Nitrosonium cation

In most of the cases discussed so far, the rate-determining step of the reaction is nitrosation of the free amine. When diazotisation is carried out in concentrated acids, the nitrosonium cation, NO +, is the nitrosating species. In this case, an exchange mechanism has been proposed, as... 

In principle the ISO-NOP sensor works as follows. The sensor is immersed in a solution containing NO and a positive potential of —860 mV (vs Ag/AgCl reference electrode) is applied. NO diffuses across the gas permeable/NO-selective membrane and is oxidized at the working electrode surface producing a redox current. This oxidation proceeds via an electrochemical reaction followed by a chemical reaction. The electrochemical reaction is a one-electron transfer from the NO molecule to the electrode, resulting in the formation of the nitrosonium cation ... 

Productive bimolecular reactions of the ion radicals in the contact ion pair can effectively compete with the back electron transfer if either the cation radical or the anion radical undergoes a rapid reaction with an additive that is present during electron-transfer activation. For example, the [D, A] complex of an arene donor with nitrosonium cation exists in the equilibrium with a low steady-state concentration of the radical pair, which persists indefinitely. However, the introduction of oxygen rapidly oxidizes even small amounts of nitric oxide to compete with back electron transfer and thus successfully effects aromatic nitration80 (Scheme 16). 

Donor/acceptor association and the electron-transfer paradigm form the unifying theme for the C—C bond cleavage of various benzpinacols and diary-lethane-like donors in the presence of different electron acceptors (such as chloranil (CA), dichlorodicyanobenzoquinone (DDQ), tetracyanobenzene (TCNB), triphenylpyrylium (TPP+), methyl viologen, nitrosonium cation, etc.). Scheme 13 reminds us how this is achieved by either CT photolysis of the D/A pair or via diffusional quenching of the excited electron acceptor A by the electron donor D. 

Fig. 10 ORTEP diagram showing the complexation of nitrosonium cation with a single phenyl moiety of bicumene. Reproduced with permission from Ref. 194.

In a similar vein, the cleavage of the photodimer of anthracene occurs in the dark in the presence of nitrosonium cation to afford the anthracene cation radical as its 7t-dimer via a thermal electron transfer196 (equation 66). 

The disproportionation of nitrogen dioxide has been independently confirmed by the direct observation of nitrosonium cation as the EDA complex of hexamethybenzene by spectroscopic means as well as by the spectral comparison with the authentic [HMB, NO+] complex,240 i.e.,... 

Donor-acceptor association. It is experimentally well established that nitrosonium cation forms vividly colored charge-transfer complexes with a wide variety of aromatic donors243 (equation 85). 

Fig. 19 Temperature-dependent interconversion of the hydroquinone ether (MA) cation radical (Amax = 518nm) and its EDA complex with nitrosonium cation (imax = 360 nm) according to equation (86) in the temperature range from +40°C to -78°C (incrementally).

The oxidation of hydroquinones254 and quinone dioximes255 (denoted as QH2) involves removal of two electrons and two protons. This redox stoichiometry is experimentally established both in the stoichiometric oxidations with NO and with two equivalents of nitrosonium cation (equations 97a,b). 

Musker29 carried out a systematic study of the oxidation of several cyclic 22 and acyclic 23 bis-sulfides using nitrosonium salts. Several unstable dications were characterized as sulfoxides 24. These oxidations proceed through stepwise transfer of two electrons from a bis-sulfide to the nitrosonium cation and the intermediate formation of the corresponding radical cation. Radical cations of 1,5-dithiacyclooctane 11 and 1,5-dithiacyclononane are sufficiently stable to be isolated as individual compounds (Scheme 8).50... 

The most important physiological nitrogen substrate of peroxidases is undoubtedly nitric oxide. In 1996, Ishiropoulos et al. [252] suggested that nitric oxide is able to interact with HRP Compounds I and II. Glover et al. [253] measured the rate constants for the reactions of NO with HRP Compounds I and II (Table 22.2) and proposed that these reactions may occur in in vivo inflammatory processes. The interaction of NO with peroxidases may proceed by two ways through the NO one-electron oxidation or the formation of peroxidase NO complexes. One-electron oxidation of nitric oxide will yield nitrosonium cation NO+ [253,254], which is extremely unstable and rapidly hydrolyzed to nitrite. On the other hand, in the presence of high concentrations of nitric oxide and the competitor ligand Cl, the formation of peroxidase NO complexes becomes more favorable. It has been shown [255]... 

All of these processes are interpreted through intermediate nitrosonium cations, which are generated upon cleavage of the endocyclic N—O bond with LA. A detailed consideration of all these transformations is beyond the scope of the present monograph. 

In the authors opinion, the nitrosonium cations A are chlorinated by TiCl4(LA) and undergo cyclization with one of the methoxycarbonyl groups to give buty-rolactones (151). This process can be accompanied by ortho-cyclization giving rise to oximes (152) as by-products. 

A plausible Lewis structure for the nitrosonium cation, NO+ is drawn below ... 

It is known that the nitrosonium cation is a strong oxidant (54). In (55) it was found by multinuclear NMR ( H, 13C, 19F and 14N) that the interaction of nitrosonium tetrafluoroborate with 2,2,6,6-tetramethyl-4-R-piperidine-1 -oxyl radicals 22a-e resulted in formation of 4-R-2,2,6,6-tetramethylpiperidine-l-oxoammonium tetrafluoroborates (Scheme 16). Cations 23a-e could be classified as nitrosonium complexes of biradicals 24a-e. 

To date the structure and reactivity of numerous complexes derived from aromatic compounds and nitrosonium cation have been studied (5, 56-63). However, relatively few studies are available on the nitrosonium complexes of cyclophanes (5, 57, 59, 61, 62), cf ref. (63). The interaction of [2.2]paracyclophane with nitrosonium tetrachloroaluminate was studied by H and 13C NMR spectroscopy using deuterium isotope perturbation technique (64). It was found that the resulting nitrosonium complexes containing one (25) or two NO groups (26) are involved in fast interconversion (on the NMR time scale) (Scheme 17). 

The affinity of [2.2]paracyclophane for nitrosonium cation is much greater than that of para-xylene, presumably owing to stacking interaction between the aromatic rings in the 7i-complex. Low isotope effect on the aromatic carbon... 

Data pertaining to affinity of organic N-bases towards nitrosonium cation are numerous 74-80), however, those for N-heteroaromatic compounds are limited 77-80). 

Axial and oblique structures of EDA complexes with diatomic acceptors 225 Charge-transfer in weak and strong aromatic EDA complexes 226 Charge-transfer structures of aromatic complexes with the nitrosonium cation 228... 

Charge transfer versus electron transfer in the interaction of aromatic donors with the nitrosonium cation 230... 

The nitrosonium cation can serve effectively either as an oxidant or as an electrophile towards different aromatic substrates. Thus the electron-rich polynuclear arenes suffer electron transfer with NO+BF to afford stable arene cation radicals (Bandlish and Shine, 1977 Musker et al., 1978). Other activated aromatic compounds such as phenols, anilines and indoles undergo nuclear substitution with nitrosonium species that are usually generated in situ from the treatment of nitrites with acid. It is less well known, but nonetheless experimentally established (Hunziker et al., 1971 Brownstein et al., 1984), that NO+ forms intensely coloured charge-transfer complexes with a wide variety of common arenes (30). For example, benzene, toluene,... 

The nitrosonium cation bears a formal relationship to the well-studied halogens (i.e. X2 = I2, Br2, and Cl2), with both classes of structurally simple diatomic electron acceptors forming an extensive series of intermolecular electron donor-acceptor (EDA) complexes that show well-defined charge-transfer absorption bands in the UV-visible spectral region. Mulliken (1952a,b 1964 Mulliken and Person, 1969) originally identified the three possible nonbonded structures of the halogen complexes as in Chart 7, and the subsequent X-ray studies established the axial form II to be extant in the crystals of the benzene complexes with Cl2 and Br2 (Hassel and Stromme, 1958, 1959). In these 1 1 molecular complexes, the closest approach of the... 

CHARGE-TRANSFER STRUCTURES OF AROMATIC COMPLEXES WITH THE NITROSONIUM CATION... 

The oxidative conversions of the aromatic donors hexamethylbenzene, anthracene, dianthracene, bicumene and methoxytoluene by the nitrosonium cation, as described above, are rather unequivocal examples in which the establishment of an electron-transfer equilibrium is a clear prerequisite for the further (follow-up) reactions. There are other donors, including... 

The mechanistic conundrum presented by such a dichotomy between electron-transfer and electrophilic processes can only be rigorously resolved by the experimental proof of whether the cation radical (or the electrophilic adduct) is, or is not, the vital reactive intermediate. However, in a thermal (adiabatic) reaction between arene donors and the nitrosonium cation, such reactive intermediates cannot be formed in sufficient concentrations to be observed directly by conventional experimental methods since their rates of follow-up reactions must perforce always be faster than their rates of formation, except when they are formed in a reversible equilibrium like the... 

Neutral (cyclobutadiene)Fe(CO)3 complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate and with the nitrosonium cation to generate the corresponding (cyclobutadiene)Fe(CO)2L complexes15. These types of complexes are presumably intermediates in the reaction of (cyclobutadiene)Fe(CO)3 complexes with perfluorinated alkenes and alkynes to generate the insertion products 266 or 267 respectively (Scheme 70)15,238. 

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