Photochemical aromatic nucleophilic

Photochemical Aromatic Substitution Reactions 6.4.1 Nucleophilic Substitution... 

Arnold., D.R. and Mangion, D., The photochemical aromatic-olefin substitution, nucleophile combination reaction, Book of Abstracts, 216th ACS National Meeting, Boston, August 23-27,1998, American Chemical Society, Washington, 1998. 

Oxepin has also been converted photochemically to phenol in 74% yield. This reaction occurs under irradiation conditions by which benzene oxide is excited to a triplet state, e.g. by irradiation in acetone as solvent.207 A rare example for a nucleophilic catalysis of the aromatization of an oxepin/benzene oxide to a phenol has been reported for /err-butyl oxepin-4-carboxylate which undergoes a rearrangement reaction in the presence of trimethylamine to give a mixture of /m-butyl 3-hydroxybenzoate (94%) and 4-hydroxybenzoate (6%).243... 

The photolysis of chlorinated aromatic compounds occurs by several processes which follow predictable routes 13). They frequently undergo photochemical loss of chlorine by dissociation of the excited molecule to free radicals or, alternatively, through a nucleophilic displacement reaction with a solvent or substrate molecule. Either mechanism is plausible, and the operation of one or the other may be influenced by the reaction medium and the presence of other reagents. 

The aryl-thallium bond is thus apparently capable of displacement either by electrophilic or by suitable nucleophilic reagents. Coupled with its propensity for homolytic cleavage (spontaneous in the case of ArTlIj compounds, and otherwise photochemically induced), ArTlXj compounds should be capable of reacting with a wide variety of reagents under a wide variety of conditions. Since the position of initial aromatic thallation can be controlled to a remarkable degree, the above reactions may be only representative of a remarkably versatile route to aromatic substitution reactions in which organothallium compounds play a unique and indispensable role. 

Photoexcited aromatic compounds undergo substitution reactions with (non-excited) nucleophiles. The rules governing these reactions are characteristically different and often opposite to those prevailing in aromatic ground state chemistry 501a,b>, in contrast to the well known ortho/para activation in thermal aromatic substitutions, nitro groups activate the meta position in the photochemical substitution, as shown in (5.1) 502). 

In order to measure the absorption spectra, the radical anions were generated electrochemically in the optical path of a spectrophotometer. The absorption spectrum of 3,5-dinitroanisole radical anion (Figure 11, curve c) is very similar to that of the 550-570 nm species produced photochemically. So we believe this species to be the radical anion formed by electron transfer from the nucleophile to the excited 3,5-dinitroanisole and decaying by interaction with its surroundings including the nucleophile radical cation. The behaviour described seems to be rather general for aromatic nitro-compounds since it is observed with a series of these compounds with various nucleophilic reagents. 

The photochemical nucleophile-olefin combination aromatic substitution (photo-NOCAS) reaction received considerable attention from many groups not only because of its synthetic value because the yields of nucleophile-olefm-arene (1 1 1) adducts can be high but also because of interesting mechanistic details (Scheme 48). 

The photochemical nucleophile olefin combination, aromatic substitution (photo-NOCAS) reaction, formulated below for 2,3-dimethylbutene-methanol-p-dicyano-benzene, has some synthetic utility. The final step, loss of cyanide ion, is not shown. 

The major classes of photochemical reaction for aromatic compounds are nucleophilic substitution and a range of processes that lead to non-aromatic products—valence isomerization, addition or cycloaddition reactions, and cyclization involving 6-electron systems. These five general categories of reaction will be described in the following sections, together with a few examples of more specific processes. 

Radical cations can be formed by irradiation of unsubstttuted aromatic hydrocarbons such as naphthalene, and this makes possible the photochemical displacement of hydride ion by a nucleophile such ascyanide f3.10). Oxygen is not necessary for the success of this type of reaction if a good electron-acceptor is present, such as p-dicyanobenzene (3.11), which enhances the initial photoionization and also provides for reaction with the displaced hydrogen. 

R. A. Rossi and R. H. de Rossi, Aromatic Substitution by the S t Mechanism, American Chemical Society (1983). This extensive review includes a discussion of photochemical nucleophilic substitutions and their mechanisms. 

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