3-Nitroanisole. nucleophilic

Anisole. The simultaneous (homolytic and nucleophilic) annihilations of AN+ as described by the time-resolved spectroscopic results lead to the distinctive mixtures of o- and p-nitroanisoles, in which the isomeric composition is strongly dependent on the nitrating reagent in the following way. 

The nucleophilic aromatic substitutions of 2-fluoro-4-nitroanisole with amines have been shown to be useful as biochemical photoprobes30. Nitrophenyl ethers such as 4-nitroveratrole and 3- or 4-nitroanisole have also been explored as possible photoaflinity labels31. 

Other factors, however, should also be effective. A specific influence of the solvent 101,116) added detergents 12 ) and remote electron donating substituents 119) has been observed. Steric hindrance, which certainly is of influence in the nucleophilic photosubstitution reactions of a-nitronaphthalenes, has been found to alter the reactivity of nitroanisoles... 

The photohydrolysis of 2-fluoro-4-nitroanisole to 2-methoxy-5-nitrophenole is sensitized by benzophenone and completely quenched by sodium sorbate The excited state multiplicity in photoaminations has also been studied. Photolysis of mNA in liquid ammonia yields m-nitroaniline. If the amination is carried out in a large excess of benzophenone, 2-methoxy-4-nitroaniline is formed instead and thus an excited singlet state as reacting species is envisaged in the unsensitized photoamination loo.ioi). it may well be that uptake of the nucleophile present in high concentration successfully competes with intersystem crossing. 

A competition between substitution and two kinds of quenching processes, either dependent or independent on the added nucleophile, is envisaged for the photosubstitutions of p-nitroanisole (pNA) ni.ns). With cyanide ions in aqueous aerated solution pNA is transformed on irradiation into 2-cyano-4-nitroanisole in high yield ns,ii8) probably via addition of cyanide to excited pNA. 

Figure 2. Quantum yield as a function of nucleophile concentration (m-nitroanisole in NH3/CH3OH).

It was recognized at an early stage that the fact that m-nitrophenyl phosphate and m-nitroanisole give a much cleaner and more efficient reaction than their ortho- and para-isomers, in itself, is not sufficient to conclude that the rate of reactions of the excited meta-compound with the nucleophile is higher. Such a more efficient photoreaction might also be explained by a longer lifetime of the excited metacompound, by a different intersystem crossing efficiency, etc. 

These optimistic views received a first blow by the discovery of another category of nucleophilic aromatic photosubstitutions occurring in liquid ammonia as a solvent and nucleophile. In this medium the nitroanisoles still show the pattern that had become familiar, i.e., photosubstitution of OCH3 by NH2 with preference for reaction at the position meta with respect to the nitro-group. However, nitrobenzene, dinitrobenzenes, and nitrohalogenobenzenes... 

In the range of leaving groups, fluorine has been recognized as a valuable substituent that has practically no heavy atom effect and that in many cases is smoothly replaced under the influence of irradiation (Brasem et al., 1972). With 2-fluoro-4-nitroanisole it even proved capable of efficient substitution by the weak nucleophile water, a reaction that has not been equalled by any other substituent. Curiously, the photosubstitution of fluorine by cyanide is generally less efficient than that by other nucleophiles. 

Already, with one of the first nucleophilic aromatic photosubstitutions encountered, curious behaviour was found when studying the rate of reaction as a function of pH. m-Nitrophenyl sulphate shows no increase in the quantum yield of photohydrolysis with increase of hydroxide ion concentration up to values as high as 0-1 M. This behaviour, also found with one other compound (5-chloro-3-nitroanisole), is in clear contradistinction to what is... 

The behaviour of other aromatic nitrocompounds (e.g. m-nitroanisole, 3,5-dimethoxynitrobenzene and 4-nitrobiphenyl) follows the same pattern the same short-lived absorption is produced upon exciting an aromatic compound in the presence of a variety of nucleophilic agents, whereas the lifetime of the species formed depends on the nature of the reagent. 

In certain cases, light promotes substitution reactions in aromatic compounds.34 One of the fascinating features of these reactions is an almost complete change in regioselectivity from that observed in the ground-state reactions. When the nitrocatechol ether 8.1 is irradiated in alkali or in methylamine, the nucleophilic substitution takes place meta to the nitro group. The nucleophilic substitution of p-nitroanisole 8.2, however, takes place para to the X-substituent. 

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