Azobenzenes, metal catalyzed

Indoles can be formed by metal-catalyzed cyclizations of azobenzenes on disubstituted alkynes (Equation (106)). 

Another advantage of mesoporous catalysts was the possibility to use bulky peroxides like TBHP as oxidant. As expected, no conversion was observed with TBHP over medium pore zeolites like Ti ZSM-48 or TS-1. Over Ti-Beta or Ti-HMS, the product di.stribution was similar to that obtained with H2O2, i.e. AZY was the major product formed. However, relatively large amounts of azobenzene (AZO) were also detected. AZO is usually formed by reaction of nitrosobenzene with aniline. Nevertheless, the presence of AZO when TBHP was used as the oxidant might be explained by the metal-catalyzed decomposition of the peroxide to give radicals. Indeed, similar observations were made during the oxidation of toluene with TBHP over VAPO-.5 [10]. 

Late transition metal dithiolato complexes possess an aromatic nature, showing reversible redox activity and deep color. The combination of dithi-olene complexes of Ni, Pd, and Pt with the azo group, 85-87, shows reversible photoisomerization and protonation behavior in addition to the protonation-catalyzed cis-to-trans isomerization of the complexes [68,69] The trans-to-cis isomerization of 85-87 in acetonitrile occurs by irradiation of the transition from metalladithiolene n to azobenzene n orbital at A.max = 405 nm ( max = ca. 2 x 104 mol 1 dm3 cm-1), whereas the cis-to-trans isomerization proceeds by photoirradiation at 360 nm in 85, and at 310 nm in both 86 and 87. Immediate cis-to-trans transformations occur when a slight amount of acid is added to a solution containing cis-85-87. 

Metal oxide catalyzed reactions of anilines and benzylamines have been utilized to synthesize azobenzenes and A-benzylidenebenzyl amines in good yields (Scheme 32) [67, 241, 242],... 

In the presence of metal salts, aryldiazo compounds undergo various coupling reactions in aqueous media. Such reactions have been used extensively in the dye and pigment industry and in anal d-ical chemistry. For example, the AICI3-catalyzed diazo couphng of three different 3-substituted-l//-pyrazol-2-in-5-ones in water with seven different aryldiazonium salts yielded 21 different (5-hydroxy-3-substituted-lFf-pyrazol-4-yl)-azobenzene derivatives as colored products with high yields (Eq. 11.67). ... 

The nucleophilicity of an organocobalt(III) species was utilized by Hummel and Ellman to achieve condensation of an azobenzene and an aldehyde to afford an A/ -aryl-2//-indazole derivative (Scheme 10.13) [35]. A combination of a cationic Cp Co complex bearing a noncoordinating B(CgF5)4 anion and acetic acid promotes direct addition of the azobenzene to the aldehyde through azo-directed ortho-metalation and subsequent insertion of the C=0 bond into the aryl-Co bond. While these catalytic steps are likely reversible, intramolecular dehydrative condensation of the azo and the alcohol moieties in the aldehyde adduct serves as a driving force for the formation of the indazole product. The reaction shows a substrate scope comparable to that of the same reaction catalyzed by a Cp Rh catalyst [36] and is applicable to both symmetrical and unsymmetrical azobenzene derivatives and to aromatic, heteroaromatic, and aliphatic aldehydes. 

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