Vicarious mechanism

Some derivatives of the [l,2,4]triazolo[4,3-3]pyridazine ring system 33 were subjected to a special type of nucleophilic substitution called vicarious nucleophilic substitution (VSN) <2006TL4259>. In the course of this transformation a formal substitution of az aromatic hydrogen atom - occurring via an addition-elimination mechanism - takes place. 

The vicarious nucleophilic substimtion of carbo- and hetero-cyclic nitroarene hydrogen by a hydroxyl group, on reaction with silylhydroperoxide anions, has been shown to proceed via nucleophilic addition of ROO followed by base induced elimination of ROH by an ii2-type mechanism the required orientation of the hydroxylation can be controlled by the conditions selected. ... 

Scheme 3.8 represents a typical ion-radical mechanism of the so-called vicarious nucleophilic substitution of hydrogen as it was described by the pioneering chemist M kocza (M kocza 1989) in his summarizing review. ESR studies of other heterocyclic systems in conditions of the hydrogen-to-nucleophile vicarious substitution were reported by the research group at the Siberian branch of the Russian Academy of Sciences (Donskaya et al. 2002, Vakul skaya et al. 2005, 2006, Titova et al. 2005). 

Pyrolysis of guanidines 115 affords 2-dialkylaminobenzimidazoles 116 in 13-27% yield (73ZC292). A plausible mechanism is shown in Scheme 35. Acyloxy functionality serves as a vicarious leaving group. 

Abstract Synthesis methods of various C- and /V-nitroderivativcs of five-membered azoles - pyrazoles, imidazoles, 1,2,3-triazoles, 1,2,4-triazoles, oxazoles, oxadiazoles, isoxazoles, thiazoles, thiadiazoles, isothiazoles, selenazoles and tetrazoles - are summarized and critically discussed. The special attention focuses on the nitration reaction of azoles with nitric acid or sulfuric-nitric acid mixture, one of the main synthetic routes to nitroazoles. The nitration reactions with such nitrating agents as acetylnitrate, nitric acid/trifluoroacetic anhydride, nitrogen dioxide, nitrogen tetrox-ide, nitronium tetrafluoroborate, V-nitropicolinium tetrafluoroborate are reported. General information on the theory of electrophilic nitration of aromatic compounds is included in the chapter covering synthetic methods. The kinetics and mechanisms of nitration of five-membered azoles are considered. The nitroazole preparation from different cyclic systems or from aminoazoles or based on heterocyclization is the subject of wide speculation. The particular section is devoted to the chemistry of extraordinary class of nitroazoles - polynitroazoles. Vicarious nucleophilic substitution (VNS) reaction in nitroazoles is reviewed in detail. 

The nitroazoles are widely used in the reaction of vicarious nucleophilic substitution of hydrogen. Vicarious nucleophilic C-amination is, practically, the single method of direct introduction of the amino group into nitro compounds. Using the vicarious nucleophilic substitution reaction we have successfully carried out the C-amination of some representatives of nitrobenzazoles, nitroazoles, and model compounds thereof and studied the structure of aminated products and the C-amination mechanism [673-678],... 

The mechanism of vicarious C-amination of nitroazoles is presented in Schemes 3.30 (l-methyl-4-nitropyrazole), 3.31 (2-phenyl-4-nitro-l,2,3-triazole), 3.32 (l-methyl-5-nitrobenzimidazole), and 3.33 (l-methyl-6-nitrobenzimidazole) [273, 275-278],... 

First the mechanism of vicarious nucleophilic substitution of hydrogen in nitroazoles and nitrobenzene has been discussed in terms of an electron transfer [273-277]. Community of the observed effects in vicarious C-amination of nitroazoles allows... 

Guimaraes MJ, Peterson D, Vicari A, et al. 1996. Identification of a novel selD homolog from Eukaryotes, Bacteria, and Archaea Is there and autoregulatory mechanism in selenocysteine metabolism Proc Natl Acad Sci U S A 93 15086-15091. 

Nitrobenzene and many of its 2-, 3-, and 4-substituted derivatives are converted into nitroaniline derivatives by treatment with sulfenamides in the presence of t-BuOK (eq 90). In this conversion, termed vicarious nucleophilic substitution (VNS), the base presumably promotes both the formation of the nucleophilic sulfenamide anion and the -elimination of the thiocar-bamoyl group from the other examples of t-BuOK-promoted VNS reactions of nitrobenzenes have appeared in recent years. An interesting example of this process involves the synthesis of dithianylated nitrobenzenes which are hydrolyzable to aldehydes (eq 91). The treatment of mixtures of m-nitroaniUne and enoUzable ketones with t-BuOK in DMSO leads to nitroindoles by oxidative nucleophilic substitution of hydrogen (eq 92). The proposed mechanism for this transformation involves attack of the potassium enolate of the ketone on the ring, spontaneous oxidation of the a-adduct, and imine formation and tautomerization. 

Vicari AP, Ait-Yahia S, Chemin K, Mueller A, Zlotnik A, Caux C. Antitumor effects of the mouse chemokine 6Ckine/SLC through angiostatic and immunological mechanisms. J Immunol 2000 165 1992-2000. 

Mechanisms of the nucleophilic substitution of hydrogen have been reviewed, and there has been a report of the synthesis of nitroaryl derivatives of glycine using the oxidative pathway. An example of the vicarious substitution mechanism is the formation of (13), a 3-styryl-substituted boron dipyrromethene, by reaction of a... 

Active carbanions are prone to substitute hydrogen of nitroaromatic compounds the substitution usually takes place either ortho or para to the nitro group. The anionic On-adduct generated as a result of nucleophilic addition can be converted into the final product by various mechanisms [167]. Most often exemplified are vicarious nucleophilic substitution (VNS) and oxidative nucleophilic substitution of hydrogen (ONSH). 

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