Aromatic Photosubstitution

Nucleophilic aromatic photosubstitution reactions in aqueous solutions and in micellar media has been investigated extensively.42... 

Nuclear motion, the principle of least, and the theory of stereoelectronic control, 24, 113 Nucleophiles, partitioning of carbocations between addition and deprotonation. 35, 67 Nucleophilic aromatic photosubstitution, 11,225 Nucleophilic catalysis of ester hydrolysis and related reactions, 5,237 Nucleophilic displacement reactions, gas-phase, 21, 197... 

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. 

Mechanistic studies also indicate that 4-nitroveratrole (equation 20) and 4,5-dinitroveratrole (equation 21) undergo both singlet and triplet nucleophilic aromatic substitution with ethyl glycinate23. An electron transfer process competes against the nucleophilic aromatic photosubstitution for singlet excited 4-nitroveratrole, causing a decreased product yield in equation 20. 

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. 

In these bimolecular reactions the lifetime of the reacting excited state is a very important factor. In view of lifetime considerations, longer-lived triplets might be more likely candidates for the reacting excited states than singlets. Most nucleophilic aromatic photosubstitutions seem to proceed via (7r,7t )-triplet states, but cases of the intermediacy of singlet states are also known. 

Aromatic nitro compounds, which have been subject to nucleophilic photosubstitution, are... 

Substituents on an aromatic ring may show activating and directing effects in electrophilic substitution. The same situation is encountered in nucleophilic photosubstitution. 

Besides the well-estabhshed nucleophilic photosubstitutions of various groups in aromatic nitro compounds, a small number of other substitutions at excited aromatic nitro compounds have become apparent. 

The Excited State From Which the Nucleophilic Aromatic Photosubstitution Starts. Kinetics. 236... 

Gradually it was recognized that nucleophilic aromatic photosubstitution is a fairly general reaction (Havinga et al., 1967 Havinga and Kronenberg, 1968). It can be realized also with polycyclic and heterocyclic aromatic systems. Various solvents (water, alcohols. 

In many of the cases studied a nitro-group is present as a substituent in the aromatic reactant and one gets the impression that this is favourable to the reaction. On the other hand, quite a few examples are known where no nitro-group plays a role, e.g., in the reactions of anisoles (Bcirltrop et al., 1967 Nilsson, 1971 Lok and Havinga, 1973), in the photocyanation of aromatic hydrocarbons (Vink et al., 1972a), and in the photosubstitution of aromatic ketones (Letsinger and Colb, 1972). 

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. 

Curiously, the earliest examples of aromatic photosubstitution found, the photohydrolysis and photoamination of meta-) nitro-phenyl esters and ethers, already show many of the aspects, the principles and the fundamental questions characteristic of the whole field. 

Although many questions remained to be answered, it looked some years ago as if a consistent picture of the nucleophilic aromatic photosubstitutions was emerging from the experimental data and from charge density calculations. This picture would present an understanding of the photosubstitution process somewhat equivalent to what had been reached in the course of time for the well-known thermal aromatic substitution reactions. 

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