P-Dicyanobenzene

Photoamination of 7-methoxy-l,2-dihydronaphthalene in the presence of p-dicyanobenzene similarly affords the amine 153147 and l-benzamido-9,10-anthraquinone reacts with butylamine under UV irradiation in air to yield 154148. 

Various substituted cyclopropanes have been shown to undergo nucleophilic addition of alcoholic solvents. For example, the electron transfer reaction of phenylcyclopropane (43, R = H) with p-dicyanobenzene resulted in a ring-opened ether 44. This reaction also produced an aromatic substitution product (45, R = H) formed by coupling with the sensitizer anion. This reaction is the cyclopropane analog of the photo-NOCAS reaction, but preceded it by almost a decade. 

Substituted cyclopropane systems also undergo nucleophilic addition of suitable solvents (MeOH). For example, the photoinduced ET reaction of 1,2-dimethyl-3-phenylcyclopropane (112, R = Me) with p-dicyanobenzene formed a ring-opened ether by anti-Markovnikov addition. The reaction occurs with essentially complete inversion of configuration at carbon, suggesting a nucleophilic cleavage of a one-electron cyclopropane bond, generating 113. The retention of chirality confirms that the stereochemistry of the parent molecule is unperturbed in the radical cation 112 " ". 

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. 

DCNB p-dicyanobenzene DME dimethoxyethane DMF A.A-dimethylformamide DMSO dimethylsulfoxide Et ethyl -... 

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. 

Photo-NOCAS reactions of p-dicyanobenzene with 2-methylpropene in acetonitrile afforded novel 3,4-dihydroisoquinoline derivatives, as shown in Scheme 132 [482], This photoreaction is initiated by a single electron transfer from olefin to p-dicyanobenzene. Acetonitrile as a nucleophile combined with the alkene radical cation and the resulting radical cation adds to the radical anion of 1,4-di-cyanobenzene. Cyclization to the ortho position of phenyl group followed by loss... 

Hirano et al. reported on the stereoselective cyclization to give tetralin derivatives using the phenanthrene-p-dicyanobenzene sensitizer system. Pandey independently reported the intramolecular photocyclization of methoxybenzene derivatives bearing silyl enol ether chromophore via their heterodimer radical cations in the presence of 1,4-dicyanonaphthalene gave benzo-annulated cyclic ketones in 70% yields [490] (Scheme 133). 

Figure 13. Arrangements of molecules in crystalline p-dicyanobenzene [77] and p-diisocyanobenzene [78] demonstrating a marked difference in packing (see text). 

The efficiency of nucleophilic addition of methanol to the radical cation of arylalkenes can be improved by use of a redox sensitizer system, and also by utilizing solvent and additive effects. Pac and his coworkers found that the aromatic hydrocarbon-electron acceptor sensitizer system, such as phenan-threne-p-dicyanobenzene system, acts as an excellent redox sensitizer for the anti-Markownikoff addition of methanol to 1,1-diphenylethene and indene (Scheme 7) [29]. 

Bmax maximum field. DCNB p-dicyanobenzene. TCNB 1,2,4,5-tetracyanobenzene. At 2,4,6-trimethylphenyl group. See the reference(s) cited in this column. No MFE was observed. XnC=0 xanthone. MC=0 4-methoxybenzophenone. AcC=0 acetophenone. The magnetic field dependence of the MIE was observed. 

The kinetics and mechanism of the phosphorus-catalysed dimeri2ation of acrylonitrile to give 1,4-dicyanobut-l-ene and 2,4-dicyanobut-l-ene have been studied."" The reactions of aryhminodimagnesium (138) with p -substituted p-cyanobenzophenones, l-cyano-9-fluorenenone, o-, m-, and p-dicyanobenzenes, and o-, m-, and p-nitrobenzonitriles have been examined. The elfect of pressure on the reaction of 3-methyl-l-(4-tolyl)triazene (139) and benzoic acid in chloroform and acetonitrile has been studied. The elfect of acids on the rate of urethane formation from alcohols and isocyanates in the presence of alkyltin carboxylates has been examined." A Hammett (T value has been reported for the amidine group N=CHNMe2 and used for the prediction of the basicity of sites in bifrmctional amidines. ... 

FeCp2 in THF. Aromatic substrates such as p-dicyanobenzene and p-methylbenzoate whose standard redox potentials are more negative than —2 V relative to FeCp2 can be reduced (Scheme 18). 

There are many examples of such reactivity and some of these have been reviewed by Roth and coworkers, a research group that is extremely active in this area. An example that is typical of the processes encountered involves the cyclization of the diene geraniol (1). In this case the sensitizer is 9,10-dicyanoanthracene (DCA) and the reactions are carried out in methylene chloride. The authors state that a contact radical-ion parr is involved, i.e. the radical cation of the diene is in close proximity to the radical anion of the DCA. Reaction within this yields the cyclopentane derivatives 2 and 3 in the yields shown. The ring formation is the result of a five centre CC cyclization within the radical cation of 1. When a more powerful oxidant such as p-dicyanobenzene is used as the sensitizer in acetonitrile as solvent, separated radical-ion pairs are involved. This leads to intramolecular trapping and the formation of the bicyclic ethers 4 and 5 . The bicyclic ether incorporates an aryl group by reaction of the radical cation of the diene with the radical anion of the sensitizer (DCB). This type of reactivity is referred to later. Other naturally occurring compounds such as (/fj-f-bj-a-terpineol (6) and (R)-(- -)-limonene (7)... 

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