Dicyanobenzenes, formation

P 32] Pyrene (20 mM), 1,4-dicyanobenzene (40 mM) and sodium cyanide (1 M) were reacted in propylene carbonate and water. A 100 pi solution of pyrene (20 mM), 1.4-dicyanobenzene (40 mM) in propylene carbonate and a 100 pi solution of sodium cyanide (1 M) in water were fed by programmable dual-syringe pumps via fused-silica capillary tubes into a micro-channel chip [29]. Both solutions were fed with equal flow velocity. A 300 W high-pressure mercury lamp was used as light source. After passing an optical filter made of a CUSO4 solution, the whole chip was irradiated after formation of a stable oil/water interface inside. The oil phase was collected at the exit. 

The synthesis was repeated in 1984 [136], and, because of the stability of phthalocyanines and their catalytic utiHty, these materials have continued to attract attention. Metal phthalocyanines have been used to catalyze selective oxidations under mild conditions, and have been studied as functional models of enzymes. Their formation from dicyanobenzene within a zeolite involves the condensation of the ligand precursors and the supply of two reducing equivalents, which are frequently suggested to be provided by intrazeoHtic water ... 

The symbol rx is the reaction rate of xylene, rM, f i>, and rB are the formation rates of m-tolunitrile, dicyanobenzene, and other products (C02, HCN), respectively. PM and Px are the partial pressures of m-tolunitrile and xylene, respectively. The symbols kx, km, ku k2, ks, k4, and h, are the specific rate constants of the reaction paths of xylene, m-tolunitrile, respectively. The values of the specific rate constants for each reaction path are listed in Table I. 

One of the first examples of metal ions facilitating macrocycle formation was the synthesis of metal phthalocyanines (95) from 1,2-dicyanobenzene or 2-cyanobenzamide (Scheme 53).229-231 This example of macrocycle synthesis has been applied to a very wide range of metal salts of various valencies, and is of great commercial interest. As this area of chemistry has been reviewed frequently and is dealt with in Chapter 21.1, only a brief consideration of the most important template reactions will be presented here. 

The kinetics and mechanism of the phosphorus-catalysed dimerization of acrylonitrile to give 1,4-dicyanobut-l-ene and 2,4-dicyanobut-l-ene have been studied.114 The reactions of aryliminodimagnesium (138) with //-substituted p-ey nobcnzophcnones, l-cyano-9-fluorenenone, o-, m-, and /-dicyanobenzenes, and o-, m-, and p-nitrobenzonitriles have been examined.115 The effect of pressure on the reaction of 3-methyl-l-(4-tolyl)triazene (139) and benzoic acid in chloroform and acetonitrile has been studied.116 The effect of acids on the rate of urethane formation from alcohols and isocyanates in the presence of alkyltin carboxylates has been examined.117 A Hammett a value has been reported for the amidine group N=CHNMe2 and used for the prediction of the basicity of sites in bifunctional amidines.118... 

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. 

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 1966, Walker, Bednar, and Lumry postulated the formation of a 1 2 excited complex state in the system indole-pentane- butanol [101]. Two years later, Beens and Weller found fluorescence emission at 475 nm from an excited complex composed of two molecules of naphthalene and one of 1,4-dicyanobenzene. They postulated the unsymmetrical structure (DD+ A- ) from the solvent dependence of the wavelength of the peak maximum (high dipole moment in contrast to DAD structure) [102]. Later, several other groups detected such termolecular species. For a review on earlier contributions, see Ref. [103]. 

Irradiation of a solution of (l ,3.y)-chrysanthemol 283 in the presence of phenanthrene and dicyanobenzene leads to formation of 3,6-dihydropyran 284. The reaction proceeds via formation and cyclization of the radical cation 285 followed by reaction of the radical intermediate 286 with dicyanobenzene (Scheme 72) <1996JA10954>. 

Scheme 10.3 Formation of MePc complexes in the supercages of FAU-type zeolites via tetramerization of 1,2-dicyanobenzene around transition metal exchanged zeolite.

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... 

One of the problems facing spin chemists performing these measurements is that the observed field effects can be rather small. Thus the method of detection should be as sensitive as possible. In some systems, it is possible to use the inherent fluorescence of one of the species involved in the reaction as a probe of RP activity. The most common of these approaches is the situation with the formation of RlPs that can often lead to spin-selective exciplex formation via the singlet RIP. Systems involving conjugated aromatic molecules, for example, anthracene and pyrene as electron donors/acceptors, amines as electron donors, and substituted benzenes (e.g., dicyanobenzenes) as electron acceptors, have been commonly employed and are now extremely well... 

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. 

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-... 

One of the first examples of metal ions facilitating macrocycle formation was the synthesis of metal phthalocyanines (95) from 1,2-dicyanobenzene or 2-cyanobenzamide (Scheme 53). 

Zimmerman and Hofacker have studied the photochemically induced SET reactivity of the 1,4-dienes (74). The sensitizers used were dicyanoanthracene and dicyanonaphthalene. The radical cations of the 1,4-dienes undergo regioselective cyclization to the cyclic radical cations (75) which ultimately afford the final products (76). The SET-induced photochemistry of other non-conjugated dienes such as geraniol (77) has been studied. The results demonstrate that with DCA as the sensitizer in methylene chloride a contact radical-ion pair is involved and this yields the cyclopentane derivatives (78) and (79) in the yields shown. The cyclization is the result of a five-centre cyclization. With the more powerful oxidant dicyanobenzene as the sensitizer and in acetonitrile as solvent, separated radical-ion pairs are involved and this leads to the formation of the bicyclic ethers (80) and (81). DCA-sensitized reactions of the dienes (82) and E,E-(S3) and the bicyclohexane (84) have been studied. At low conversion the irradiation of (84) under these conditions affords a mixture of the dienes (82) and , -(83) in ratios that are independent of temperature. 

Irradiation of optically pure (l/ ,25 )-(+)-cw-chrysanthemol (22) in the presence of dicyanobenzene as an electron acceptor leads to the formation of (/ )-5-(l-(p-cyanophenyl)-l-methylethyl)-2,2-dimethyloxacyclohex-3-ene (23 Ar = p-cyano-phenyl). The success of this reaction depends upon the relief of ring strain which can be rationalised in terms of nucleophilic attack of the radical cation on the terminal carbon of the vinyl group, and simultaneous replacement of the isopropyl radical as an intramolecular leaving group in an 8 2 reaction. 

The photo-NOCAS process has also been reported with P-myrcene (57) as the reactant. The resultant radical cation, generated using dicyanobenzene as the sensitiser, affords the five products (58-62) shown and cyclization within the myrcene radical cation is an essential feature of this reaction sequence. SET photochemistry of aliphatic electron donors can provide a source of radicals. Thus irradiation of donors such as (63), (64), (65) and (66) results in bond fission and the formation of alkyl radicals which undergo addition to alkenes e.g. 67) or alkynes e.g. 68) to give the adducts (69) and (70), respectively. ... 

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