Molecular Oxygen as the Oxidant

Alternative synthetic routes to poly(arylene sulfide)s have been pubHshed (79—82). The general theme explored is the oxidative polymerization of diphenyl disulfide and its substituted analogues by using molecular oxygen as the oxidant, often catalyzed by a variety of reagents ... 

Oxidizing enzymes use molecular oxygen as the oxidant, but epoxidation with synthetic metalloporphyrins needs a chemical oxidant, except for one example Groves and Quinn have reported that dioxo-ruthenium porphyrin (19) catalyzes epoxidation using molecular oxygen.69 An asymmetric version of this aerobic epoxidation has been achieved by using complex (7) as the catalyst, albeit with moderate enantioselectivity (Scheme 9).53... 

Optically Active Lactones from Metal-Catalyzed Baeyer-Villiger-Type Oxidations Using Molecular Oxygen as the Oxidant... 

The protocol for the oxidation of alcohols (10.6.20) has been improved by the use of molecular oxygen as the oxidant in the presence of a catalytic amount of the perruthenate catalyst (10.6.21). Yields are extremely high with relatively short... 

TABLE 11.12. Hydroxylation of phenol using molecular oxygen as the oxidant (pH = 6.5, temperature = 298 K, reaction time = 19 h) ... 

Oxidation as a process to transform biomass into value-added chemicals is a key one. Here, we focus on oxidations using molecular oxygen as the oxidant, with the aim of illustrating selected interesting reactions that could be important in the efforts to develop sustainable chemistry since they only require abundant bio-resources as reactants and have water as the only, or at least the main, byproduct. 

The oxidations in dilute solution discussed in this section mostly involve molecular oxygen as the oxidizing agent. The oxidative step in several catecholamine assay procedures, which usually involves the participation of an inorganic oxidizing agent, and which also occurs in dilute solution, is considered in Section V, E. 

In bulk chemicals manufacture economic considerations usually dictate the use of molecular oxygen as the oxidant. In fine chemicals, on the other hand, other oxidants may be commercially feasible (see table 1). Indeed, other oxidants (e.g. 30% hydrogen peroxide) may even be preferred for reasons of selectivity and ease of handling, i.e. it is not a question of price per se but price/performance ratio. Although molecular oxygen is the least expensive oxidant it requires elaborate safety precautions, and the associated costs, in order to avoid working within explosion limits. 

Attempts to use molecular oxygen as the oxidant failed except in solvents that undergo efficient autoxidation to the corresponding hydroperoxide (e.g., THF). Mechanistic studies, including isotopic labeling studies, indicate that fBuOOH is the source of the oxygen atom incorporated into the product, and the reaction proceeds via a hydride-shift pathway that avoids formation of an enol intermediate (Scheme 12). 

Traditionally, these products were produced using a three-step, chlorine-based, oxidative coupling process (Fig. 1.46). In contrast, Monsanto scientists [133] developed a process involving one step, under mild conditions (< 1 h at 70°C). It uses molecular oxygen as the oxidant and activated charcoal as the catalyst (Fig. 1.46). The alkylaminomercaptobenzothiazole product is formed in essentially quantitative yield, and water is the coproduct. We note that activated charcoal contains various trace metals which may be the actual catalyst. 

Fig. 4.102 Enantioselective epoxidation of phenyl-conjugated olefins employing aldehyde and molecular oxygen as the oxidant.

With molecular oxygen as the oxidant, addition of perfluorinated solvents can help to increase the reaction rate as demonstrated in the Co(acac)2-catalysed oxidation of ethylbenzene to acetophenone. The function of the perfluorohexane solvent is to increase the otherwise low oxygen concentration in the ionic liquid phase. 

This enzyme-based biosensor uses glucose oxidase (GO) as a chemical recognition element, and an amperometric graphite foil electrode as the transducer. It differs from the first reported glucose biosensor discussed in the introduction to this chapter in that a mediator, 1,1 -dimethylferricinium, replaces molecular oxygen as the oxidant that regenerates active enzyme. The enzymatic reaction is given in Eq. 7.15, and the electrochemical reaction that provides the measured current is shown in Eq. 7.16. 

Novel Pathways and Reactants This is a very broad area. We will thus restrict discussion to few examples. The first regards the important reaction of phenol synthesis and the possibility to realize it in one step directly from benzene using molecular oxygen as the oxidant. Various aspects of direct phenol synthesis from benzene are discussed in Chapter 13. We highlight here only recent results that exemplify how starting from the previously cited activity of Re complexes in the epoxidation in homogeneous phase could lead to investigation of the behavior of Re complexes when inserted into the channels of zeolites (ZSM-5) and in gas-phase selective oxidations. This has opened a new unexpected direction. 

Finally, titanium silicates have also been extensively investigated for the epoxida-tion of olefins. The reaction of ethylene over a silver-supported catalyst to ethylene oxide is one of the few large-scale industrial oxidation reactions with molecular oxygen as the oxidant. Numerous studies have shown TS-1 to be effective at selectively forming propylene oxide (PO) from propylene using hydrogen peroxide as the oxidant. This is a more environmentally friendly route to PO than the currently used chlorhydrin route, and it is likely that this process will see commercialization in the near future. 

Considering oxidations employing air or molecular oxygen as the oxidizing reagent, the reactivity of different chemical groups in carbohydrates can be predicted on the basis of thermodynamic and kinetic models. Under ambient conditions, monomeric molecules can be more easily oxidized than polymeric materials but, in any case, the kinetics are very slow, and most of the processes require catalysis. On the basis of reaction models underpinned by experimental data, the reactivity of carbohydrates is more predictable in the case of chemical catalysis than in the case of enzymatic catalysis. 

Using molecular oxygen as the oxidizing agent, the Itoh group has achieved the enantioselective preparation of 3-allyl-3-hydroxyoxindole 90 (85% ee) under phase-transfer conditions with the cinchonidine derived catalyst 89 [54]. The oxindole 90 was further manipulated to a key intermediate that has been applied in a prior synthesis of the hexahydropyrroloindole CPC-1 [55] (Scheme 24). 

Synthetic approaches to xanthones (2005—2012) 12COC2818. Transition-metal catalyzed reactions using molecular oxygen as the oxidant (2007-2012) 12CSR3381. 

Which of the steps in the following biochemical pathways use molecular oxygen as the oxidizing agent ... 

---