Photoinduced radical reactions nucleophilic aromatic substitution

One way of carrying out nucleophilic aromatic substitution reactions under mild conditions is the Ar RNl process, which is initiated by (usually, but not necessarily, photoinduced) electron transfer to an aryl halide, e.g., from an enolate Cleavage of the resulting aryl radical anion with loss of a halide anion gives an aryl radical that combines with the enolate, thus forming the desired aryl-carbon bond. 

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. 

The reactivity of sulfur-centered nucleophiles such as thiourea anion [46] and thioacetate anion [17] in photoinduced SRN1 reaction has been reported as a one-pot, two-step method for the synthesis of several sulfur-aromatic compounds from moderate to good yields. Without isolation, the ArS ions obtained by the aromatic substitution are quenched with Mel to yield ArSMe in a one-pot procedure, together with Ar2S in variable yields, from an SRN1 between ArS- and aryl radicals (Scheme 10.3). 

In the presence of methanol as solvent and 1,4-dicyanobenzene as acceptor, photoinduced electron transfer from 1,4-bis(methylene)cyclohexane gives 4-(methoxymethyl)-1 -methylenecyclohexane and 4-(4-cyanophenyl)-4-(methoxy-methyl)-l-methylenecyclohexane which arise by nucleophilic attack of the solvent on the radical cations, followed either by reduction and protonation, or by combination with the radical anion of the electron acceptor.These observations are in accordance with the proposed mechanism of the nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction. The same group has also investigated the use of cyanide ion as nucleophile and report that irradiation of a mixture of 1,4-dicyanobenzene in the presence of biphenyl as donor, KCN, and 18-crown-6 gives a mixture of (79) and (80). These workers have also extended the scope of NOCAS to fluoride ion. In particular, use of 2,3-dimethylbut-2-ene and 2-methylbut-2-ene gives 4-cyanophenyl substituted... 

Within the year a wide range of photoreactions in which an aromatic residue undergoes change in substitution has been published. As previously, the diversity of the various processes makes any classification of the reactions unrealistic, and so their order of presentation here is somewhat arbitrary. Aromatic photosubstitution reactions have been reviewed by Parkanyi although the treatment is not extensive, the processes of free radical, electrophilic, and nucleophilic photoinduced substitutions of arenes are well covered.Arene photoreactions initiated by electron transfer with electron donors or acceptors are the subject of a review by Pac and Sakurai. The requirements for the efficient photogeneration of the ion radicals are considered and the synthetic utility of the photoreactions, which include reduction, cyanation, and amination, is discussed. 

The photocyanation of pyrene in a microchaimel through an oil-water interface was investigated by Ueno et al. [13]. The microchips employed are made of polystyrene by embossing with a silicone template. The phase transfer reaction proceeds in four steps as depicted in Figure 16.2. In the first step, a photoinduced electron transfer in the oil phase (polycarbonate) occurs from the aromatic hydrocarbon pyrene (DH) to the electron acceptor 1,4-dicyanobenzene (A). The cationic DH " radical is subsequently the target of the nucleophilic attack of the cyanide anion at the oil-water interface. The cyanated product DCN is insoluble in water and goes back into the oil phase. Experiments without a cyanide source (NaCN) in the aqueous phase show no reaction. Hence it can be excluded that the nudeophilic-substituted cyanide originates from the electron acceptor 1,4-dicyanobenzene. 

The photoinduced photochemical substitution reactions of pentafluoronitrobenzene with several nucleophiles in aqueous media and the electrochemical detection of aromatic radical anions are taken as evidence for the presence of radical anion intermediates within the radical chain reaction steps. - ... 

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