1.2- Dicyanobenzene anion radical

With respect to photoinitiation, generally, it is important to be very careful in one s choice of sensitizers. For example, attempts to initiate the cyclization of homobenzylic ethers failed if 1,4-dicyanobenzene was used as a sensitizer. Rapid regeneration of the starting material by back-electron transfer from the dicyanobenzene anion-radical to the substrate cation-radical was the cause of cyclization inefficiency. To slow this unproductive process, a mixture of A-methylquinolinium hexafluorophosphate (sensitizer), solid sodium acetate (buffer), and tert-butylbenzene (cosensitizer) in 1,2-dichloroethane was employed. This dramatically increased the efficiency of the reaction, providing cyclic product yields of more than 90% in only 20 min (Kumar and Floreancig 2001, Floreancig 2007). 

The potassium salt of the phthalodinitrile (ort/zo-dicyanobenzene) anion radical also reacts with an electrophile according to the electron transfer scheme. If the electrophile is tert-butyl halide, the reaction proceeds via the mechanism, including at the first-stage dissociative electron transfer from the anion radical to alkyl halide, followed by recombination of the generated tertiary butyl radical with another molecule of the phthalodinitrile anion radical. The product mixture resulting in the reaction includes 4-tert-butyl-1,2-di-cyanobenzene, 2-tert-bytylbenzonitrile, and 2,5-di(tert-butyl)benzonitrile (Panteleeva and co-authors 1998). 

Ti of 10-methylphenothiazine to dicyanobenzene, givining a triplet radical ion pair involing the lO-methylphenothiazine cation and dicyanobenzene anion radicals. In the latter reaction, the Si- Ti ISC of 10-methylphenothiazine is much faster than the reaction from Sj. 

However, the study of the entropy of formation of anion radicals formed by electroreduction of selected organic species such that the charge is localized at an unshielded heteroatom (e.g., / -dicyanobenzene [97]) shows that this parameter is linearly dependent on the acceptor number of solvents. 

Phthalonitrile, which gives two, one-electron waves on polarography in this medium, undergoes decyanation on CPE at the potential of the second wave. There is evidence that the anion radicals of 1,3-dicyanobenzene and 1,3,5-tricyanobenzene may dimerize and lose cyanide to form biphenyl derivatives [151]. 

We will start by considering an intramolecular electron transfer reaction in 1,3-dicyanobenzene (DCB) radical anion... 

Fig. 35 EPR spectrum obtained by in situ electrolysis of 1,2-dicyanobenzene at (a) —1.32 V versus SCE, where the spectrum was shown to be that of 1,2-dicyanobenzene radical anion and (b) —2.35 V versus SCE, where the spectrum corresponds to the benzonitrile anion radical.

Electroreductions of organic catalysts 9-phenylanthracene (9-PA) and l,2-dicyanobenzene > (1,2-DCB or o-phthalonitrile) were chemically reversible and diffusion controlled at cyclic voltammetric (CV) scan rates between about 0.2 and 50 V/s in aqueous 0.1 M CTAB containing 0.1 M tetra-ethylammonium bromide (TEAB). The anion radicals of 9-PA and 1,2-DCB were stabilized against reaction with proton donors by the cationic micellar... 

Alkene radical cations may transfer protons to cyanoaromatic radical anions, followed by coupling of the resulting radicals. For example, 1,4-dicyanobenzene and other cyano-aromatic acceptors form substitution products (e.g., 73) with 2,3-dimethylbutene via coupling and loss of... 

RBSctions of Radical Anions With Radicals. The coupling of arene or alkene radical anions with radicals is an important reaction, and one that has significant synthetic potential. For example, radicals formed by nucleophilic capture of radical cations couple with the acceptor radical anion, resulting in (net) aromatic substitution. Thus, the l-methoxy-3-phenylpropyl radical (113 R = H) couples with dicyanobenzene radical anion loss of cyanide ion then generates the substitution product 132.2 + ... 

Similarly, radical 128 couples with dicyanobenzene radical anion generating the substitution product 133 after loss of cyanide ion. 

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. 

Photoaddition and substitution of electron-deficient aromatic compounds such as o-dicyanobenzene (o-DCNB), p-DCNB, and TCNB by use of group 14 organometallic compounds are classified to the reaction of the radical anions of electron-deficient aromatic compounds with carbon radical species generated... 

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... 

In several photochemical electron transfer reactions, addition products are observed between the donor and acceptor molecules. However, the formation of these products does not necessarily involve direct coupling of the radical ion pair. Instead, many of these reactions proceed via proton transfer from the radical cation to the radical anion, followed by coupling of the donor derived radical with an acceptor derived intermediate. For example, 1,4-dicyanobenzene and various other cyanoaromatic acceptors react with 2,3-dimethylbutene to give aromatic substitution products, most likely formed via an addition-elimination sequence [140]. 

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). 

Generation of the radical cation of aromatic substrates in the presence of sodium boron hydride offers another path for reduction, alternative to that via the radical anion seen in Sect. 2.1.2, and, as one may expect in view of the different mechanism, with a different regiochemistry [174-175], Thus, e.g. irradiation of the xylenes in the presence of m-dicyanobenzene and NaBH4 yields the corresponding 1,4-dihydro derivatives rather than the 2,5-dihydro derivatives obtained with dissolved metals [175]. 

A subsequent study ° from the Arnold group showed an intriguing stereoelectronic effect in oxidative benzylic carbon-hydrogen bond cleavage reactions of substrates 8 and 9 (Scheme 3.7). In this study, electron transfer reactions were conducted in the presence of a nonnucleophilic base. Radical cation formation also weakens benzylic carbon-hydrogen bonds, thereby enhancing their acidity. Deprotonation of benzylic hydrogens yields benzylic radicals that can be reduced by the radical anion of dicyanobenzene to form benzylic anions that will be protonated by solvent. This sequence of oxidation, deprotonation, reduction, and protonation provides a sequence by which epimerization can be effected at the benzylic center. In this study, tram isomer 10 showed no propensity to isomerize to cis isomer 11 (equation 1 in Scheme 3.7), but 11 readily converted to 10 (equation 2 in Scheme 3.7). The reactions were repeated in deuterated solvents to assure that these observations resulted from kinetic rather than thermodynamic factors. Trans isomer 9 showed no incorporation of deuterium (equation 3 in Scheme 3.7) whereas cis isomer 11 showed complete deuterium incorporation. The authors attributed this difference in reactivity to... 

The reaction of phenanthrene and unsaturated compounds, such as furan, 1,1-diphenylethylene and indene, in the presence of electron acceptors and a nucleophile leads to products incorporating the nucleophile (Majima et ai, 1981). Thus furan gives [96], Excitation generates the radical cation of phenanthrene (Phent) and the radical anion of 1,4-dicyanobenzene. Phent oxidises furan to its radical cation which reacts in the manner as shown in Scheme 16. 

Indene gives [97] and [98]. The benzylic radical [99] is probably reduced by the dicyanobenzene radical anion to give its anion which is protonated by the methanol to give [97]. In these reactions phenanthrene is acting purely as a sensitiser. Phenanthrene has been found (Yamada et al., 1977) to react with... 

Mariano and coworkers have investigated the effect of substituents on the a-CH kinetic acidity of several tertiary aromatic amine radical cations generated by electron transfer from the parent amines to the excited state of dicyanobenzene [128]. Laser excitation of 60 40 methanol-acetonitrile solutions of dimethylaniline (DMA) and dicyanobenzene (DCB) result in the formation of the DMA cation radical with an absorption maximum at ca 460 nm and the DCB radical anion with an absorption maximum at ca 340 nm, which decay by back-electron-transfer at diffusion-controlled rates k = 1.1 x 10 s ). Bases such as tetrabutyl-... 

---