Alkenediazonium ions

The diazonium ions 2.13 with electron-withdrawing substituents are not hetero-aromatic compounds and therefore do not strictly come within the scope of this book. They are formally related to the alkenediazonium ions. Nevertheless, they are discussed here because in their properties they bear a close resemblance to heteroaromatic and arenediazonium ions rather than to alkenediazonium ions. In par-... 

The few known cases of azo coupling reactions of alkane- and alkenediazonium ions will be reviewed in the forthcoming second book (Zollinger, 1995, Sec. 6.1). We will not discuss systematically the chemical, spectroscopic, or other properties of the azo compounds formed in azo coupling reactions (see Zollinger, 1991, Ch. 7). However, two phenomena are important for this book, as discussed below. 

Alkenediazonium ions 16, 161, 306 Alkoxy-de-diazoniation 166 ff., 198 f., 212, 227, 278, see also Ethoxy- and Methoxy-de-diazoniation... 

Oxazolidinones react with nitrous acid, the resulting jV-nitroso compounds being decomposed by alkali to yield a variety of products alkynes, aldehydes and vinyl ethers. These transformations are rationalized by postulating the intermediacy of transient alkenediazonium ions (Scheme 27) (79CB2120). 

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... 

In conclusion it can be said that alkane- and alkenediazonium ions react with nucleophiles by a variety of pathways, one of them, in certain cases, being the azo coupling reaction. Small changes in the substrate structure, as well as in the reaction conditions, can drastically change the reaction pathway, indicating that the energy requirements for the competing reactions are rather similar. 

This result is rather surprising as H/D exchange of diazo ketones in acidic D2O indicates reversible C-protonation. The explanation may be that the 2-hydroxy-alkenediazonium ions 2.11 and 2.12 are thermodynamically much more stable than the alkanediazonium ion 2.13 (see also Sect. 2.10). 

Further and important support for the hypothesis of alkenediazonium ions in the decomposition of nitroso oxazolidinones is provided by investigations by Kirmse et al. (1979) with the N-labeled nitroso compounds 5,5-dimethyl-3-nitroso-[3- N]-l,3-oxazolidin-2-one (2.259, R = H R" = R " = CH3) and 5,5-pentamethyl-ene-3-nitroso-[3-i N]-l,3-oxazolidin-2-one (2.259, R = H, R"-R "= -[CH2]5-). The authors determined the N content of products of decompositions conducted in the presence of lithium azide. The results are consistent with A-coupling of the intermediate with azide ions (for coupling of arenediazonium ions with azide ions see Zollinger, 1994, Sect. 6.4). 

The work of Newman s and Curtin s groups is, however, worth remembering for its historical significance One might conclude that alkenediazonium ions have an unexpectedly and surprisingly low stability. This is not generally true, however, because the first alkenediazonium salts were isolated in the mid-1960 s, and they had decomposition points above 100 °C. 

Another stable salt is the protonated product 2.273 of phenyl(pyridin-4-yl)diazo-methane (2.272), which was discovered by Reimlinger (1963, 1964). The relatively low NN stretching frequency (2060 cm ) indicates, however, that this compound may not be classified as an alkenediazonium ion, but rather as a diazoalkane, protonated at a relatively remote heterocyclic N-atom (2-104). 

Seven alkenediazonium hexachloroantimonates and three alkenediazonium tetra-chloro(4-toluene-sulfinato)stannates were synthesized by Bott (1975). All except the 2-chlorobut-l-enediazonium hexachloroantimonate can be isolated, and all decompose in the solid state, depending on substituents, in the range 50-132°C. It seems that the stability, as reflected in the decomposition temperature, is related to the stability of the vinyl cations formed. A systematic investigation of these problems, which could define the potential of alkenediazonium ions for synthetic applications, would be desirable. At present, there are very few reactions known of alkenediazonium ions that have such a potential (see Sect. 9.5). 

Theoretical papers on alkenediazonium ions will be discussed in the context of the structure of diazonium ions in Section 5.3. 

The dominant property of the alkynediazonium ion 2.291 is the addition of nucleophiles to the triple bond. Addition of water or methanol to the CH2CI2 — SbCls solution leads to alkenediazonium ions 2.292 with OH, CH3O or Cl in the 2-position (2-112). The C—N bond in the alkynediazonium is more stable than the corresponding C — N bond of the alkenediazonium ion. With water, the final product of addition, dediazoniation of the alkenediazonium ion, and addition of HCl is 2-chloro-l-phenylethanone (2.294). Without HCl, 2-hydroxy-l-phenylethanone (2.293) is formed (2-113). 

The higher stability of the alkynediazonium ion towards dediazoniation, relative to that of the alkenediazonium ion, is consistent with structure calculations obtained by Glaser (1987, 1989 see Sect. 5.3). It is unlikely, therfore, that alkyne cations can be obtained by dediazoniation of alkynediazonium ions. An alkynyl cation was formed, however, by spontaneous nuclear decay in l,4-bis(tritioethynyl)benzene, as found by Angelini et al. (1988). 

This discussion of the potential existence of enols of a-diazocarbonyl compounds brings us to the structure of alkenediazonium ions. The mesomeric structures 5.13 b and 5.13 c of an a-diazocarbonyl compound have a similarity with alkenediazonium ions (5.29a-5.29b). Scheme 5-6 shows one of Bott s synthetic routes to alkene diazonium salts (see Sect. 2.10). 

The investigations of Kirmse s group (1973) also include the reaction of azide ions with some alkenediazonium ions. Formation of alkenylidenecarbenes (RR C = C ) is known from other reactions with alkenediazonium ions (e.g., Patrick et al., 1972). Product ratios and the results of N(a)- and N()ff)-labeling indicate that alkenylidenecarbenes are formed in the reaction of azide ions with 2,2-dialkyl- and 2-cycloalkylethene-l-diazonium ions, but not with 3-methylbut-2-enediazonium ions (as expected). 

We will discuss the dediazoniation of alkenediazonium ions separated from that of alkanediazonium ions (Chapt. 7) because alkenediazonium ions behave quite differently with respect to dediazoniation. As the diazonio group is bonded to an sp -C-atom, this difference is not surprising. The reactivity of alkenediazonium ions is, however, also significantly different from that of arenediazonium ions in... 

The influence of substituents on the weight of the two pathways in Scheme 9-46 certainly demonstrates the difficulty of rationalizing the reactivity of alkenediazonium ions. 

When these solutions were slowly heated to room temperature, dediazonia-tion took place and the corresponding 1,3-dioxolium salts 9.125 could be isolated. The lower stability of )ff-(acyloxy)alkenediazonium ions relative to that of jff-alkoxy-ethenediazonium ions is understandable on the basis of the lower electron donating power of the substituents. 

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