Carbocations from diazoniums

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). 

Carbocations stemming from diazonium cations, such as 57, often give spurious product ratios (see Ref. 84)... 

Formation of carbocations from aliphatic diazonium ions... 539... 

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-... 

Because a neutral molecule is eliminate4 rather than an anion, there is no electrostatic attraction (ion pairing) between the products of the dissociation step. As a result, the carbocations generated by diazonium-ion decomposition frequently exhibit somewhat different behavior from those generated from halides or sulfonates under solvolytic conditions. ... 

FIGURE 22.5 The diazonium ion generated by treatment of a primary alkylamine with nitrous acid loses nitrogen to give a carbocation. The isolated products are derived from the carbocation and include, in this example, alkenes (by loss of a proton) and an alcohol (nucleophilic capture by water). 

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... 

Carbocations may be obtained from the decomposition of other cations, e.g. diazonium cations from the action of NaN02/HCl on RNH2 (cf. p. 119),... 

With simple aliphatic amines, the initial diazonium cations (56) will break down extremely readily to yield carbocations (cf. p. 107) which are, for reasons that are not wholly clear, markedly more reactive than those obtained from other fission processes, e.g. RBr- R Bre. Where the prime purpose is the formation of carbocations, the nitrosation is better carried out on a derivative of the amine (to avoid formation of H20) under anhydrous conditions ... 

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... 

With propanamine, loss of nitrogen from the diazonium ion gives the very poorly stabilized propyl cation, which then undergoes a variety of reactions that are consistent with the carbocation reactions discussed previously (see Sections 8-9B and 15-5E) ... 

This diazo ester is formed because loss of N2 from the diazonium ion results in formation of a quite unfavorable carbocation. 

Like electrophilic addition to diazo compounds [7] from which diazonium ions and, subsequently, carbocations are generated, transition-metal compounds that can act as Lewis acids are potentially effective catalysts for metal carbene transformations. These compounds possess an open coordination site that allows the formation of a diazo carbon-metal bond with a diazo compound and, after loss of dinitrogen, affords a metal carbene (Scheme 5.2). 

Deamination of 1,1-dimethylpropylamine gives products that result from 1,1-dimethylpropyl cation. Because 2,2-dimethylpropylamine gives the same products, it is likely that 1,1-dimethylpropyl cation is formed from 2,2-dimethylpropylamine by way of its diazonium ion. A carbocation rearrangement is indicated. 

Loss of nitrogen from this diazonium ion is accompanied by a hydride shift to form a secondary carbocation. 

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)... 

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. 

The leaving group does not have to be H2O, but can be any departing species whose loss creates a carbocation, including N2 from aliphatic diazonium... 

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