Arenediazonium cations

It was noted early by Smid and his coworkers that open-chained polyethylene glycol type compounds bind alkali metals much as the crowns do, but with considerably lower binding constants. This suggested that such materials could be substituted for crown ethers in phase transfer catalytic reactions where a larger amount of the more economical material could effect the transformation just as effectively as more expensive cyclic ethers. Knbchel and coworkers demonstrated the application of open-chained crown ether equivalents in 1975 . Recently, a number of applications have been published in which simple polyethylene glycols are substituted for crowns . These include nucleophilic substitution reactions, as well as solubilization of arenediazonium cations . Glymes have also been bound into polymer backbones for use as catalysts " " . 

More recently, elegant mechanistic studies on intramolecular Meer-wein reactions by Beckwith et al. considerably extended the utility of diazonium salts. They showed that many electron donors could convert an arenediazonium cation into an aryl radical which cyclized in good yield to form dihydrobenzofurans and indolines. The final radical was functionalized as a halide, sulfide, or ferrocene. Thus, the credentials of diazonium salts as electron acceptors were well established, and the stage was set to investigate the interaction between diazonium salts and TTF. 

Singh, P. R., Kumar, R., Khanna, R. K. Radical nucleophilic substitution mechanism in the reactions of arenediazonium cations with nitrite ion. Tetrahedron Lett. 1982, 23, 5191-5194. 

Through comparative studies for a range of [Tp M(CO)3] analogues with different substituents in the pyrazolyl ring, Lalor demonstrated that the oxidation by arenediazonium cations occurred in response to the steric rather than the electronic effect of the 3-methyl substituents. However, further steric crowding in either the hydrotris(pyrazolyl)borate ligand or the diazonium cation promoted a reversion to the carbonyl-substitution pathway, producing aryldiazenido complexes Tp M(NNAr)(CO)2, which are also the products observed for the... 

Couplings between arenediazonium cations and phenols take place most rapidly in slightly alkaline solution. Under these conditions an appreciable amount of the phenol is present as a phenoxide ion, ArO , and phenoxide ions are even more reactive toward electrophilic substitution than are phenols themselves. (Why ) If the solution is too alkaline (pH > 10), however, the arenediazonium salt itself reacts with hydroxide ion to form a relatively unreactive diazohydroxide or diazotate ion ... 

Couplings between arenediazonium cations and amines take place most rapidly in slightly acidic solutions (pH 5—7). Under these conditions the concentration of the arenediazonium cation is at a maximum at the same time an excessive amount of the amine has not been converted to an unreactive aminium salt ... 

There has been a theoretical study of the reaction of arenediazonium cations with azide anions, which yields arylazides. No evidence for the formation of arylpen-tazoles was found. Arenediazonium salts have been identified as intermediates in the palladium-catalysed Sonagashira reaction of arylamines with alkynes to give arylalkynes. Palladium catalysis has also been used in the synthesis of diarylheptanols by the reaction of 4-hydroxybenzenediazonium ions with dihydropyrans. ... 

Hypophosphorous acid is an important reducing agent, both in chemistry labs and in industry. Its best known use in organic chemistry is in the reduction of arenediazonium cations (ArN2 ) to arenes (ArH). In industry, its chief application is electroless plating where a metal such as nickel or copper is deposited on a surface by chemical reduction, as opposed to passing an electfic current (hence the name). 

Masllorens, J., Moreno-Manas, M., Pla-Quintana, A., Roglans, A. (2003) First Heck Reaction with Arenediazonium Cations with Recovery of Pd-triolefinic Macrocyclic Catalyst. Org. Lett. 5 1559-1561. 

The simplest method yet uncovered for the replacement of the diazonium group by H (or H) is the use of PhSH (or PhS H) so far, the reaction seems to be limited to arenediazonium cations. Further reductive processes that are mediated by thiols are the conversion of 1 -nitro-alkenes into alkenes (+ NaNOj - - PhSSPh + Sg)... 

The synthesis and mutagenic effect upon Salmonella tvphimurium of l-deoxy-l-(p-tolylamino)-D-fructose, nine related glycosylamines, and their M-nitroso derivatives have been reported. It was suggested that mutagenicity was associated with production of the arenediazonium cation.The Amadori reaction between D-glucose and glycine led to the expected fructose-glycine condensation product which was shown to exist mainly in the... 

In Summary Arenediazonium cations attack activated benzene rings by diazo coupling, a process that furnishes azobenzenes, which are often highly colored. 

Salts of diazonium ions with certain arenesulfonate ions also have a relatively high stability in the solid state. They are also used for inhibiting the decomposition of diazonium ions in solution. The most recent experimental data (Roller and Zollinger, 1970 Kampar et al., 1977) point to the formation of molecular complexes of the diazonium ions with the arenesulfonates rather than to diazosulfonates (ArN2 —0S02Ar ) as previously thought. For a diazonium ion in acetic acid/water (4 1) solutions of naphthalene derivatives, the complex equilibrium constants are found to increase in the order naphthalene < 1-methylnaphthalene < naphthalene-1-sulfonic acid < 1-naphthylmethanesulfonic acid. The sequence reflects the combined effects of the electron donor properties of these compounds and the Coulomb attraction between the diazonium cation and the sulfonate anions (where present). Arenediazonium salt solutions are also stabilized by crown ethers (see Sec. 11.2). 

The reaction of molecular nitrogen with aryl cations, i. e., the reverse reaction of (heterolytic) dediazoniations of arenediazonium ions, is a direct introduction of the... 

Another redox reaction leading to arenediazonium salts was described by Morkov-nik et al. (1988). They showed that the perchlorates of the cation-radicals of 4-A,A-dimethylamino- and 4-morpholinoaniline (2.63) react with gaseous nitric oxide in acetone in a closed vessel. The characteristic red coloration of these cation-radical salts (Michaelis and Granick, 1943) disappears within 20 min., and after addition of ether the diazonium perchlorate is obtained in 84% and 92% yields, respectively. This reaction (Scheme 2-39) is important in the context of the mechanism of diazotization by the classical method (see Sec. 3.1). 

The experimental work of the groups of Swain and Zollinger on the dediazoniation mechanism of arenediazonium ions, which started in 1975, provided good evidence for the existence of aryl cations as steady state intermediates (see Sec. 8.3). These results also initiated theoretical work on aryl cations, in part combined with further calculations on the structure and reactivity of arenediazonium ions. Publications that contain data on arenediazonium ions and aryl cations will therefore be discussed in the chapter on dediazoniation reactions (Sec. 8.4). In the rest of this section we will concentrate on investigations that are concerned with the geometries and electron densities of diazonium ions but not, or only marginally, with energetics of the dediazoniation reaction. 

The volumes of activation for some additions of anionic nucleophiles to arenediazonium ions were determined by Isaacs et al. (1987) and are listed in Table 6-1. All but one are negative, although one expects — and knows from various other reactions between cations and anions — that ion combination reactions should have positive volumes of activation by reason of solvent relaxation as charges become neutralized. The authors present various interpretations, one of which seems to be plausible, namely that a C — N—N bond-bending deformation of the diazonium ion occurs before the transition state of the addition is reached (Scheme 6-2). This bondbending is expected to bring about a decrease in resonance interaction in the arenediazonium ion and hence a charge concentration on Np and an increase in solvation. 

In a classic study in 1940, Crossley and coworkers demonstrated that the rates of nucleophilic substitution of the diazonio group of the arenediazonium ion in acidic aqueous solution were independent of the nucleophile concentration, and that these rates were identical with the rate of hydrolysis. Since that time it has therefore been accepted without question that these reactions proceed by a DN + AN mechanism, i.e., that they consist of a rate-determining irreversible dissociation of the diazonium ion into an aryl cation and nitrogen followed by rapid reactions of the cation with water or other nucleophiles present in solution (Scheme 8-6). 

Scaiano and Kim-Thuan (1983) searched without success for the electronic spectrum of the phenyl cation using laser techniques. Ambroz et al. (1980) photolysed solutions of three arenediazonium salts in a glass matrix of 3 M LiCl in 1 1 (v/v) water/acetone at 77 K. With 2,4,5-trimethoxybenzenediazonium hexafluorophos-phate Ambroz et al. observed two relatively weak absorption bands at 415 and 442 nm (no e-values given) and a reduction in the intensity of the 370 nm band of the diazonium ion. The absence of any ESR signals indicates that these new bands are not due to aryl radicals, but to the aryl cation in its triplet ground state. 

All these results are consistent with the hypothesis that aryl cations react in aqueous media at diffusion-controlled rates with all nucleophiles that are available in the immediate neighbourhood of the diazonium ion. On this basis Romsted and coworkers (Chaudhuri et al., 1993a, 1993b) used dediazoniation reactions as probes of the interfacial composition of association colloids. These authors determined product yields from dediazoniation of two arenediazonium tetrafluoroborates containing ft-hexadecyl residues (8.15 and 8.16) and the corresponding diazonium salts with methyl groups instead of Ci6H33 chains. ... 

As mentioned at the end of Section 8.3, the MO investigation by Apeloig and Arad (1985) of the influence of trimethylsilyl substituents on the phenyl cation led to the discovery of a further reagent, in addition to arenediazonium ions, that is able to form aryl cations, namely 2,6-bis-(trimethylsilyl)phenyltriflate. This was a significant success in the field of predictions on aryl cations by theoretical work. 

We will conclude this section on theory with such a case. In Section 8.3 it was shown that the influence of substituents on the rate of dediazoniation of arenediazonium ions can be treated by dual substituent parameter (DSP) methods, and that kinetic evidence is consistent with a side-on addition of N2. We will now discuss these experimental conclusion with the help of schematic orbital correlation diagrams for the diazonium ion, the aryl cation, and the side-on ion-molecule pair (Fig. 8-5, from Zollinger, 1990). We use the same orbital classification as Vincent and Radom (1978) (C2v symmetry). 

Szele and Zollinger (1978 b) have found that homolytic dediazoniation is favored by an increase in the nucleophilicity of the solvent and by an increase in the elec-trophilicity of the P-nitrogen atom of the arenediazonium ion. In Table 8-2 are listed the products of dediazoniation in various solvents that have been investigated in detail. Products obtained from heterolytic and homolytic intermediates are denoted by C (cationic) and R (radical) respectively for three typical substituted benzenediazonium salts and the unsubstituted salt. A borderline case is dediazoniation in DMSO, where the 4-nitrobenzenediazonium ion follows a homolytic mechanism, but the benzenediazonium ion decomposes heterolytically, as shown by product analyses by Kuokkanen (1989) the homolytic process has an activation volume AF = + (6.4 0.4) xlO-3 m-1, whereas for the heterolytic reaction AF = +(10.4 0.4) x 10 3 m-1. Both values are similar to the corresponding activation volumes found earlier in methanol (Kuokkanen, 1984) and in water (Ishida et al., 1970). 

In Section 8.3 the mechanism of heterolytic dediazoniation of arenediazonium ions was discussed, and it was shown that the hypothesis of Crossley et al. (1940) that the aryl cation is the characteristic metastable intermediate in those reactions was not consistent with some experimental facts found in 1952 by Lewis and Hinds. Nevertheless, these facts did not have significant influence on the scientific community, which continued to accept the original and apparently convincing hypothesis of the rate-limiting formation of an aryl cation as an intermediate as correct . The incom-patabilities of various mechanistic hypotheses with experimental facts were, however, discussed in some detail only two decades later (Zollinger, 1973 a). Another year passed before I performed a crucial experiment that refuted a number of hypotheses (Bergstrom et al., 1974, 1976). ... 

The photolysis of arenediazonium salts has been widely used for intramolecular cyclizations in the synthesis of 1-phenylethylisoquinoline alkaloids by Kametani and Fukumoto (review 1972). An example is the photolysis of the diazonium ion 10.73, which resulted in the formation of O-benzylandrocymbine (10.74) (Kametani et al., 1971). The mechanism of this cyclization is obviously quite complex, since the carbon (as cation or radical ) to which the diazonio group is attached in 10.73 does not react with the aromatic CH group, but with the tertiary carbon (dot in 10.73), forming a quinone-like ring (10.74). In our opinion the methyl cation released is likely to react with the counter-ion X- or the solvent. 

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