Stable Aliphatic Diazonium Ions

All diazonium ions, whether aromatic or aliphatic, contain two nitrogen atoms in a manner similar to the two nitrogens in dinitrogen molecules. There is a triple bond between the two atoms, as can be concluded from NMR spectra and X-ray structure analysis (see Zollinger, 1994, Sect. 4.2). As dinitrogen is a very stable molecule, it is hardly surprising that the C —N bond is rather weak, and dediazoniation is a common reaction of diazonium ions. In solution at room temperature, an aromatic diazonium ion loses N2 in a first-order reaction with a half-life of some hours. Most salts of aromatic diazonium ions can be kept in the solid state almost indefinitely, if not heated or subjected to mechanical shock (see Zollinger, 1994, Sect. 2.3). 

Aliphatic diazonium ions, on the other hand, are extremely unstable. Although they were obtained as very unstable intermediates in 1848 when Piria treated aspartic acid (2-aminosuccinic acid) with nitrosating reagents, leading to malic acid (2-hydroxysuccinic acid), they were not identified directly for almost 120 years. Piria s reaction allowed, in some cases, the substitution of a primary amino group by a hydroxy group. This reaction is also characterized by eliminations and rearrangements (see Chapt. 7). Aliphatic diazonium ions were postulated and subsequently identified as intermediates in the acid-catalyzed decomposition of aliphatic diazo compounds (see Sect. 7.2). 

The reason for the quite different stability of aliphatic and aromatic diazonium ions is the strong n — n interaction of C(l) and N(a) in the aromatic series, which is, of course, not present in alkane diazonium ions (see Zollinger, 1994, Sects. 8.3 and 8.4). 

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). 

Diazo Chemistry II Aliphatic, Inorganic and Organometallic Compounds. By Heinrich Zollinger Copyright 1995 VCH Veriagsgesellschail mbH ISBN 3-527-29222-5 

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Azo coupling reactions with stable aliphatic diazonium ions (see Sect. 2.1) have not been investigated, as far as we are aware. 

In some of these papers (e.g., Weiss et al., 1984) it is mentioned that various attempts to synthesize stable aliphatic diazonium salts failed, although the identified reaction products indicated the involvement of aliphatic diazonium ions as intermediates (see also Olah et al., 1966). 

When primary aromatic amines are treated with nitrous acid, diazonium salts are formed. The reaction also occurs with aliphatic primary amines, but aliphatic diazonium ions are extremely unstable, even in solution (see p. 448). Aromatic diazonium ions are more stable, because of the resonance interaction between the nitrogens and the ring ... 

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... 

The first direct observation453 of an aliphatic diazonium ion was achieved by protonation of trifluoromethyldiazomethane in HSO3F at — 60°C. The 2,2,2-trifluoroethyldiazonium ion 206 is stable for 1 h at — 60°C. Similarly bis(trifluor-omethyl)methane diazonium ion 207 has been prepared and characterized.454 These ions were studied by H NMR spectroscopy. Similar diazonium structures have been assigned to protonated 2-diazo-5a-cholestan-3-one.455 None of these studies, however, showed nitrogen protonation. 

There exists the possibility of an electron transfer from the tt system, leading to structure D, which would have two unpaired electrons. A molecular orbital calculation at the 4-3IG level has indicated that the singlet structure C is more stable than the triplet structure by about 20 kcal/mol, but it is highly distorted with the C-C-C bond angle at the divalent carbon calculated to be 145°." " Phenyl cations are formed by the thermal decomposition of aryldiazonium ions." The relatively high stability of arylidiazonium ions, even in comparison with primary aliphatic diazonium ions, reflects the instability of the phenyl cation. The cation is also extremely reactive. 

Problem 18.23 (a) Outline the steps in the formation of a diazonium cation from nitrous acid and a 1° amine. (b) Why are diazonium ions of aromatic amines more stable than those of aliphatic amines M... 

In the reactions of diazoalkanes considered so far the operation of acid catalysis has not-hgen questioned. One reason has been that the compounds considered are in the main sufficiently stable to require a relatively strong acid for reaction, and little difficulty has arisen in distinguishing the acid-catalysed reaction from competing thermal reactions. For more reactive substrates, the possibility of diazonium-ion formation by proton transfer from an acid as weak as a molecule of a normal hydroxylic solvent has to be taken into account, and separation of acid and thermal reactions is no longer straightforward. In fact many thermal reactions of primary and secondary aliphatic diazoalkanes are known which yield different sets of products in hydroxylic and aprotic solvents and yield mixtures of these products in solvents of intermediate acidity, such as acetamide. It is useful to consider these reactions in the light of experience of other reactions in which the presence of diazonium ions is well authenticated. 

Aryldiazonium ions are much more stable than their aliphatic counterparts, and are therefore much less likely to be involved in rearrangements. They are readily prepared by nitrosation of phenylamines. No evidence of scrambling of the nitrogen atoms in solution has been detected, so that rearrangements of the diazonium ion 192 to a cyclic structure 193 can be ruled out. 

Primary aliphatic amines (RNH2) react with nitrous acid in the same way that primary arylamines (ArNH2) do, to form diazonium ions. But alkyldiazonium ions RN2 are much less stable than aryldiazonium ions ArN2 and readily lose nitrogen even at 0°C. Explain the difference. 

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