M-Nitroanisole

When o-, m- and p-nitroanisole with 14C-labelled at the methoxy group were irradiated under identical conditions in methanol in the presence of sodium methoxide, only m-nitroanisole underwent methoxy exchange, with the limiting quantum yield (

labelled isotope experiments support a a complex intermediate and indicate an Sjv23Ar mechanism (direct substitution in the triplet state) for this reaction (equation 12) and for 4-nitroveratroles (equation 13). Further evidence from quenching and lifetime experiments also support a direct displacement SAr2Ar mechanism for the photosubstitution reaction of nitroaryl ethers with hydroxide ions13. 

The photohydrolysis of m-nitroanisole (mNA) has been thoroughly studied. The quantum yield of m-nitrophenol (mNP) formation reaches a limiting value of 0.23 at 0.07 M OH , and there is no wavelength dependency at pH 12 over the 254—334 nm range ). A linear plot of vs. [ OH]-i justifies the assump-... 

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. 

The first systems with meta-activation deliberately investigated by sensitization (and quenching) experiments were m-nitroanisole in liquid ammonia (van Vliet et al., 1970) cmd l-methoxy-6-nitro-naphthalene in alkaline medium (Beyersbergen van Henegouwen and Havinga, 1970), In these two cases indications of a singlet reaction were found. 

With m-nitroanisole in liquid ammonia the benzophenone-sensitized reaction yields inter alia 2-methoxy-4-nitroaniline as a product and no m-nitroaniline, which is formed in very high yield upon direct irradiation in liquid ammonia as well as in NH3/CH3OH. In the latter instance l/4> varies linearly with 1/[NH3], suggesting that the reaction is either singlet or triplet but not of a mixed type. 

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. 

The use of PTC conditions has proved beneficial in several cases and the method of choice in large scale operations. m-Dinitrobenzene was converted to m-nitroanisole in 81 % yield on the mole scale by treatment for 2 h with MeONa in chlorobenzene at 80 C in the presence of a catalytic amount of trioc-tylmethylammonium chloride (Aliquat 336).155 No reaction occurred in the absence of the onium salt catalyst.155 Analogous results were obtained by performing the reaction in HMPA.83... 

A mixture of the tetranitroanisole isomers may be obtained (according to Claes-sen [7]) by the nitration of m- nitroanisole. The compounds are not stable since their nitro groups in the meta position are readily hydrolysed or substituted (van Duin and van Lennep [8]). Their sensitiveness to impact is similar to that of TNT. The expansion they give in a lead block is about 135% of that given by TNT. 

The quantum yields are the same, independent of whether the irradiation takes place with light absorbed in the first or the second ir- -it absorption band (exceptions were found later, e.g., in the case of the azulenes (Section 2), and in the photo-amination of nitrobenzene, (van Vliet et al., 1969). At long wavelength [X > 330 nm (for the photohydrolysis of m-nitroanisole)] a small but significant decrease in is observed, tentatively ascribed to absorption of part of the light by a hidden n - IT band. The obvious conclusion is that the reacting species is the aromatic molecule in its lowest excited... 

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