Azoxybenzene, synthesis

Zr-containing mesoporous silicas offer some potential for the oxidation of aniline with H2O2 as the oxidant. The Zr is introduced as Zr(OiPr)4 during the micelle-templated hydrothermal synthesis of the MCM-41-type structure (206). The materials are calcined prior to use in catalytic experiments. Azobenzene and azoxybenzene are formed ... 

Ru(II)Cl2(cod)(PhNO)2, prepared by reacting [RuCl2(cod)]n with nitrosobenzene, has been shown to be an active catalyst for the synthesis of azoxybenzene from nitro-soarene and carbon monoxide (Eq. 11.57) [105]. 

Azobenzenes and azoxybenzenes are readily reduced to hydrazobenzenes [60]. The reduction of azo compounds was found to be chemically reversible, since the corresponding hydrazo compounds afford easily azo compounds under similar experimental conditions (note that the mercury electrode - easily anod-ically oxidized - cannot be conveniently used for such experiments). The use of redox cells with both porous electrodes permits the synthesis of azocompounds (a one-pot electrolysis) from azoxy compounds. 

Thermotropic Polymeric Liquid Crystals (PLCs) which are formed by regularly alternating mesogenic elements and flexible spacer groups in the main chain are currently the focus of intensive investigation The standard meso-gens, such as biphenyl, stilbene, azo or azoxybenzene derivatives, which form the core of low molecular mass liquid crystals (LMLCs), are used in synthesis of PLCs as well. 

Formation of carbamates or ureas by carbonylation reactions of nitrosoarenes is not an important reaction. Only in one case has the synthesis of phenylcarbamates been reported to be catalysed by a rhodium complex and the selectivity was very poor (< 33 %) [169]. RhCl(COD)(PhNO) was the best catalyst, but several ruthenium, palladium and platinum complexes were also tested. Since the main product of these reactions was almost always azoxybenzene, they are discussed in Chapter 4. 

The direct catalytic synthesis of azo or azoxy compounds from nitro compounds is not a common reaction. Apart from Fe(CO)5, Ru3(CO)i2 has been reported to catalyse reaction 17 [113], but mononitro compounds failed to give appreciable amounts of azo compounds with this catalyst. Very recently, however, it has been reported that good selectivities (up to 72.5 %) in azoxybenzene can be obtained by using Pd(Phen)(OTf)2 as catalyst in the presence of an excess (best 12.5 1 with respect to palladium) of sodium benzoate [114]. Conditions were 60 bar and 135 °C in methanol as solvent (note that no water was added). The principal by-product was methyl phenylurethane, with minor amounts of aniline and diphenylurea being also formed. The study was principally aimed at evidencing the effect of benzoate anion on the synthesis of carbamates, so no attempt has apparently been made to optimise the experimental conditions for the synthesis of azoxybenzene and better yields can be probably obtained under different conditions. 

Although the catalytic synthesis of azo compounds from nitro compound is not a common reaction, azo- and azoxybenzene are rather common products in... 

Details for the high yield preparation of 5-imino-2-pyrroIidone derivatives and their 5-atkylimino analogues have been provided by Koberda and Lange (Scheme 70). 1,2-Oxazine and its N-substituted derivatives e.g. (181) are elusive and have not knowingly been prepared. An attempted synthesis of these compounds by the cycloaddition of nitrosobenzene to a-pyrone gave instead azoxybenzene and a 2 1 adduct (182) in 24% yield, (Scheme 71). ... 

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