Photosubstitution reactions of aromatic compounds

Abstract

In nucleophilic aromatic photosubstitution, just as in aromatic substitution in the ground state, substituents can have directing and activating effects. Four rules, which describe orientation of nucleophilic substitution in the excited state, can now be formulated. They are: (a) meta-activation by the nitro group (b) ortho/para-activation by the methoxy group (and probably also by other electron-donating groups) (c) 'x-reactivity' in bi-and tricyclic aromatics (i.e. position 1 in naphthalenes and azulenes, 9 in phenanthrene, 2 and 4 in biphenyls, etc.) (d) merging (resonance) stabilization during product formation.In most nucleophilic aromatic photosubstitutions the reaction proceeds via a it,ir excited triplet state, which interacts with the nucleophile. In some cases the reaction starts from a ir,it excited singlet and in some others there are indications that the aromatic molecule in its excited state undergoes dissociation, producing an ion which subsequently reacts with the nucleophile.Kinetic measurements have revealed the absolute necessity of using rate constants instead of quantum yields as a measure of reactivity. Rate constants for the process in which the triplet excited molecule undergoes deactivation by the nucleophile, leading to substitution product, have been determined for some naphthalene derivatives.From flash photolytic investigations in the nanosecond region evidence has been obtained for the occurrence of a complex (possibly a sigma-complex) formed from the nucleophile and the aromatic molecule in its reactive excited state. The complex may dissociate back into the starting materials or lead to substitution products, while a third route gives rise to the radical anion. The latter cannot be intermediate in the route to substitution product; instead, it may undergo protonation and be transformed into reduction products.INTRODUCTION In this paper a summary is given of our present knowledge on aromatic photosubstitution. The following aspects will be treated: (a) orientation rules; (b) the nature of the reactive excited state and of the primary step; kinetic results; (c) intermediates in aromatic photosubstitution.

High yield photoreagents for protein crosslinking and affinity labeling

“…Here we shall describe a set of reagents that meet most of the above criteria. They are based on the nucleophilic aromatic photosubstitution reactions of nitrophenyl ethers (15)(16)(17)(18)(19).…”

Synthesis of nitropolychlorinated dibenzo-p-dioxins (NPCDDs) and their photochemical reaction with nucleophiles

“…In conclusion, the nitrodibenzo-p-dioxin congeners studied here behave as typical nitro-substitutedethers such as the 3-and 4-nitroveratroles (8,17) in their photochemical reaction with the poorly oxidizable nucleophile ethoxide ion. One triplet excited state is involved, and the photochemical kinetics are completely consistent with substitution by the SN2Ar* mechanism.…”

“…The diazonium salt was added dropwise to a suspension of CuCl (3.6 g, 36 mmol) in concentrated HC1 (33 mL), and the solution was then heated to 80°C for 30 min, diluted with an equal volume of water, and kept at 0°C overnight. After filtration and air drying, the yellow product was crystallized from benzenepetroleum ether, affording 2.21 g (67%) of pure 2,6-dinitrochlorobenzene, mp 8 (19), which gave a very low yield. In a 50-mL round-bottomed flask equipped with a stir bar and a condenser (connected to a drying tube) were placed 3,4,5-trichlorofluorobenzene (1.0 g, 5.0 mmol), fuming nitric acid (3 mL), and fuming sulphuric acid (3 mL) containing 20% of sulphur trioxide.…”

Electrostatic effects on the photoreactions of nitrophenyl ethers with nucleophiles

“…Nucleophilic aromatic photosubstitutions in substituted nitroaromatic compounds have been the object of intense research since their discovery in 1956. 6 A mechanistic borderline between polar S N Ar * 7 and S N Ar * -SET (reactions that take place through single electron transfer from the nucleophile to the excited nitroaromatic substrate) has been established. [8][9][10][11][12] Another interesting borderline exists between the S N Ar*-SET process and the electron transfer initiated photoreduction of the nitro group.…”