Aliphatic Diazonium Ions

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

No counter ion effect is discernible for samples with other anions. The proton-coupled NMR spectrum consists of a quartet (/= 163 Hz), as expected for CH3 —Ni. The multiplicity of the NMR spectra of methene- and ethenediazenium ions (see Table 5-1) show the expected coupling for slow exchange. The triplet of doublets in the NMR spectrum of methenediazenium ion is consistent with the assigned structure. The NMR chemical shift of this ion is comparable to that of the terminal C-atoms in allene. The chemical shift and the splitting of the 2,2,2-trifluoroethanediazonium ion are consistent with its structure the shift change relative to the methanediazonium ion is considerable but fairly well understandable. 

Our knowledge on the electronic structure of alkenediazonium salts was modest for a long time. It was assumed, however, that mesomeric structures involving substituents at the C-atom had to be considered due to the fact that practically all isolated alkenediazonium salts (see Sect. 2.10) have electron-donating substituents (see Sect. 5.3). 

By far the most important azo coupling reaction, i.e. the reaction of arenediazonium ions with aromatic coupling components, has been known for many decades and has been extensively reviewed Therefore, in this section we shall concentrate on reactions of aliphatic diazonium ions, diazoalkanes and diazocarbonyl compounds which have not been known previously as well as on coupling reactions which have gained in importance more recently, specifically those of heterocyclic diazonium ions. 

Kirmse and coworkers have studied the reaction of alkanediazonium ions with amines and with lithium azide Cyclopropanediazonium ions give azo coupling products 6 and 7 with dimethylamine and with ethylamine, respectively (1) K However, no azo coupling of 1 with phenols was observed. In the reaction 

Curtin and coworkers have observed the azo coupling of the bridgehead diazonium ion 6 with 2-naphthol at low temperature (2) in 50 % yield. 

Other alkanediazonium ions, including the bridgehead bicyclo[2.2.1]heptane-l-diazonium ion 9, do not undergo azo coupling reactions but give only nucleophilic substitution productsIn addition to the examples given above, some alkene-diazonium ions generated from nitrosooxazolidones can also add azide ions  

It is interesting to compare the behavior of short-lived diazonium ion intermediates with that of the relatively stable and isolable alkenediazonium ions first prepared 

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Aliphatic primary amines also undergo the diazotization reaction in weakly acidic solution however the resulting aliphatic diazonium ions are generally unstable, and easily decompose into nitrogen and highly reactive carbenium ions. The arenediazonium ions are stabilized by resonance with the aromatic ring ... 

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). 

I hope therefore that, in the not too distant future, an expert on the modeling of organic reactions in solution will investigate the dediazoniation of aromatic or aliphatic diazonium ions in water. 

Aromatic diazonium salts can, of course, be isolated (see Chapter 13), but only a few aliphatic diazonium salts have been prepared (see also Ref. 383). For reviews see Laali, K. Olah, G.A. Rev. Chem. Intermed., 1985, 6, 237 Bott, K. in Patai Rappoport The Chemistry of Functional Groups, Supplement C, pt. 1 Wiley NY, 1983, p. 671 Bott, K. Angew. Chem. Int. Ed. Engl., 1979, 18, 259. The simplest aliphatic diazonium ion CH3N2 has been prepared at — 120°C in superacid solution, where it lived long enough for an nmr spectrum to be taken Berner, D. McGarrity, J.F. J. Am. Chem. Soc., 1979, 101, 3135. 

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... 

Diazonium salts can be regarded as combinations of carbocations R with N2 and, because of the considerable stability of nitrogen in the form of N2, we would expect diazonium salts to decompose readily with evolution of nitrogen and formation of carbocations. This expectation is realized, and diazonium salts normally decompose in this manner in water solution. The aliphatic diazonium ions decompose so rapidly that their presence can only be inferred... 

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. 

Whereas cleavage of trisubstituted triazenes gives rise to secondary amines in excellent yields, cleavage of disubstituted triazene 77 gives rise to aliphatic diazonium salts. The aliphatic diazonium ion formed undergoes substitution with nu-... 

Finally, we note that deamination could in principle be achieved by generating an aliphatic diazonium ion in the presence of a hydride source this could lead to an alkane either via the diazene, or by Sn2 attack with loss of N . Protonating an aliphatic diazo compound, or treating an /V -alkyl, N -aryltriazene with protic or Lewis acid in the presence of a suitable reducing agent (NaBHsCN, NaBH(OAc)3 or RsSiH) might achieve this transformation. 

The deamination of aliphatic amines The deamination of primary amines is the best known reaction involving aliphatic diazonium ions. The ions are probably produced within a hydroxide ion pair formed by dissociation of an intermediate diazohydroxide. Alcohols are characteristic products of the reaction, even in non-aqueous solvents for which stringent precautions are taken to remove any free water formed (Bailey and Burr, 1953). The partial racemization of the product from optically active amines rules out the operation of a concerted S i mechanism. 

The usual reaction of aliphatic diazonium ions is a decomposition into nitrogen and a carbonium ion (see p. 40) ... 

Now it becomes attractive to bypass (291) in the ionization of (288) by participation of the migrating group R (Fig. 7b). Products are derived from (290) only. Anchimeric assistance depends strongly, however, on the leaving group X. With readily ionizing substrates, such as aliphatic diazonium ions, (291) may still intervene. 

Aliphatic diazonium ions are very unstable, rapidly decomposing to carbocations. 

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). 

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). 

All inorganic diazonium ions (X-Ni ) discussed in Section 3.1 are characterized structurally, first, by a small group X consisting of one to five atoms, and, second, by little or no stabilization of the X —N(a) bond. These inorganic diazonium ions are, therefore, comparable to simple aliphatic diazonium ions, e.g., H3C Ni. ... 

We will discuss work on diazoalkanes, diazoalkenes, and related compounds first and investigations on aliphatic diazonium ions later in this section. 

Azo Coupling Reactions of Aliphatic Diazonium Ions and Related Processes... 

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

In an earlier review (Zollinger, 1994, Chapt. 8) we explained that the dediazoniation of aromatic diazonium ions to aryl cations and dinitrogen, or to aryl radicals and dinitrogen in the presence of suitable electron donors, is based primarily on the high stability of N2. Analogous aliphatic diazonium ions show similar behavior, but the existence of diazenides (R —Nf) and diazoalkanes also opens the possibility of generating carbanions, e.g., in the Wolff-Kishner reduction and related reactions (see the classic book of Cram, 1965, and the monographs of Staley, 1985, and Buncel and Durst, 1980-1987), and carbenes (see Chapt. 8 of this book). 

As mentioned in Section 1.1, the first diazotization of amines, followed by dediazoniation, was carried out by Piria in 1848, well before Griess discovered and isolated aromatic diazo compounds (1858). Piria added an impure HNO3 —HCl solution to a mixture of asparagine and aspartic acid in water and obtained malic acid (7-1). It was not possible for Piria, however, to realize that the primary reaction products were diazonium ions. Yet, Piria s process was one of the few types of reaction via aliphatic diazonium ions that became important for synthetic purposes, after Ingold s group (Brewster et al., 1950) discovered that a-amino acids undergo clean retentive deamination (see Sect. 7.7). 

Attention towards an aliphatic diazonium ion as a transient intermediate in nitrosation of alkylamines was subsequently further distracted by investigations of Linnemann and Siersch (1867), who found that in the reaction of propylamine even more propan-2-ol was detected than the propan-l-ol expected. Kinetics of nitrosa-tions of alkylamines were determined as early as 1928 by Taylor, but it was not until Hammett, in his pioneering book on physical organic chemistry (1940, p. 295), that the analogy to the aromatic series was established. He suggested that diazonium ions are also intermediates in the reactions of aliphatic primary amines. 

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