Nitrosonium reagents

Mercuric acetate and mercuric trifluoroacetate are the usual reagents.66 The synthetic utility of the mercuration reaction derives from subsequent transformations of the arylmercury compounds. As indicated in Section 7.3.3, arylmercury compounds are only weakly nucleophilic, but the carbon-mercury bond is reactive toward various electrophiles. The nitroso group can be introduced by reaction with nitrosyl chloride67 or nitrosonium tetrafluoroborate68 as the electrophile. Arylmercury compounds are also useful in certain palladium-catalyzed reactions, as discussed in Section 8.2. 

Radical cations can be generated by many chemical oxidizing reagents, including Brpnsted and Lewis acids, the halogens, peroxide anions or radical anions, metal ions or oxides, nitrosonium and dioxygenyl ions, stable aminium radical cations, semiconductor surfaces, and suitable zeolites. In principle, it is possible to choose a reagent with a one-electron redox potential sufficient for oxidation-reduction, and a two-electron potential insufficient for oxidation-reduction of the radical ion. 

Nitrosonium tetrafluoroborate has been prepared from nitrosyl fluoride (NOF) and boron trifluoride in Freon-113. With this reagent, in a nitro-methane solution, pyridine was converted into jV-nitrosopyridinium tetra-fluorobate, m.p. 155°-158°C [33]. 

By using the same reagent combination of nitrosonium tetrafluoroborate and pyridinium poly(hydrogen fluoride) (PPHF), diarylacetylenes could be converted into diaryltetrafluoro-ethanes in moderate to good yields (Table 19).87... 

Unsubstituted alkyl azides react with nitrosonium tetrafluoroborate without formation of alkyl fluorides, but azidonitriles give fluoronitriles with this reagent.69 Depending on the chain length, rearrangements can occur extensively. Interaction between the nitrile group and boron trifluoride has been invoked to explain these reactions. 

Laszlo introduced an oxidative cleavage of dithioacetals by their clayfen (4) and claycop reagents under mild conditions with excellent yields [46, 47]. These reagents are convenient sources of the nitrosonium ion NO+, a soft reactive Lewis acid species, well adapted for attack of the soft sulfur atom. 

Nitrosonium salts NO+Y (Y = BF4, PF6 etc.) react with basic precursors which are either coordinatively unsaturated or have displaceable groups. The use of these electrophilic reagents has only recently been developed and their sensitivity to conditions, such as the nature of the solvent, has tended to restrict their application (equation 9). 

A solution of sodium nitrite in concentrated sulphuric acid, which provides a nitrosonium hydrogen sulphate reagent, is a very effective diazotising medium which is particularly valuable for even more weakly basic amines, such as 2,4-dinitroaniline or the corresponding trinitro compound, picramide. 

A rich variety of chemical oxidizing reagents have been applied for the generation of radical cations. The principal reagent types include Bronsted and Lewis acids the halogens certain peroxide anions or radical anions numerous metal ions or oxides nitrosonium and dioxygenyl ions stable organic (aminium) radical cations ... 

Nitrosonium salts as reagents in inorganic chemistry. (M. T. Mocella, M. S. Okamoto, and E. K. Barefield, Syn. React. Inorg. Metal-org. Chem., 1974,4, 69). 

The direct oxidation of hydroxyls on inulin allows the potential introduction of carbonyl and carboxyl groups, altering the properties of the polysaccharide and opening additional commercial applications (Bragd et al., 2004). The primary hydroxyl in the C-6 position on the fructofuranoside subunits can be selectively oxidized using 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO). This forms a stable radical that can be oxidized by hypobromite, or similar reagent, to give a nitrosonium... 

One-electron oxidants convert tertiary amines into aminium ions. Chlorine dioxide is an example of such a reagent this converts triethylamine and other tertiary alkylamines into aminium ions, which normally react further by the loss of hydrogen from an a-carbon atom. An example of a synthetic (plication of this reaction sequence is shown in Scheme 24, the inteimediate iminium ion being intercepted intramolecularly." Nitrosonium tetrafluoroborate and dioxygenyl hexafluoroantimonate, 02 SbF6, are also good one-electron oxidants which can be used at low temperature." ... 

Only a few examples of reactions of oximes and oxime methyl ethers with iodine monofluoride and bromine trifluoride have been reported (Table 6. entries 6, 8 -12). The yields in these reactions are lower, except in the case of the activated aryl ketone (entry 12). Furthermore, iodine monofluoride does not react with oxime methyl ethers. due to the low basicity of the nitrogen atom. Therefore, these reagents are not good alternatives to the reaction utilizing nitrosonium tetrafluoroborate and pyridinium poly(hydrogen fluoride). 

Tetrafluoro(perfluorophenyl)silicates, M [CgF5SiF4]. obtained by the interaction between trifluoro(perfluorophenyl)silanc and metal fluorides MF (M — K. Cs) or tetramethylam-monium fluoride in nonaqueous acetonitrile or diglyme,are excellent reagents for penta-fluoroarylation with formation of pentafluoro-substituted derivatives of benzene. Thus, the reaction of these salts with nitrosonium and nitronium tetrafluoroborates leads to per-fluoro(nitrosobenzene) (14) and perfluoro(nitrobenzenc)(15), respectively, in quantitative yield. In the case of reaction with perfluoropyridine, perfluoro(4-phenylpyridine) (16) (33% yield) and pentafluorobenzene (.37 % yield) arc formed. Reaction of tetramcthylammonium tetrafluo-ro(perfluorophenyl)silicate with bromine or iodine pentafluoride gives the corresponding bro-mopentafluorobenzene and pentafluoro(tetrafluoroiodo)benzene. ... 

H2O—NO or NOCl) that can easily transfer to the ring. The nitrosonium ion is very weakly electrophilic compared with the reagents involved in nitration, sulfonation, halogenation, and the Friedel-Crafts reaction nitrosation ordinarily occurs only in rings bearing the powerfully activating dialkylamino (—NR2) or hydroxy (—OH) group. 

The scope of the Ritter reaction can also be extended by avoiding strong acid as the carbenium ion initiator. In at least one case it is possible to omit this reagent entirely because of the facility of 5 1 reaction. Thus, reflux of r-butyl bromide in acetonitrile or propionitrile for 24 h led to formation of A -f-butylamide products. Less commonly used initiators have included cation exchange resins and clayfen. In this latter case, the clay-supported iron(III) nitrate is believed to give rise to nitrosonium ions, which react with the alkyl halide substrate to produce a carbenium ion. Yields of amide from this technique are generally modest (21-35%). 

Alkylation of thioamides with Meerwein s reagent (trialkoxonium fluoroborate) proceeds via a thioiminoester and subsequent base hydrolysis with sodium carbonate (equation 32). For the synthesis of 2-pyridones from 2-thiopyridones, chloroacetic acid has been used. ° Mild reaction conditions are provided by nitrosyl species which are derived from a variety of reagents excess NaNOa/HCP in aqueous medium, nitrosonium tetrafluoroborate in dichloromethane, dinitrogen tetroxide in acetonitrile at low temperature and f-butyl thionitrate. ... 

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