Aerobic oxidation nitrobenzene

The aerobic oxidation of nitrobenzene to 2-nitrophenol, catalysed by H5PV2Moio04o polyoxometalate, under pseudo-zero-order conditions is first order in the catalyst and... 

Even more rare are oxidations with dioxygen [102]. Khenkin et al. found that heating a 0.01 M solution of H5[PVjMOjjO J in neat nitrobenzene at 140°C under 2 bar Oj selectively (>99%) yielded 2-nitrophenol at a 5% maximum yield [102a]. Liu et al. have developed a Cu-catalyzed oxygenase-type aerobic oxidation of arenes [102b]. The yield of 2,3,5,6-tetrachlorophenol reached 89% (Eq. 14.29). 

The aerobic oxidation of nitrobenzene catalyzed by H5[PV2Moio04o] proceeded regioselectively to form 2-nitrophenol in 5% yield (14). On the other hand, a mixture of 2-, 3-, and 4-nitrophenols with a nearly statistical distribution ( 1 0.9 0.45 ratio) was obtained for the aerobic oxidation of nitrobenzene in the presence of l,l -azobis(cyclohexanecarbonitrile) as radical initiator [158]. This study provides the first example of an aerobic regioselective hydroxylation of an arene compound. Such a unique regioselective hydroxylation resulted from an intramolecular interaction between the POM and substrate. 

These complexes have also been used in direct aerobic oxidation catalysis. A recent interesting addition in this area is the chemoselective oxidation of nitrobenzene to p-nitrophenol under aerobic conditions (Scheme 5.23) [82]. Based on... 

The conversion of nitrobenzene to 4-NODPA and 4-NDPA increased with temperature. However, the molar yield of azobezene also increased with rising temperature indicating that under aerobic conditions there must be an oxgyen dependant route for the production of azobenzene that does not consume nitrobenzene. That azobenzene was formed by two separate pathways in the presence and absence of O2 was confirmed by a series of deuterium labeling experiments. When aniline-tf5 was used as solvent and the reaction was conducted in the absence of O2, 4-NODPA-d5, 4-NDPA- 5 and azobenzene-d5 were observed. However, when the identical reaction was conducted in the presence of O2 a significant amount of azobenzene- /jo was produced indicating diat under aerobic conditions the predominant route for the formation of azobezene is via the oxidative coupling of aniline (5). 

In contrast to the case where aniline is used as the nucleophile, the benzamide reaction can be improved by utilizing dioxygen in the reaction mixture since 11 is resistent to autoxidation. Under aerobic conditions the nitrobenzene radical anion is readily trapped by O2 generating superoxide and nitrobenzene (Figure 10) (11). This reaction pathway inhibits the formation of azoxybenzene by diverting the electron transfer cascade and ultimately utilizing dioxygen as the terminal oxidant. Thus, under aerobic reaction conditions 12 is the only observed reaction product. 

Af,A/ -Dimethylaniline is marginally catalytically oxidized by 3.3 x 10 mM benzene solutions of Ru(TMP)(0)2 under aerobic conditions at 50°C perhaps to the p-hydroxy-derivative the turnover of 2.1 after 30 h, was limited by decomposition of the catalyst to Ru(TMP)CO via a decarbonylation process, presumably of the phenolic product . The preliminary findings could be consistent with initial attack of an electrophilic 0X0 moiety at the para-position of the aniline, as suggested for the phenol oxidation (see above), or attaek at the -NMez moiety (see Section 3.7). Of note, methoxybenzene, toluene, chlorobenzene, bromobenzene and nitrobenzene were not oxidized by Ru(TMP)(0)2 . 

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