Nitro compounds vicarious nucleophilic

Aromatic nitro compounds undergo nucleophilic aromatic substitutions with various nucleophiles. In 1991 Terrier s book covered (1) SNAr reactions, mechanistic aspects (2) structure and reactivity of anionic o-complexes (3) synthetic aspects of intermolecular SNAr substitutions (4) intramolecular SNAr reactions (5) vicarious nucleophilic substitutions of hydrogen (VNS) (6) nucleophilic aromatic photo-substitutions and (7) radical nucleophilic aromatic substitutions. This chapter describes the recent development in synthetic application of SNAr and especially VNS. The environmentally friendly chemical processes are highly required in modem chemical industry. VNS reaction is an ideal process to introduce functional groups into aromatic rings because hydrogen can be substituted by nucleophiles without the need of metal catalysts. 

The 2-alkylene-substituted aromatic nitro compound 1011, which is readily accessible via vicarious nucleophilic substitution (VNS) [88-90] of 4-fluoronitroben-... 

Tetranitro derivative 90 (z-TACOT Section 12.10.15.5) treated with methanolic sodium methoxide at ambient temperature does not lead to simple product of nucleophilic substitution of a nitro group but provides compound 92. Its formation can be rationalized by introduction of the methoxy group into the 1-position, followed by scission of the remote triazole ring of 91 to give the final product. Compound 90 subjected to the vicarious nucleophilic substitution (VNS) conditions using either hydroxylamine or trimethylhydrazinium iodide gives a very insoluble red solid, which was identified as l,3,7,9-tetraamino-2,4,8,10-tetranitrobenzotriazolo[2,l- ]benzotriazole 93 (Scheme 5) <1998JOC3352>. 

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],... 

There has been some interest in the chemistry of aminonitroazoles when used as high energetic compounds. Data concerning direct introduction of the amino group into the nitroazole cycle have been absent in the literature till the present time. We have studied the vicarious nucleophilic substitution of l-methyl-4-nitropyrazole and also l-mcthyl-4-nitroimidazole (Table 3.10), 4-nitro-2-phenyl-1,2,3-triazole (Table 3.24), and nitrobenzimidazoles (Table 3.25) under the effect of 1,1,1-trim-ethylhydrazinium halides and 4-amino-1,2,4-triazole by NMR spectroscopy (DMSO-<76) (Scheme 3.5) [220, 272-278] ... 

Another kind of substitution of nitro compounds was Vicarious substitution (72 73) which takes place when the attack is carried out by a nucleophile... 

Makosza M, Kwast E (1995) Vicarious nucleophilic substitution of hydrogen in nitro derivatives of five-membered heteroaromatic compounds. Tetrahedron 51 8339-8354... 

Keywords Alkyl hydroperoxides Amination Ammonia Benzimidazole Benzisoxazole Carbanions Heterocycles Hydroxylation Indole Nitro compounds Nucleophiles Nucleophilic substitution Oxidation Phenazine Potassium permanganate Pyridine Quinoline Sulfones Vicarious... 

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). 

The alkylation of aromatic nitro compounds by carbanions having a leaving group at the nueleophilic center is called Vicarious Nucleophilic Substitution of Hydrogen (VNS) (Seheme 36.1). This reaction is one of the scarce general processes that result in the formal nucleophilic aromatic substitution of hydrogen. 

Taking into account the vicarious nature of the nucleophile the authors suggested that ring closure occurs inside alkenic aH-adduct 290 with participation of the nitro group. However, in principle, the reaction might proceed via the formation of nitroso compound 291 with subsequent electrocyclization into oxazine 292 and hydrolytic exchange of the p-tosyl group (Scheme 85). 

The nitro group in the ortho- or para-positions is one of the strongest electron-withdrawing substituent, activating an aromatic ring for a nucleophilic attack [5]. No wonder that a great deal of both oxidative and eliminative Sn reactions have been observed in the series of nitroaromatic compounds [11, 15-18, 25, 27, 28, 30-32, 36, 37, 97-100]. The VNS protocol has been particularly effective for the Sn reactions of a large number of nitro-substituted aromatic and heteroaromatic compounds [11-18, 25, 27, 28, 30-32, 36, 37, 45, 98-100], as illustrated by nucleophilic alkylation of 3-nitropyridine with vicarious methyl chloroacetate (Scheme 22) [100]. 

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