Tetracyanobenzenes

Fig. 11 Fragment of the crystal structures of tetracyanobenzene/bromide complex showing different modes of anion coordination to the acceptor moieties (coordinates from [24])...

Donor/acceptor association and the electron-transfer paradigm form the unifying theme for the C—C bond cleavage of various benzpinacols and diary-lethane-like donors in the presence of different electron acceptors (such as chloranil (CA), dichlorodicyanobenzoquinone (DDQ), tetracyanobenzene (TCNB), triphenylpyrylium (TPP+), methyl viologen, nitrosonium cation, etc.). Scheme 13 reminds us how this is achieved by either CT photolysis of the D/A pair or via diffusional quenching of the excited electron acceptor A by the electron donor D. 

As a final example the spectra of molecular charge-transfer complexes are considered next. Electron acceptors such as pyromellitic dianhydride, chloranil and tetracyanobenzene... 

The oxidation of 3,6-dehydrohomoadamantane (52) with NO+BF4, photo-excited tetracyanobenzene, and under anodic conditions has been found to involve a common radical cation intermediate. The study has shown that the activation of propellane cTc-c bonds with strong oxidizing electrophiles occurs by a sequence of single-electron transfer steps. These findings are supported by ab initio computations showing that the isomeric radical cations can equilibrate with low barriers and lead to a common product. ... 

TCNB = tetracyanobenzene, TCPA = tetrachlorophtha1ic anhydride, DCNQ. = d i ch 1 orod i cyanoqu i none, HMB = hexamethyl benzene,... 

Keywords tetracyanobenzene, benzyl cyanide, two-component crystal, photo-reaction... 

Nakadaira and coworkers171 described the reaction of 6-trimethylsilylhex-l-ene with 1,2,4,5-tetracyanobenzene in which both the cyclic adduct 336 and the acyclic product 337 were formed (equation 32), and other workers further investigated the mechanistic details of this and related alkylations172. 

True CT complexes are formed between unsaturated electron acceptors, among which the derivatives with cyano and nitro groups predominate. The most common strong electron acceptors are 1,2,4,5-tetracyanobenzene, 7,7,8,8-tetracyanoquinodimethane (1), tetracyano-p-benzoquinone (2), tetracyanoethylene (TCNE) (3) and many electron-donor... 

Ion radicals play a role as mediators in these two-electron transfers. Each one-electron step achieves a maximal rate, and both rate constants become close. Coulombic repulsion of positive (or negative) charges makes the double-charged ion formation difficult. Therefore, donors (or acceptors) are preferable for which some possibility exists to disperse the charge. Extension of the 77-system reduces intramolecular coulombic repulsion in the dianion state. Electron-donor (or electron-acceptor) substituents should be located at diametrically opposite sites of the molecule. Examples are ll,ll,12,12-tetracyano-9, 10-an-thraquinodimethane, TCNQ, DCNQI, and tetracyanobenzene. 

In TCNQ, the cyano groups are in the most remote (face-to-face) positions. This diminishes coulombic repulsion of two surplus electrons in the dianion and, consequently, facilitates an electron transfer along the conductive chain. In tetracyanobenzene, the ring substituents are close to each other. The (anion radical)-to-(dianion) transformation energy appears to be larger than that for TCNQ. Tetracyanobenzene forms only dielectric salts. In contrast, TCNQ is able to give salts with high conductivity. 

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. 

For non-electrophilic strong oxidants, the reaction with an alkane typically follows an outer-sphere ET mechanism. Photoexcited aromatic compounds are among the most powerful outer-sphere oxidants (e.g., the oxidation potential of the excited singlet state of 1,2,4,5-tetracyanobenzene (TCB) is 3.44 V relative to the SCE) [14, 15]. Photoexcited TCB (TCB ) can generate radical cations even from straight-chain alkanes through an SET oxidation. The reaction involves formation of ion-radical pairs between the alkane radical cation and the reduced oxidant (Eq. 5). Proton loss from the radical cation to the solvent (Eq. 6) is followed by aromatic substitution (Eq. 7) to form alkylaromatic compounds. 

Thus, an initial PET reaction between tetracyanobenzene 40 and styrene 41 causes the formation of a radical ion pair. The radical cation of the olefin is trapped by a carbon nucleophile (acetylacetone) and, after deprotonation, the resultant radical reacts with 40 to give an anion that in turn undergoes cyanide loss and yields a... 

In the second case, 2-tert-butyl dimethylmaleate (as a mixture of isomers) was obtained in 62% yield by a PET reaction between 2-tert-butyl-2-phcnyl-l,3-dioxolane anddimethylacetilendicarboxylate, catalyzed by 1,2,4,5-tetracyanobenzene(TCB) [80],... 

Zhi-Feng, L., Yong-Miao, S., Jia-Jun, Y., Hong-Wen, H., and Jian-Hua, X. (2008) Photoinduced three-component reactions of tetracyanobenzene with alkenes in the presence of 1,3-dicarbonyl compounds as nucleophiles. Journal of Organic Chemistry, 73, 8010-8015. 

Formal substitution of a benzyl for a cyano group by a radical resulting from the deprotonation of the radical cation is observed also with other derivatives, e.g., acenaphthene in the reaction with dicyano and tetracyanobenzene [197] or 4,4/-dimethoxybibenzyl in the reaction with 1,4-dicyanonaphthalene. Furthermore, the benzyl radical may result from C-C bond (e.g., bibenzyls) or carbon-heteroatom (in particular Si, Ge, Sn, B) fragmentation. As an example, an oc,oc-dimethylbenzyl group is introduced when using tetramethylbibenzyl [198]. 

Bodige, S. G., Rogers, R. D. and Blackstock, S. C. (1997). Supramolecular networks via pyridine A-oxide C—H O hydrogen bonding in the crystal structures of 2,2 -dithiobis(pyridine A-oxide) and its complexes with 1,2,4,5-tetracyanobenzene and pyromellitic dianhydride. J. Chem. Soc., Chem. Commun., 1669. 

Although aliphatic nitriles take part in alkylation reactions with tri- and tetracyanobenzenes (Sect. 2.1.3), irradiation of DCA and an amine in aqueous acetonitrile takes a different course giving 9-amino-10-cyanoanthracene, a reaction in which the amine serves as the donor, but then the DCA radical anion reacts with acetonitrile functioning as an electrophile leading via an isomerization-elimination process to the final product [32],... 

Lewis and Petisce [44] have investigated PET reactions between a number of cyano aromatic electron acceptors and electron donating methyl aromatic systems. Botb substitution as well as dimer products have been observed depending on the electron affinity of the acceptor [44,45]. When weak electron acceptors, e.g. m-dicyanobenzene and benzonitrile, were used dibenzyl derivatives were formed predominantly. In contrast, strong electron acceptors produced predominantly substitution products. For example, use of tetracyanobenzene with p-xylene produced predominantly in-cage substitution product while use of m-dicyanoben-... 

When an alkene molecule loses an electron, a cation radical is formed. The very reactive cation radical (CH3)2C—CHJ is generated from 2-methyl-propene in light in the presence of TiCl4. It can be detected by ESR in the frozen parent compound at 123 K [172], We assume that at higher temperatures these formations are dimerized to dications. The existence of a donor-acceptor complex is a necessary condition for the mechanism generating cation radicals (see Chap. 3, Sect. 5). a-Methylstyrene is cationically polymerized when illuminated in the presence of tetracyanobenzene in methylene chloride. From the two compounds, of which a-methylstyrene is the donor (D) and tetracyanobenzene the acceptor (A), the donor-acceptor complex is generated in the singlet and triplet states it dissociates to solvated ion radicals [173]... 

In the reaction to produce [74] it will be seen that an amide was used. Reactions of carbamates (Tsujimoto et al., 1979b) and ureas (Miyamoto et al., 1978) with l,l-di(4-cyanophenyl)ethene have been postulated as occurring via radical ions, although as yet there is no spectroscopic evidence for such intermediates. Tetracyanobenzene (Ohashi et al., 1979b) and 1,2-dicyano,3,4,5. 

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