Nitro groups, neighbouring

Neighbouring group participation by a nitro group was suggested130 for the reaction in Scheme 16 because the reaction stopped at the tosylate (32) when the nitro group was in the para position on the benzene ring attached to the nitrogen of the substrate. 

An enamine (formed in situ) reacts with a neighbouring amino group (also formed in situ by sodium borohydride-nickei boride-hydrazine hydrate reduction of a nitro group) to give a high yield of the precursor of a biologically important indole. 

This mechanism, however, turned out abortive because still earlier it was firmly -established, by the method of spin saturation transfer (cf. Ref. that the nitro group in this ion, if not exclusively, then at iKist mostly, migrates to the neighbouring positions by a 1,2-shift. Similar results have been obtained in studying the mechanism of the shifts for the other migrants in ions of this type CH3... 

The nitro group also increases the shifts of neighbouring protons and so we can assign the spectrum. The rather high field of the proton on the pyridine ring at 6.6 ppm is explained by the electron-donating effect of the amino group. 

The mechanism of the novel transformation of a-nitro- to a-hydroxy-ketones has been probed. The reaction, which proceeds under basic aqueous conditions, requires that the Q -nitro substrate be CH-acidic in the a -position, and that it be readily depro-tonated under the conditions employed. NO2 -OH exchange occurs with retention of configuration, with the hydroxyl oxygen being predominantly derived from the solvent. A mechanism involving neighbouring-group participation, via a Favorskii-like cyclopropanone intermediate, is proposed. 

Rate measurements for thermolysis of 4-azidothiazoles in p-xylene solution have found neighbouring-group effects from nitro (19-fold rate increase), phenylimmomethyl (16-fold), formyl (4.5-fold) and acetyl (2.2-fold) substituents in the 5-position.60 See also under Other Small Rings above.50... 

The reactivity of halogen compounds, of course, strongly depends on the halogen. Fluoroaliphatic compounds are nonreactive unless they contain another reactive functional group. Chloro-compounds are fairly reactive, k > 108 M 1 s 1, and their reactivity increases in the presence of neighbouring electron withdrawing groups. Bromo-and iodo-compounds are more reactive in that order. Thiols and disulphides are very reactive, whereas thiol anions and thioethers are only fairly reactive. Nitro- and nitroso-compounds are very reactive toward eaq. 

Solvent specific effects do not produce remarkable change in the hydrogen bond strength [159]. Addition of 1 water molecules variously located in the neighbouring of the catechol groups of 4-nitro-catechol has not modified the relative stability of its three possible conformations with respect to vacuum... 

A /3-subtituted vinyl group carrying two electron-withdrawing groups reacts under mild conditions with a neighbouring nitro function and potassium cyanide to produce a good yield of a pyridine oxide. 

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