Photosubstitution Reactions of Aromatic Compounds

Photo-induced aromatic substitution reactions occur through an electron transfer process, which creates an aromatic radical anion or aromatic radical cation as intermediate. This intermediate couples with the electrophile or nucleophile radical to give the product. This mechanism is called Sr I (where the abbreviations stand for substitution, radical, nucleophilic, and first order). Photoirradiation of aromatic compounds in the presence of nucleophiles gives nucleophilic-substituted products different from those of thermal reaction. For example, 3,4-dimethoxynitrobenzene on UV irradiation in presence of hydroxide ion gives 3-hydroxy-substituted product, while on heating gives 4-hydroxy-substituted product [57]. 

Photoirradiadon of phenylacetate dianions 62 with aryl bromides and iodides in liquid ammonia gives isomeric arylated phenyl acetic acids 63 and 64 [58]. 

Irradiation of azulene 65 in presence of aryl iodide gives 1-arylazulene 66 [59]. 

Photochemical nucleophilic-substimtion reactions of cyanobenzene with allenes take place in a radical coupling process at the less heavily substituted radical site by donor-acceptor property. For example, 1,2,4,5-tetracyanobenzene 67 reacts with 1,1-dimethyIallene 68 in the presence of diphenyl to give 69 as major product [60]. 

Possibly diphenyl acts as a co-donor to drive the reaction in the forward direction. The major product of the reaction is formed in a stepwise process as follows  

---

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]. 

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

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. 

Nucleophilic aromatic photosubstitution in general is a rapidly growing field 88-89) which originated from research on light-induced reactions of aromatic nitro compoimds but is by no means restricted to this class of compounds. 

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. 

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. 

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. 

Nucleophilic aromatic photosubstitution reactions of furans and thiophenes substituted with iodo or bromo and electron withdrawing groups in the presence of aromatic compounds have been studied extensively. The aryl nucleophihc substitution reaction mechanism of these compounds is illustrated in Scheme 12. 

Nakagaki, R. and Mutai, K., Photophysical properties and photosubstitution and photoredox reactions of aromatic nitro compounds. Bull. Chem. Soc. Jpn., 69, 261,1996. 

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. 

There is no difference in the ability of photogeneration of radicals between systems that show photosubstitution and systems that do not, for example amines with 3,5-dinitroanisole and m-dinitro-benzene, respectively. This indicates that the formation of radicals from excited aromatic nitro-compounds in the presence of nucleophiles has no direct relation with the photosubstitution reaction. 

---