Nucleophilic substitution aromatic, photochemical

Photochemical nucleophilic aromatic substitution 4-Nitrophenyl-a-D- galactopyranoside... 

Nitrophenyl ethers capable of undergoing photochemical nucleophilic aromatic substitution (Cornelisse et al., 1975, 1979 Comelisse and Havinga, 1975 Havinga and Comelisse, 1979) are recent additions to the list of photochemical reagents (Jelenc et al., 1978 Fig. 2.8). 

Klan et al. recently studied temperature-sensitive, regioselective photochemical nucleophilic aromatic substitution of 4-nitroanisole by the hydroxide anion in... 

The second general reaction which proceeds by an SrnI mechanistic pattern involves aryl halides. Aryl halides react with a variety of potential electron donors to give nucleophilic aromatic substitution by a chain mechanism of the SrnI class. Many of the reactions are initiated photochemically and most have been conducted in liquid ammonia solutions. 

Jiang, Z.Q., Marquet, J., Cervera, M., and GaUardo, I., Reductivity activated polar nucleophilic aromatic substitution. 4. Thermal and photochemical behavior of polychloro and polyfluoroni-trobenzenes in front of soft nucleophiles. An. Quim. Int. Ed., 92, 95,1996. 

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). 

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. 

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. 

There has been a study of photo-induced intramolecular cyclization of some o-haloarylheterylamines which may lead to pyridof 1,2-a]benzimidazole derivatives.51 Several studies have been reported of photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reactions with fluoride,52 cyanide,53 or acetonitrile54 acting as the nucleophile, hi the example illustrated in Scheme 3,... 

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. 

In some cases the nucleophilic capture of a radical cation is followed by coupling with the radical anion (or possibly with the neutral acceptor), resulting ultimately in an aromatic substitution reaction. Thus, irradiation of 1,4-dicyanobenzene in acetonitrile-methanol (3 1) solution containing 2,3-dimethylbutene or several other olefins leads to capture of the olefin radical cation by methanol, followed by coupling of the resulting radical with the sensitizer radical anion. Loss of cyanide ion completes the net substitution reaction [144]. This photochemical nucleophile olefin combination, aromatic substitution (photo-NOCAS) reaction has shown synthetic utility (in spite of its awkward acronym). 

The photochemical Nucleophile-Olefin Combination, Aromatic Substitution (Photo-NOCAS) reaction, in Handbook of Organic Photochemistry and Photobiology, 2nd edn (eds W. Horspool and F. Lenci), CRC Press, Boca Raton, pp. 40-1-40-17. 

Photochemical Aromatic Substitution Reactions 6.4.1 Nucleophilic Substitution... 

Arnold, D. R., Chan, M. S. W., McManus, K. A., Photochemical Nucleophile Olefin Combination, Aromatic Substitution (Photo NOCAS) Reaction. 12. Factors Controlling the Regiochemistry of the Reaction with Alcohol as the Nucleophile, Can. J. Chem. 1996, 74, 2143 2166. 

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