Carbocations, from diazonium salts

A carbocation may lose a proton to form an alkene. For example, 1-propyl carbocation generated from diazonium salt may lose a proton (H+) to form an alkene (propene). Alternatively, 1-propyl carbocation may rearrange to more stable secondary carbocation, which may also lose a proton to give propene (Scheme 2.1). 

Intramolecular insertion has been reported. The positively charged carbon of the carbocation (23), generated from the diazonium salt of the triptycene compound (22), reacted with the CH3 group in close proximity with it. ... 

The first widely used intermediates for nucleophilic aromatic substitution were the aryl diazonium salts. Aryl diazonium ions are usually prepared by reaction of an aniline with nitrous acid, which is generated in situ from a nitrite salt.81 Unlike aliphatic diazonium ions, which decompose very rapidly to molecular nitrogen and a carbocation (see Part A, Section 4.1.5), aryl diazonium ions are stable enough to exist in solution at room temperature and below. They can also be isolated as salts with nonnucleophilic anions, such as tetrafluoroborate or trifluoroacetate.82 Salts prepared with 0-benzenedisulfonimidate also appear to have potential for synthetic application.83... 

When 4-/-butylcyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate (3) is heated at 60 °C in chloroform, 1-fluorocyclohexene 4, 1-chlorocyclohexene 5 and l-(o-iodophenyl)cyclohexene 6 are formed with accompanying iodobenzene leaving group (eq 2).3 These three substitution products are best accounted for by formation of an ion pair involving cyclohexenyl cation 7. The cyclohexenyl cation 7 formed picks up fluoride from tetrafluoroborate and chloride from chloroform solvent, and recombines with the iodobenzene generated (eq 3). This kind of reactions with a counteranion and solvent are characteristic of unstable carbocations and are known in the case of phenyl cation generated from the diazonium salt in the Schiemann-type reaction.4... 

Diazoketones are relatively easy to prepare (see 16-89). When treated with acid, they add a proton to give a-keto diazonium salts, which are hydrolyzed to the alcohols by the S l or Sn2 mechanism.Relatively good yields of a-hydroxy ketones can be prepared in this way, since the diazonium ion is somewhat stabilized by the presence of the carbonyl group, which discourages N2 from leaving because that would result in an unstable a-carbonyl carbocation. 

Covalent enzyme modification can also be achieved by Sn2 substitution reactions. Enzymic protonation of a diazoketone yields a diazonium salt which then serves as a leaving group in a substitution reaction with an enzyme nucleophile [Eq. (2e)[. An indistinguishable mechanistic possibility in similar reactions is the liberation of nitrogen gas from the diazonium salt to produce a carbocation which then reacts with the nucleophile in an Sn 1 process. 

The l-alkyl-3-aryltriazenes (7.15 see Scheme 7-3) are easily obtained from aromatic diazonium salts and alkylamines. They exist in a tautomeric equilibrium (see Zollinger, 1994, Sect. 13.4) and, under acid catalysis, they dissociate into both possible combinations of amine and diazonium ion. The aliphatic amine and aromatic diazonium ion will, however, react further with each other, whereas in the combination alkanediazonium ion -h aromatic amine the diazonium ion will decompose rapidly into the carbocation and dinitrogen. This system has been used little for mechanistic or preparative deamination studies, obviously because a very complex product pattern is inherent in it. The carbocation may react with the aromatic and the aliphatic amine at the amino group. A modified method was described by Southam and Whiting (1982) using anhydrous acetonitrile as medium at —10 to -5°C. ... 

Decomposition of aliphatic diazonium salts often yields a more complicated mixture of products (see Table 8) than the one formed in solvolysis. Streitwieser explains this difference that in the first case the reaction proceeds not only through carbocations but also directly from diazonium ions the latter react un-... 

Fluorodikylammes react with nitrous acid to produce the corresponding unstable fluoroahphatic diazonium ions Placement of the tnfluoromethyl group at a carbon position a, P, or Y to a diazonium ion was used to probe the inductive effect on the chemistry of the transient carbocation resulting from dediazoniation [7] If the fluoroalkyl group is bound to the same carbon as the amino group, conversion to the more stable diazo compound occurs For example, 4-diazo-1,1,1,2,2-pentafluoro-3-pentafluoroethyl-3-tnfluoromethylbutane is obtained from the reaction of the poly-fluoroalkylamine salt with sodium nitnte [S, 9] (equation 8)... 

The reactivity of the ethenediazonium salt 9.100 towards the nucleophiles mentioned shows that it has the properties of the corresponding carbocation, since it can ethylate the nucleophile and is prone to attack at the C()ff)-atom of the original ethene-l-diazonium ion. The thermal decomposition pattern is typical of that for an oxonium salt. Reactions with amines are similar to those of ketene acetals. No product that could be explained in terms of an azo coupling reaction, e.g., with 2-naphthol, could be observed. The electrophilicity of the diazonio group is, therefore, low. N-Azo coupling products with azide ions have been postulated with good arguments, however, by Kirmse and Schnurr (1977) with certain short-lived ethene diazonium intermediates produced from nitroso oxazolidones. 

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