of the Scholl reaction. The mechanism of the Scholl reaction was assumed to proceed through a radical-cation intermediate derived from the single-electron oxidation of the monomer and its subsequent electrophilic addition to the nucleophilic monomer. The reaction releases two hydrogens, both as protons, to form the... 

The product ratio of 8K/9K is similar to that of the Lewis acid-mediated reaction of 4a-c. These products of the C2-C3 bond cleavage (8K and 9K) may be formed via alkylideneallyl cation intermediate, which is formed by the oxygen protonation of 4. Thus, the product ratio of 10/(8K + 9K) is controlled by the protonations at the olefinic carbon and at the acetal oxygen of 4. 

In an effort to explore the factors that govern anodic C-C bond formation, Swenton and coworkers have also been exploring the intramolecular coupling of phenols and olefins (Scheme 28) [44]. In these reactions, initial oxidation of the phenol followed by loss of a proton and a second oxidation led to the formation of a cationic intermediate (26). This intermediate was trapped by the olefin to form a second cation that was in turn trapped by methanol to form the final product 28. When R2 was equal to methyl (25b) or phenyl (25c) the reaction led to a good yield of the cyclized product. Reactions where the R2 was equal to a hydrogen (25a and 25d) were not so successful. The cyclizations were compatible with the incorporation of the olefin into a third ring (25e). 

Two possible mechanisms are proposed. Primarily the enol radical cation is formed. It either undergoes deprotonation because of its intrinsic acidity, producing an a-carbonyl radical, which is oxidized in a further one-electron oxidation step to an a-carbonyl cation. Cyclization leads to an intermediate cyclo-hexadienyl cation. On the other hand, cyclization of the enol radical cation can be faster than deprotonation, producing a distonic radical cation, which, after proton loss and second one-electron oxidation, leads to the same cyclo-hexadienyl cation intermediate as in the first reaction pathway. After a 1,2-methyl shift and further deprotonation, the benzofuran is obtained. Since the oxidation potentials of the enols are about 0.3-0.5 V higher than those of the corresponding a-carbonyl radicals, the author prefers the first reaction pathway via a-carbonyl cations [112]. Under the same reaction conditions, the oxidation of 2-mesityl-2-phenylethenol 74 does not lead to benzofuran but to oxazole 75 in yields of up to 85 %. The oxazole 75 is generated by nucleophilic attack of acetonitrile on the a-carbonyl cation or the proceeding enol radical cation. 

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