Charge-transfer complexes from tetracyanobenzenes

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. 

Photochemical generation of the radical cations derived from A -vinylcar-bazole/acceptor charge-transfer complexes and subsequent polymerization is well known. Perhaps somewhat more interesting are the cationic photopolymerizations of styrene and a-methylstyrene. With these monomers of relatively weak electron donor character photolysis of the charge-transfer complexes formed with tetracyanobenzene and pyromellitic dianhydride produces monomer radical cation species from both singlet and triplet states, and the photophysics of the primary processes have been elucidated in some detail. ... 

Full details have been published of the substitution reactions which result from irradiation of the charge-transfer complexes of 1,2,4,5-tetracyanobenzene and toluene81 and of 7,7,8,8-tetracyanoquinodimethane and toluene.82 The former reaction yields 1-benzyl-2,4,5-tricyanobenzene (44) via, it is suggested, the intermediate 1,4-acyclic adduct (45), and this reaction is quenched by trifluoro-acetic acid. In contrast, the formation of aa-dicyano-4-dicyanomethylbibenzyl... 

Finally, electron transfer within an excited charge-transfer complex may lead to products of reaction without the appearance of the ion radicals presumed to have been formed. Thus, irradiation of tetracyanobenzene in mesitylene at the system s charge-transfer band led to the formation of 2,4,5-tricyanodiphenylmethane in a reaction involving as the initial step the transfer of a proton from the cation radical of mesitylene to the tetracyanobenzene anion radical (Yoshino et al., 1971). Direct detection of the cation radical (and the anion radical) was not made. 

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