Nitrosyl exchange

As a result of their very low reactivity with the HO anion, the organic nitrates show a practically pH-independent (and very slow) hydrolysis at pH 7 and above [27], In contrast, they will react rapidly with alcohols in nonpolar media by nitrosyl exchange (Fig. 9.2,b) [28]. Formally, H+ and NO+ (nitrosyl cation) are exchanged, but the actual reaction mechanism is quite complex. The reaction leads to an equilibrium, which depends on the partners and presumably also on conditions. However, the biological relevance of this reaction remains to be understood. 

Doyle and Bryker (1979) reported high yields of arenediazonium tetrafluoroborates when aromatic amines were reacted with tert-butyl nitrite and trifluoroboro etherate in CH2C12. It is likely that nitrosyl fluoride is formed as nitrosating reagent by fluoride-alkoxy exchange. 

No accurate rate measurements have yet been carried out, but qualitative experiments showed that the exchange was acid-catalysed so that it is likely that the protonated form of the nitrosamine (LIII) acts itself as a nitrosating agent. This would account for all the earlier cross-nitrosations that have been observed without necessitating the prior formation of nitrous acid, nitrosyl chloride or any other carrier of NO+. 

Butler, A. R., Glidewell, C., Hyde, A. R., and Walton, J. C. (1985). Formation of paramagnetic mononuclear iron nitrosyl complexes from diamagnetic di- and tetranuclear iron-sulphur nitrosyls Characterization by EPR spectroscopy and study of thiolate and nitrosyl ligand exchange reactions. Polyhedron 4, 797-809. 

O-exchange studies of Ye et al. (1991) support, we believe, the catalysis by nitrite reductase of redox reversibility between nitrite and NO as depicted in the first line of Eq. (3). They observed by analyzing the 0 content of product N2O that all eight strains of denitrifying bacteria studied could catalyze the exchange of 0 between water and nitrite or NO by way of an electrophilic (nitrosyl donor) species of NO. The rates and extent of these exchange reactions depended on whether the bacterium made use of a heme- or Cu-type nitrite reductase. Contrary to the conclusions of Ye et al. (1991), we do not believe that this study otherwise informs about the pathway of denitrification or whether NO is an intermediate. 

Ruggiero, C. E., Carrier, S. M., Antholine, W. E., Whittaker, J. W., Cramer, C. J., and Tolman, W. B. (1993). Synthesis and structure and spectroscopic characterization of mononuclear copper nitrosyl complexes Models for nitric oxide adducts of copper proteins and copper exchange zeolites. J. Am. Chem. Soc. 115, 11285-11298. 

Treatment of [W(NO)2Cl2] with AgBF4 in MeCN leads to cw-[W(NO)2(MeCN)4](BF4)2. NMR measurements have shown that this complex undergoes stereospecific exchange of coordinated MeCN via a dissociative pathway. The two labile MeCN groups are those trans to the nitrosyl ligands.399... 

In a lecture presented to the Faraday Society,332 Norrish commented on why the higher vibrational levels of NO were observed in the nitrosyl halide experiments but not by absorption in NO irradiation experiments. He reasoned that, as the emission of NO from the AZH + state populates the first five vibrational levels of NO almost equally,341 the fast-exchange reaction (4) can quickly eliminate all the vibrational levels above the first. However, in the nitrosyl halide experiments, the NO may be formed preferentially in very high levels, such as v — 10 or 11, almost exclusively. Thus, reaction (4) cannot occur initially, and depopulation must be by reaction (2). Reaction (2) is considerably slower than reaction (4), because of the increased difference in vibrational energy between the reactants and products resulting from an-harmonic effects. 

In addition to rapid exchange of thiolate ligands with R SH and for R S , the mononuclear complexes [Fe(NO)2(SR)2] also undergo (23) rapid exchange of the nitrosyl ligands in the presence of isotopically labeled nitrite a W-shaped N203 intermediate was proposed (23) [Eq. (22)]. 

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