Aromatic compounds Photosubstitution

Aromatic compounds activated by electron donating groups undergo photosubstitution preferentially in the ortho or para position (5.3) 503). 

Nucleophilic substitution is the widely accepted reaction route for the photosubstitution of aromatic nitro compounds. There are three possible mechanisms11,12, namely (i) direct displacement (S/v2Ar ) (equation 9), (ii) electron transfer from the nucleophile to the excited aromatic substrate (SR wlAr ) (equation 10) and (iii) electron transfer from the excited aromatic compound to an appropriate electron acceptor, followed by attack of the nucleophile on the resultant aromatic radical cation (SRi w 1 Ar ) (equation 11). Substituent effects are important criteria for probing the reaction mechanisms. While the SR wlAr mechanism, which requires no substituent activation, is insensitive to substituent effects, both the S/v2Ar and the Sr+n lAr mechanisms show strong and opposite substituent effects. 

It should be emphasized that the wide scope of nucleophilic aromatic photosubstitution does not imply that it will work indiscriminately with any combination of aromatic compound and nucleophile. On the contrary, there are pronounced selectivities. The general picture now arising shows a field with certainly as much variability and diversification as chemists, in the course of growing experience, have learned to appreciate in the area of classical (thermal) aromatic substitution. It is one of the aims of this article to contribute to a description and understanding of the various reaction paths and mechanisms of nucleophilic aromatic photosubstitution, hopefully to the extent that valuable predictions on the outcome of the reaction in novel systems will become feasible. 

The overall picture certainly became no simpler when quite a few examples were encountered where the nature of the nucleophile is decisive for the position at which the photosubstitution occurs. Just one recent example, taken from the work of Lammers (1974) is given in equation (8). To account for this influence of the nucleophile, the formation of an excited complex between aromatic compound and nucleophile has been suggested as the primary intermediate (de Vries, 1970). 

It is found that generally the polycyclic aromatic compounds undergo nucleophilic aromatic photosubstitution more easily and cleanly than benzene derivatives. Here, even the unsubstituted hydrocarbons prove capable of undergoing light-induced substitution Winketal, 1972a 1972b). 

A third orientation rule presents itself from a survey of the nucleophilic photosubstitutions of bi- and tricyclic aromatic compounds (Vink et al., 1972a Vink et aL, 1972b Lok et al., 1973). 

To test the first hypothesis, solutions of 3,5-dinitroanisole and hydroxide ions were flashed and the absorption spectra at different time intervals after excitation were compared. The absorption ( max 400-410 nm) that remains after all time-dependent absorptions have decayed can be shown to be due to 3,5-dinitrophenolate anion, the photosubstitution product of 3,5-dinitroanisole with hydroxide ion. When the absorption band of the 550-570 nm species is subtracted from the spectrum of the solution immediately after the flash, there remains an absorption at 400-410 nm, which can also be ascribed to 3,5-dinitrophenolate anion. The quantity of this photoproduct does not increase during the decay of the 550-570 nm species. Therefore the 550-570 nm species cannot be intermediate in the aromatic photosubstitution reaction of 3,5-dinitroanisole with hydroxide ion to yield 3,5-dinitrophenolate. Repetition of the experiment with a variety of nucleophiles on this and other aromatic compounds yielded invariably the same result nucleophilic aromatic photosubstitution is, in all cases studied, completed within the flash duration (about 20jLts) of our classical flash apparatus. 

In the absence of nucleophile, neither the 412 nm species nor the formation of the radical anion, nor that of the photosubstitution product is found. It is concluded therefore that the 412 nm species results from some kind of interaction between the (excited) aromatic compound and the nucleophilic reagent. The character of this aromatic compound-nucleophile-complex is as yet unknown. However, in our present view, the nature of the complex has to allow for the formation of both the radical anion and the photosubstitution product(s). An attractive possibility for this complex remains the a-complex, in formal analogy with the Meisenheimer complexes in the thermal nucleophilic reactions with aromatic compounds. An exciplex forms another possibility. 

Heteroaromatic Photosubstitutions C. Parkanyi, Bull. Soc. Chim. Belg., 1981,90,599-608. Photosubstitution Reactions of Aromatic Compounds J. Cornelisse and E. Havinga, Chem. Rev., 1975, 75, 353-388. 

The efficiency and selectivity of direct photoaddition reactions to aromatic compounds are usually not so high. Havinga reviewed direct photosubstitution reactions on aromatic compounds [55]. 

The photoaddition of radicals to benzene rings usually gives substitution products as major products. The photosubstitution and reductive photoaddition occur competitively in the case of polycyclic aromatic compounds. The reductive addition becomes predominant over the substitution with increasing the number of the component rings. 

Many aromatic compounds undergo heterolytic photosubstitution on irradiation. Nucleophilic aromatic substitutions are particularly frequent. The orientation rules are reversed compared to those known for ground-state reactivity that is, electron-withdrawing substituents orient incoming groups... 

E. Havinga, Heterolytic photosubstitution reactions in aromatic compounds in Reactivity of the Photo-... 

Aromatic compounds similarly undergo photoprotonation, as evidenced by their fluorescence quenching in acidic media, acid-catalyzed ipso photosubstitution of alkoxy benzenes, and photochemical deuterium exchange of aromatic ring protons [100,101]. McClelland has summarized in a recent review the observation of transient spectra from cyclohexadienyl cations upon laser flash photolysis of several aromatic compounds in HFIP [5,98,102-105]. 

Photosubstitution of polycyano-aromatic compounds has been achieved by use of organosilicon compounds such as allylic silanes and arylmethylsilanes, and in some cases using alkyl silanes. The primary process in these reactions is the generation of neutral radicals R via radical cations of organosilicon compounds as described above. Addition of R to the radical anions of polycyano-aromatic com-... 

This chapter deals with the photoisomerization, photoaddition and cycloaddition, photosubstitution, intramolecular photocyclization, intra- and inter-molecular photodimerization, photorearrangement reactions of aromatic compounds and related photoreactions. 

The photochemistry of aromatic compounds is classified into the same categories adopted in the previous reviews in the series. The photoisomerization of arylalkenes, photoaddition and cycloaddition to aromatic rings, photosubstitution, photorearrangement reactions have less appeared in the period (2010-2011) considered. On the other hand, the photo-chromism including photoisomerization of azobenzenes and intramolecular photocyclization and cycloreversion of 1,2-diarylethenes, and the photodimerization have been widely developed. Supramolecular Photochemistry as a series of Molecular and Supramolecular Photochemistry was edited by Ramamurthy and Inoue in the period. In addition, it should be noteworthy that so many photochemical reactions in solid and/or crystalline states have appeared and developed. 

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