Nitrosyl transfer reactions

The first product of nitrosyl transfer to nitrite in Eq. (2), E N203, contains N-N bonded N2O3 which is itself a well-known and powerful nitrosyl donor. It is reasonable to suppose therefore that nitrosyl transfer reactions with N- and O-nucleophiles could involve both E NO (or E HONO) and E N205. In addition, the involvement of a second molecule of nitrite for denitrification would require that the substrate saturation curve should be sigmoidal to reflect a term second-order in nitrite concentration. No such effect has been reported to our knowledge. The use of bimolecular substrate kinetics in dilute solutions to generate an intermediate subject to solvolysis seems metabolically unwise hut not impossible. 

Reaction of metal nitrosyls with azide ion proceeds with formation of N2 and N20 (56). This can be viewed as the result of a nitrene transfer reaction in analogy with the Curtius rearrangement (62) and its organome-tallic counterpart (63). 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

In spite of the above speculation, the actual mechanisms used by NO-pro-ducing nitrite reductases for reduction of nitrite and its activation for nitrosyl transfer are poorly understood. The fact that both the heme and Cu types of enzyme catalyze the reaction is remarkable in view of the fact that nitrosyl compounds of Fe complexes are well known, whereas Cu-nitrosyl compounds are rare [see Garber and Hollocher (1982) and Kim and Hollocher (1984) for further discussion]. On the other hand, both Fe and Cu can coordinate O and this might be more relevant for the activation of an O atom of nitrite for N-O bond breaking. In the case of the Cu-type nitrite reductase of A. cycloclastes, the... 

Platinum(II) nitrato complexes can be synthesized by metathesis, by addition of N204 to Pt02(PPh3)2, by nitrosylation of Pt02(PPh3)2 or by ligand transfer reactions (equations 467-471). 

Oxygen-transfer reactions have been shown to occur from cobalt(III)-nitro complexes to alkenes coordinated to palladium.472 Thus ethylene and propene have been oxidized stoichiometrically in quantitative yields to acetaldehyde and acetone respectively, with the concomitant reduction of the nitro- to the nitrosyl-cobalt analog. A catalytic transformation with turnover numbers of 4-12 can be achieved at 70 °C in diglyme. The mechanism shown in Scheme 11 has been suggested. 

These synthetic methods are based on oxygen transfer reactions, which are an important general feature of transition metal nitrosyl chemistry. The following reactions illustrate this route 15, 42) ... 

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