Molecular Oxygen as Terminal Oxidant

Ishi reported on aerobic oxidation of sulfides in the presence of N-hydroxyphtha-limide (NHPI) and alcohols [56]. The reaction works at atmospheric pressure of oxygen however, it requires 80-90 °C, and the selectivity for sulfoxide over sulfone is moderate ( 85-90%). 

The binary system Fe(N03)3-FeBr3 was used as an efficient catalytic system for the selective aerobic oxidation of sulfides to sulfoxides [57]. The reaction works with air 

The mechanism of this aerobic oxidation involves the oxidation of bromide to bromine. The procedure may therefore be limited to sulfides that lack olefinic functionality. 

A method for mild and efficient aerobic oxidation of sulfides catalyzed by HAuCl4/AgN03 was reported by Hill [58]. The active catalyst is thought to be Au(III)Cl2N03(thioether). A very high selectivity for sulfoxide was observed in these oxidations and no sulfone was detected. Isotope labeling studies with H2 0 shows that water and not Og is the source of oxygen in the sulfoxide product. 

They react rapidly with sulfides with transfer of an oxygen to give sulfoxides. 

---

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

There are several typical oxidation products from alkenes, which can be reached via catalytic routes using molecular oxygen as terminal oxidant. We are only considering liquid phase processes catalyzed by transition metal ions or complexes typically below 100-150 C. Many of these homogeneous catalytic reactions occur at or around room temperature. In addition to a single solvent containing the dissolved catalyst complex, phase-transfer conditions involving liquid-liquid or solid-liquid systems will in some cases be described. Likewise,... 

Highly efficient rhodium-catalyzed direct arylations were accomplished through the use of 2,2, 6,6 -tetramethylpiperidine-N-oxyl (TEMPO) as terminal oxidant [17]. Thereby, a variety of pyridine-substituted arenes was regioselectively functionalized with aromatic boronic acids (Scheme 9.5). However, in order for efficient catalysis to proceed, 4equiv. of TEMPO were required. The use of molecular oxygen as terminal oxidant yielded, unfortunately, only unsatisfactory results under otherwise identical reaction conditions. However, a variety of easily available boronic acids could be employed as arylating reagents. 

As an extension of this oxygenative C-C bond fission, cyclic carbonates are prepared in good yield from terminal epoxides and DMF using bismuth bromide as a catalyst and molecular oxygen as an oxidant (Scheme 5.16). This is the first successful use of bismuth bromide as the catalyst for oxidative functionalization [97CC95]. Ph3BiF2 is examined as a catalyst for this reaction [98RCB1607]. 

Mechanism and sulphur oxidation Apart from its intrinsic interest the economic importance of acid corrosion and more lately interest in ore leaching, has stimulated considerable work on the oxidation of sulphur, Fe and Mn. It must be stressed that the Thiobacilli are obligate aerobes, i.e. that depend on molecular oxygen as a terminal electron acceptor. Possible reactions for the oxidation of sulphur are... 

Abstract Palladium-catalyzed oxidation reactions are among the most diverse methods available for the selective oxidation of organic molecules, and benzoquinone is one of the most widely used terminal oxidants for these reactions. Over the past decade, however, numerous reactions have been reported that utilize molecular oxygen as the sole oxidant. This chapter outlines the fundamental reactivity of benzoquinone and molecular oxygen with palladium(O) and their catalyst reoxidation mechanisms. The chemical similarities... 

In practice in the literature of the past 20 years the important results with ruthenium in epoxidation are those where ruthenium was demonstrated to afford epoxides with molecular oxygen as the terminal oxidant. Some examples are presented (see later). Also ruthenium complexes, because of their rich chemistry, are promising candidates for the asymmetric epoxidation of alkenes. The state of the art in the epoxidation of nonfunctionalized alkenes is namely still governed by the Jacobsen-Katsuki Mn-based system, which requires oxidants such as NaOCl and PhIO [43,44]. Most examples in ruthenium-catalysed asymmetric epoxidation known until now still require the use of expensive oxidants, such as bulky amine oxides (see later). 

Other metal-centered catalysts that have been studied include (te)strapped chiral porphyrins derived from L-proline, which can induce modest (< 30%) enantioselectivity <02EJIOC1666>. Supported amidate-bridged platinum blue complexes, which have not yet been applied to chiral epoxidation, but which show promise of utilizing molecular oxygen as the terminal oxidant, have... 

From an economic and environmental perspective, catalytic aerobic alcohol oxidation represents a promising protocol. The use of molecular oxygen as the primary oxidant has several benefits, including low cost, improved safety, abundance, and water as the sole by-product. In this way, many catalytic systems have been used for the aerobic oxidations in ionic hquids as green solvents. Different types of catalysts or catalytic systems useful for the oxidation of alcohols with as terminal oxidant in ionic liquids as solvent will be discussed below. 

In 2002, Ansari et al. reported the conversion of primary and secondary alcohols to corresponding aldehydes and ketones in the ionic liquid, l-butyl-3-methylimidazo-lium hexafluorophosphate ([bmim][PFg]). These conversions catalyzed by TEMPO/ CuCl system in the presence of molecular oxygen as a terminal oxidant at 65°C (Scheme 14.28) [27]. 

Metal-catalyzed epoxidations are becoming important on the industrial scale, since the ability to use molecular oxygen as the terminal oxidant offers considerable operational and environmental benefits. The crucial feedstock propylene oxide 16 can be produced using molecular oxygen and a catalytic system of palladium(II) acetate and a peroxo-heteropoly compound in methanol <04CC582>. A discussion of some quantum chemical calculations with regard to the industrially relevant peroxometal epoxidation catalysts has recently appeared in the literature <04SCR645>. 

Esters are very useful chemical intermediates, in terms of atom economy and versatility, and can be helpful in further transformations. Esterification is one of the fundamental transformations in organic synthesis and is widely used in laboratories and industry [1]. Oxidative esterification of aldehydes with alcohols is an attractive method for the synthesis of esters because aldehydes are readily available raw materials on a commercial scale. Although several facile and selective esterification reactions have been reported [2], the development of a catalytic method for the direct oxidative esterification of aldehydes with alcohols under mild and neutral conditions in the presence of molecular oxygen as the terminal oxidant is highly desirable from both economic and environmental aspects. 

---