PHOTOCHEMISTRY OF AROMATIC NITRO COMPOUNDS

Considerable interest has been shown in the photochemistry of aromatic nitro-compounds, especially those reactions that involve hydrogen transfer to the nitro-group. Nitrobenzene is photoreduced... 

Topics that have formed the subjects of reviews this year include contemporary issues in electron transport research, dynamics of bimolecular photoelectron transfer reactions, photophysical properties of functionalised fullerene derivatives, carbon-carbon bond formation via radical ions, photoinduced electron transfer processes in ketone, aldehyde, and ester synthesis, photochemical reactions between arenenitriles and benzylic donors, photo-oxidation of conjugated dienes, photoredox reactions of aromatic nitro compounds, electron transfer-mediated photochemistry of some unsaturated nitrogen-containing compounds, reactions of 02( Ag), carbon dioxide activation by aza-macrocyclic complexes, and photochromism of chalcone derivatives. ... 

Again this year several accounts have appeared which describe the cyclization of o-substituents. As these do not involve the aromatic ring directly, only a few examples will be mentioned. Dopp has continued his studies on the photochemistry of aryl nitro-compounds and has further commented upon the cyclization of o-substituents, particularly t-butyl, with the nitro function 168 this and a variety of other light-induced processes of aryl nitro-compounds are the subject of an excellent review by the same author.74 The two nitro-groups in 2,2 -dinitrodiphenylmethane undergo light-induced cyclization and the process is reported to provide a new route to the dibenzo[c,/][l,2]diazepine system (134).16 ... 

The study of the produets of the 2- and 4-(4 -azidostyryl)quinolines photolysis by the electrospr mass spectrometry showed that, as a result of the photodissoeiation of the N-N2 bond, a set of products typical of the azide photoehemistry were formed corresponding arrrines, rritroso, nitro, azo, hydrazo, and azoxy compounds (the last three in insigrriflcant amormts) [74], In the azide photolysis in the presenee of HCl, the ratio of these products changed, and the ehlorine-eontaining eompotmds were formed in addition. There were some produets, whieh eould not be identified by mass spectrometry. In addition, the formation of unsubstituted styrylquinolines was also detected. This finding indicated the photodissoeiation of the C-N3 bond, a proeess that is imeonventional in the photochemistry of aromatic azides. 

A comprehensive treatment of nitro-compound photochemistry is not attempted here and would be beyond the scope of this volume. The discussion is thus restricted to aromatic and heteroaromatic nitro compounds. The reactions preferentially reviewed are those for which at least some experimental information has been given on efforts to elucidate the multiplicity of the reacting excited state, or those in which intermediate excited triplet states seem highly probable for various reasons. 

Elimination of NO and NO2 is observed in the photochemistry of a range of nitro and nitroso compounds, nitrites and nitrates. The photochemical reactions of aromatic compounds with tetranitromethane and other related reagents have been reviewed. These reactions proceed in the simplest cases by addition of the elements of tetranitromethane to the arene, e.g. to give nitro trinitromethyl adducts, but large numbers of other products can be formed by a diverse range of multistep sequences. 

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