Selective olefins aerobic oxidation

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... 

Iron phthalocyanine encapsulated in zeolites was used as oxygen activating catalysts in the triple catalytic aerobic oxidation of hydroquinone to benzoquinone, in the allylic oxidation of olefins and in the selective oxidation of terminal olefins to ketones. The catalyst proved active in the above reactions. It is stable towards self-oxidation and can be recovered and reused. 

In this multi-authored book selected authors in the field of oxidation provide the reader with an up to date of a number of important fields of modern oxidation methodology. Chapter 1 summarizes recent advances on the use of green oxidants such as H2O2 and O2 in the osmium-catalyzed dihydroxylation of olefins. Immobilization of osmium is also discussed and with these recent achievements industrial applications seem to be near. Another important transformation of olefins is epoxidation. In Chapter 2 transition metal-catalyzed epoxidations are reviewed and in Chapter 3 recent advances in organocatalytic ketone-catalyzed epoxidations are covered. Catalytic oxidations of alcohols with the use of environmentally benign oxidants have developed tremendously during the last decade and in Chapter 4 this area is reviewed. Aerobic oxidations catalyzed by N-hydroxyphtahmides (NHPI) are reviewed in Chapter 5. In particular oxidation of hydrocarbons via C-H activation are treated but also oxidations of aUcenes and alcohols are covered. 

Simple palladium(II) salts such as chloride and acetate efficiently catalyse aerobic oxidative A-alkylation of amines and amides with alcohols. This method is suitable for a variety of sulfonamides, amides, aromatic and heteroaromatic amines as well as benzylic and heterobenzylic alcohols with a low loadings of the catalyst (0.5-1 mol%) and the alcohols. A selective carbon-carbon double bond assisted o-C-H olefination is catalysed by palladium(II) acetate. The terminal oxidant is oxygen. Addition of TFA is essential for any meaningful yield. (PdOCOCF3)+ has been proposed as the active catalyst. The observed large difference in the inter- and intra-molecular KIE values implied that the coordination of the C=C bond occurs before C-H palladation in the catalytic cycle consequently, a mechanism involving the initial coordination of allylic C=C bond to (PdOCOCF3)+ followed by selective o-C-H bond metalation has... 

Mukaiyama s conditions have also been used in other aerobic oxidation reactions of substrates including thiols (Table 5.2, entries 1—4, 10 and 11), alkanes (entries 8, 12 and 14) and alcohols (entries 9 and 13), as well as reactions involving lactone formation via a Baeyer-ViUiger oxidation (entries 5-7) and oxidative decarboxylation (entry 16) [15-17]. While nickel, iron and cobalt aU selectively oxidize thiols to sulfoxides, Co(II) is the most active (entries 1—4) [15 b]. Of particular synthetic interest, the chemoselective and diastereoselective aerobic oxidation of the complex sulfide, exomethylenecepham (entries 10 and 11), was observed with no overoxidation to the suUbne or oxidation of the olefin [16 a]. The diverse substrate scope in entries 1-9 suggest iron and nickel species tend to have similar reactivity with substrates, but cobalt behaves differently. For example, both iron and nickel displayed similar reactivity in Baeyer-Villiger oxidations, with cobalt being much less active (entries 5-7), yet the opposite trend was observed for sulfide oxidation (entries 1—4) [15]. Lastly, illustrating the broad potential scope of Mukaiyama-type oxidations, alcohol oxidation (entries 9 and 13) and oxidative decarbonylation (entry 15) reactions, which are oxidase systems, have also been reported [16b, 17b]. 

Pd/Cu-coupled catalysis has been used in many Wacker-type olefin oxidations other than those that involve Markovnikov methyl ketone formation from terminal olefins [la,b, 21]. Pd/Cu-coupled aerobic oxidation systems have also been widely appfied to other sp and sp carbon oxidations. Selected examples of these oxidations, including those involving carbon nucleophiles, oxidative carbo-nylations and oxidative coupling reaction, are pictured in Scheme 5.7 [22, 26]. 

Very recently we have developed a new, easier, and selective metal-free NHPTcatalyzed aerobic epoxidation of primary olefins [19] based on the in situ generation of peracetic acid from acetaldehyde. In this chapter, we will discuss the reaction mechanism in order to explain the significant differences in selectivity with respect to the epoxidation by peracids and we will show preliminary successful results in the synthesis of propylene oxide. 

After screening several reductants in the aerobic epoxidation of olefins catalyzed by nickel(II) complexes, it was found that an aldehyde acts as an excellent reductant when treated under an atmospheric pressure of molecular oxygen at room temperature (Scheme 6). Similar reactions have been reported in the patents. Propylene was monooxygenated into propylene oxide with molecular oxygen in the coexistence of metal complexes and aldehyde such as acetaldehyde " or crotonaldehyde, but the conversion of olefin and the selectivity of epoxide were never reached satisfactory levels. Recently, praseodymium(III) acetate was also shown to be an effective catalyst for the aerobic epoxidation of olefins in the presence of aldehyde. ... 

The use of secondary alcohols as reductants for DODH was first reported by Elhnan, Bergman, and coworkers, who employed Re-carbonyl compounds, e.g., Re2(CO)io, as pre-catalysts under aerobic conditions (Scheme 16) [36]. Optimized conditions used the glycol substrate with the mono-alcohol as the solvent, e.g., 3-octanol, at 150-175°C, with 1-2.5 mol% Re2(CO)io and TsOH as a co-catalyst (2-5 mol%). Good yields of the olefin (50-84%) were obtained with representative glycols. The sy -3,4-decanediol was converted highly selectively to trans-3-decene, implicating a sy -eUmination process in the diol to olefin conversion (Scheme 17). Erythritol was converted moderately efficiently to 2,5-dihydrofuran (62% yield), presumably the result of initial 1,4-diol dehydration followed by DODH of the THF-diol intermediate. The nature of the active catalyst was unknown at the time, but was speculated to be an oxidized Re species. 

Another interesting feature of Pd complexes is their ability to use oxygen as terminal oxidant for oxidative functionalizations. Several examples of oxidative C yi—H olefinations (oxidative Heck reactions) proceed with O2 as a reagent. Furthermore, aerobic dehydrogenative aryl couplings with high site selectivity have been developed. ... 

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