Photochemical reactions aromatic nucleophilic addition

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. 

In polar solvents, a-halomethyl aromatics give rise to photochemical reactions that can be explained by both radical and ionic mechanisms. Equation 12.77 shows the results for irradiation of 1-chloromethylnaphtha-lene (119) in methanol. The most direct pathway for formation of the methyl ether 120 is heterolytic dissociation of the C-Cl bond to give a chloride ion and a 1-naphthylmethyl carbocation, the latter then undergoing nucleophilic addition by the solvent. Indeed, naphthylphenylmethyl carbocations were detected spectroscopically following laser flash photolysis of (naphthylphenylmethyl)triphenylphosphonium chlorides. On the other hand, products 121, 122, and 123 appear to be formed via the 1-naphthylmethyl radical. Therefore, an alternative source of the carbocation leading to 120 could be electron transfer from the 1-naphthylmethyl radical instead of direct photochemical heterolysis of 119.215-216 jaj-g g p. 

Previously, Ohashi and his co-workers reported the photosubstitution of 1,2,4,5-tetracyanobenzene (TCNB) with toluene via the excitation of the charge-transfer complex between TCNB and toluene [409], The formation of substitution product is explained by the proton transfer from the radical cation of toluene to the radical anion of TCNB followed by the radical coupling and the dehydrocyanation. This type of photosubstitution has been well investigated and a variety of examples are reported. Arnold reported the photoreaction of p-dicyanobenzene (p-DCB) with 2,3-dimethyl-2-butene in the presence of phenanthrene in acetonitrile to give l-(4-cyanophenyl)-2,3-dimethyl-2-butene and 3-(4-cyanophenyl)-2,3-dimethyl-l-butene [410,411], The addition of methanol into this reaction system affords a methanol-incorporated product. This photoreaction was named the photo-NO-CAS reaction (photochemical nucleophile-olefin combination, aromatic substitution) by Arnold. However, a large number of nucleophile-incorporated photoreactions have been reported as three-component addition reactions via photoinduced electron transfer [19,40,113,114,201,410-425], Some examples are shown in Scheme 120. 

Steenken, S., Ashokkumar, M., Maruthamuthu, P., McClelland, R. A., Making Photochemically Generated Phenyl Cations Visible by Addition to Aromatics Production of Phenylcyclohexadienyl Cations and Their Reactions with Bases/Nucleophiles, J. Am. Chem. Soc. 1998, 120, 11925 11931. 

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