Aerobic epoxidation of olefins

Table II. Aerobic epoxidation of olefins catalyzed by Ni(acac)2 ...

TABLE 6.4 Solvent effect for the aerobic epoxidation of olefins catalyzed by NHPI... 

Selective Aerobic Epoxidation of Olefins over NaY and NaZSM-5 Zeolites Containing Transition Metal Ions... 

The controlled oxygenation of alkanes, alkenes, and aromatic hydrocarbons is one of the most important technologies for the conversion of crude oil and natural gas to valuable commodity chemicals. Biomimetic studies of metalloporpltyrins have led to important advances in practical catalysis, especially with ruthenium porphyrins. Reaction of wj-CPBA, periodate, or iodosylbenzene with Ru(II)(TMP)(CO) produced RuCVIjfTMPXOjj . Remarkably, Ru(VI)(TMP)(0)2 was found to catalyze the aerobic epoxidation of olefins under mild conditions. Thus, for a number of olefins including cyclooctene, norbomene, cis-, and trans- -methyl styrene 16-45 equivalents of epoxide were... 

Groves, J.T. and R. Quinn (1985). Aerobic epoxidation of olefins with ruthenium porphyrin catalysts. J. Am. Chem. Soc. 107, 5790-5792. 

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. ... 

Several efficient oxidation reactions with molecular oxygen were developed using transition-metal complexes coordinated by variuos ligands in combination with apprOTriate reductants. Recently, it was found that cyclic ketones such as 2-methylcyclohexanone and acetals of aldehyde such as propionaldehyde diethyl acetal were effectively employed in aerobic epoxidation of olefins catalyzed by cobalt(II) complexes. In the latter case, ethyl propionate and ethanol were just detected in nearly stoichiometric manner as coproducts (Scheme 12), therefore the reaction system is kept under neutral conditions during the epoxidation. 

Scheme 12. The aerobic epoxidation of olefins under neutral conditions...

In 1984, Groves and Quinn reported the formation of a trans-dioxo-ruthenium(VI) porphyrin complex (5) by the reaction of Ru (TMP)CO with either mCPBA or iodosylbenzene [168]. 5 is stable even at room temperature as a diamagnetic compound, thus, NMR spectrum of 5 can be readily available to characterize the structure. The infrared spectrum of 5 showed a strong band at 821 cm" assigned to vru=0 [168]. The most interesting feature of 5 is aerobic epoxidation of olefins, i.e., in the presence of 5, olefins are oxidized by consuming O2 [169]. Further, they found Ru(II) porphyrins catalyzed the cis-trans isomerization of epoxides under mild conditions [170]. Che et al also prepared tr n.s-dioxo-Ru (OEP) complexes in the reaction of Ru (OEP)CO and... 

Epoxidation reactions have been widely utilized for over 100 years with peradds, peroxides and, more recently, metal catalysts [7]. However, direct metal-catalyzed aerobic epoxidations are rare and generally require an aldehyde coreductant. In this case, the metal is proposed to catalyze radical formation (A-C, Scheme 5.2) followed by O2 insertion to form acyl peroxide D. Metal-catalyzed aerobic oxidation of aldehydes to peradds has previously been observed [8]. With the formation of species D, either an outer-sphere path similar to a peracid-type oxidation occurs (Path 1) or an inner-sphere metal-catalyzed oxidation in which the metal-based oxidant and substrate interact during oxygen transfer (Path 2 or 3). Mu-kaiyama and coworkers were the first to report an aerobic epoxidation of olefins catalyzed by transition metals using either a primary alcohol or an aldehyde as coreductants [9]. The role of the metal was probed through parallel studies of peracid and metal-catalyzed epoxidations of 2 which yielded different stereochemical outcomes. Therefore, a metal-centered mechanism for olefin epoxidation was proposed which implicates an oxygenase system. Path 2 or 3 (Table 5.1) [10]. 

Scheme 5.12 Asymmetric aerobic epoxidation of olefins with chiral ruthenium complex 15.

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... 

Beller et al. [85] recently described the aerobic dihydroxylation of olefins catalyzed by osmium at basic pH, as mentioned above. When using the hydroquini-dine and hydroquinine bases, they were able to obtain reasonable enantioselectivities (54% ee to 96% ee) for a range of substrates. An alternative route towards enantiopure diols, is the kinetic resolution of racemic epoxides via enantioselec-tive hydrolysis catalyzed by a Co(III)salen acetate complex, developed by Jacob-... 

The group of Masui first attempted the direct epoxidation of olefins by using oxygen and NHPI with metalloporphyrins, but they obtained poor results [15]. Ishii and coworkers proposed two different methods. In the first protocol [16,17], the epoxidizing agent is obtained in situ by the aerobic oxidation of a suitable alcoholic (benzhydrol) compound in the presence of catalytic amounts of NHPI. The resulting oxidant, which is not able to promote the epoxidation by itself, is then activated in the presence of an olefin by catalytic amounts of hexafluoroacetone (HFA) (Scheme 6.1). 

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. 

Aerobic epoxidation of different alkenes, including a number of natural terpenes, efficiently occurs under mild reaction conditions in the presence of isobutyraldehyde as a reductant and MNaY and MNaZSM-5 type zeolites (M=Co(II), Cu(II), Ni(II) and Fe(III)) as catalysts. Yields of the epoxidation products vary from 80 up to 99% depending on the olefin and catalyst. The reaction proceeds via chain radical mechanism, acylperoxy radicals being the main epoxidizing species. 

Interestingly, dinuclear Pt systems have long been known to allow for catalytic aerobic oxidation of olefins leading to the corresponding olefin oxides and carbonyl compounds [38], Formation of epoxides, in particular, was thought to be a result of an intramolecular C(sp )-0 attack similar to the one in Fig. 13 where the oxygen atom of a p-hydroxoaUcyl intermediate attacks the metal(III)-bound carbon atom (Fig. 14). No studies of C-O reductive elimination have been performed for these systems. 

Mukaiyama s work with the related 6-ketoiminato Mn(III) complex 29 has revealed that this catalytic system induces the aerobic epoxidation of unfunctionalized c/j-olefins with good enantiofacial selectivity, albeit in moderate yield and with significant trans-epoxide formation [94CL1259]. 

Some aerobic oxidation reactions progress effectively in SCCO2 without a catalyst. Aerobic oxidation of olefins (e.g. cw-cyclooctene and (/f)-(-F)-limonene) in the presence of aldehydes (e.g. 2-methyl-propionaldehyde) in SCCO2 (d = 0.75 g/mL) gave the corresponding epoxides without a catalyst. It is speculated that the stainless steel from autoclave walls triggered the formation of acylperoxy radicals from the aldehyde and oxygen, and the reaction proceeded via a non-catalytic radical... 

For the aerobic epoxidation of 1,2-disubstituted olefins, the use of smaller amount of nickel(II) complex was effective to improve yield of the corresponding epoxide (B) Dilution Method). Isovaleraldehyde was found to be remarkably effective for the epoxidation of terminal olefin, and 1,2-epoxy alkane was obtained in good to high yield (C) Isovaleraldehyde Method). 

As the results of the screening of various aldehydes, pivalaldehyde worked quite effectively for the enantioselective aerobic epoxidation of unfunctionalized olefins such as 1,2-dihydronaphthlene derivatives (Scheme 11). It is noted that i) in the case of epoxidation catalyzed by Salen-type complex, addition of a catalytic amount of A -methylimidazole effectively improved the optical yield of epoxide and ii) bulkiness in ester moiety of P-diketone-type catalyst also influenced the optical yield to higher values. 

The present system was applied to the enantioselective epoxidations of various simple olefines. 1,2-Dihydronaphthalenes which contained no function groups were converted into the corresponding optically active epoxides in good yields with good enantioselectivities (52-72% ee. Entries 1-4). The enantioselective aerobic epoxidation of l,2-benzo-l,3-... 

Introduction of bulky and chiral substituents at the 5,10,15,20 (meso)-positions of the porphyrin ring allows for aerobic, enantioselective epoxidation of olefins. Use of chiral Fe(III)- and especially Mn(ni)-... 

Chiral tridentate Schiff base ligands (0,N,0 binding) were recently apphed to the aerobic oxidative kinetic resolution of a-hydroxyesters using V(V) catalysts [54]. Tetradentate (0,N,N,0) salen-type ligands were incompetent in achieving asymmetric induction in these reactions, but use of tridentate Schiff base 22 lead to high values for a variety of a-hydroxy esters (Table 5.3). The catalyst was also chemoselective for alcohol oxidation with no observed epoxidation of olefin substrates (entries 4 and 5). 

Some new square-planar cobalt(III) complexes have been prepared by the reaction of Co(C104)2 6H20 with o-phenylenebis(oxamate) derivatives 23 under aerobic conditions. These complexes have been shown to act as catalysts for the high-yield epoxidation of unfunctionalized tri- and di-substituted olefins (c.g., 24 —> 25) under Mukaiyama conditions. Low yields are obtained with terminal olefins <97TL2377>. 

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