Nitrogen oxide cocatalysts

Nitrogen oxide (NO, ) cocatalysts [120] have received industrial interest in Pd-catalyzed aerobic oxidations such as oxidative carbonylation (see Section 8.2.2) [18], alkene oxidation [121], and arene acetoxylation [55]. Recent studies from academic literature have provided new insights into the roles of NO in these reactions. Pd-catalyzed aerobic alkene oxidation (Wacker reaction) typically affords methyl ketones arising from Markovnikov addition of water (or hydroxide) to an... 

The stepwise reduction of NO2 to NO occurs at standard reduction potentials very close to the standard potential for the reduction of O2 to H2O (Table 15.1, Eqs. (15.1-15.3)). This relationship implies that NO cocatalysts are able to capture nearly the full thermodynamic driving force of O2 as a terminal oxidant [4]. This favorable feature, together with the kinetically facUe oxidation of NO to NO2 (Table 15.1, Eq. (15.4)), contributes to the effectiveness of NO -based cocatalysts in aerobic oxidation reactions. Depending on the reaction conditions, NO2 can equilibrate with other nitrogen oxide species, such as N2O4, and NO, which could also serve as catalytically relevant oxidants in NO -catalyzed aerobic alcohol oxidation reactions (Eq. (15.5)) [5]. 

Without additives, radical formation is the main reaction in the manganese-catalyzed oxidation of alkenes and epoxide yields are poor. The heterolytic peroxide-bond-cleavage and therefore epoxide formation can be favored by using nitrogen heterocycles as cocatalysts (imidazoles, pyridines , tertiary amine Af-oxides ) acting as bases or as axial ligands on the metal catalyst. With the Mn-salen complex Mn-[AI,AI -ethylenebis(5,5 -dinitrosalicylideneaminato)], and in the presence of imidazole as cocatalyst and TBHP as oxidant, various alkenes could be epoxidized with yields between 6% and 90% (in some cases ionol was employed as additive), whereby the yields based on the amount of TBHP consumed were low (10-15%). Sterically hindered additives like 2,6-di-f-butylpyridine did not promote the epoxidation. 

Complete conversions and good enantiomeric excesses (64-100%) were achieved in the asymmetric epoxidation of chromenes and indene using UHP as oxidant and a novel dimeric homochiral Mn(III) Schiff base as catalyst. The reactions were carried out in the presence of carboxylate salts and nitrogen and oxygen coordinating co-catalysts. However, the epoxidation of styrene unfortunately proceeded with incomplete conversion and only 23% ee. Modification of the catalyst and use of pyridine 7V-oxide as cocatalyst allowed improvement of the ee to 61% (Scheme 18). ... 

The group of Jacobs reports a Mn complex with a tridentate nitrogen (N3) ligand (trimethyltriazacyclonane) catalyst that can be used for the epoxidation with H2O2 of various terminal olefins [167] and cyclohexene [168] (Table 1.6). It was found that cocatalysts such as oxalic acid suppressed solvent oxidation and the ensuing generation of radicals. 

Early reports of Cu-catalyzed aerobic alcohol oxidation focused on simple homogeneous complexes [11] with nitrogenous ligands, such as 2,2 -bipyridine and 1,10-phenanthroline. However, copper-catalyzed methods that include nitroxyl radical or azodicarboxylate cocatalysts exhibit significant synthetic advantages and have been widely investigated in the literature (Figure 6.1). 

The use of the Pt/GDE system with UVA irradiation, i.e., the PEF process, is illustrated in Fig. 2 for the treatment of clofibric acid with 1.0 mM Fe ". The much quicker TOC abatement with > 90 % mineralization at 360 min is due to the contribution of reactions (10) and (11). The photolysis of Fe(III)-oxalate by-products that are quite refractory to the OH-mediated oxidation is the crucial, distinctive event. For some pollutants, the efficiency of the PEF process can be further enhanced by using a metal ion cocatalyst. The action of Co ", Mn ", and Ag", among others, has been surveyed, but the most interesting effect has been demonstrated for the Cu VCu" couple. In the case of indigo carmine dye shown in Fig. 2, for example, the positive synergistic effect of Fe " and Cu " that promotes the quicker mineralization is accounted for by the easier oxidation of some nitrogenated complexes of Cu(II) (e.g., with oxamate) compared with the competitively formed Fe(III) complexes, which are more slowly removed by OH and can only be photodegraded. In this system, Cu" can be... 

---