Carbocations from diazonium ions

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). 

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... 

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 first step in the decomposition of nitrosoamides 123) is formation of the diazo ester 125) which fragments to a diazonium ion pair (128)129 The ion pairs thus produced differ from those obtained in the reaction of diazoalkanes with acids. The ratio of ester to ether formed in the decomposition of rV-nitroso-fV-benzhydrylbenz-amides in alcohol is lower than that found in the reaction of diphenyldiazomethane 132) with acids, and in the solvolysis of benzhydryl benzoate (I35)135,136 This effect has been attributed to the intervention of trans-diazo ester in the decomposition of 125) which leads to a greater distance between carbocation and carbox-ylate anion. In the diazoalkane reaction attack of the acid occurs at the electron-rich carbon atom to generate the carboxylate in the immediate vicinity of the incipient carbocation. 

In acetic acid, however, nitrous acid deamination resembles in many respects the acidolysis of alkanediazotates and the thermolysis of nitrosoamides. Alcohols are formed in low yield but with high retention of configuration129,130,157,1S8. The fraction of alcohol increases with increasing stability of the carbocation, suggesting that exchange of acetate for hydroxide occurs at the stage of the diazonium ion, (126) - (155) - (156). The stereochemistry of the acetates ranges from 68% net in-... 

Since nitrogen-free products result from the formation and decomposition of diazonium ions, these reactions are often referred to as deamination reactions. Alkyl diazonium ions are rarely used in synthetic work but have been studied extensively to probe the behavior of carbocations generated under conditions in which the leaving group is lost rapidly and irreversibly. 

It is clear from the data in Table 4.2 that the two pairs of stereoisomeric cyclic amines do not form the same intermediate. The collapse of the ions to product is evidently so fast that there is not time for relaxation of the initially formed intermediates to reach a common structure. Generally speaking, we can expect similar behavior for all alkyl diazonium ion decompositions. The low activation energy for dissociation and the neutral and hard character of the leaving group result in a carbocation that is free of direct interaction with the leaving group. Product composition and stereochemistry is determined by the details of the collapse of the solvent shell. 

In the dediazoniation of an aliphatic diazonium ion (2.1), a primary product is a carbocation (2.2 in 2-1). The latter will either react with a nucleophile (solvent, anion from the mineral acid used for diazotization, etc.) or rearrange (see Sects. 7.3 and 7.4). 

As already indicated in the preceeding section, nitrosation of aliphatic amines yields alkanediazonium ions with a considerable life-time only in superacids and at very low temperature. Under more usual conditions, the diazonium ion either loses N2 and the carbocation formed yields solvolysis products (in water the corresponding alcohol) and various rearrangement products or, alternatively, a proton is eliminated from the C(a)-atom to give a diazoalkane (2.3 in Scheme 2-1). 

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. ... 

Kirmse et al. (1991) came more closely to a differentiation between classical and nonclassical carbocation intermediates in deamination by the photolytic formation and nucleophilic substitution of diazonium ions from the 4-toluenesulfonyl hydrazones of 6,6-dimethyl- and 5,5,6,6-tetraalkylnorbornan-2-one 7.117, 7.118 and 7.119 (Scheme 7-36). By running the reactions in D20/Na0D, the diazonium ions were labeled with deuterium in the 2-position. For all three reagents, the alkylated exo-norbornanols (from 7.117 7.120 and 7.121) showed a distribution of deuterium in positions 1 and 2 that was very close to 1 1 (49.5 50.5 to 47.6 52.4). If these deviations from an equal distribution were caused by two classical ion intermediates. 

In 1983, Kirmse and Siegfried published a paper with the title 2-Norbornane-diazonium Ions Revisited . These authors considered it essential to reinvestigate the deamination of the endo- and exo-norbornylamine, not least within the context of the well-known discussion on nonclassical carbocations. We adopt the word revisit from this paper for this section and combine it with challenge , i. e., an expression indicating encouragement to reinvestigate deamination mechanisms in the future, because today (1995) we still have the impression that there are too many ambiguities in mechanistic interpretations of experimental results in deamination studies. 

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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