Alkenes aerobic epoxidation

The first step in the aerobic degradation of alkenes is epoxidation. Epoxidation is then followed by several alternatives. In one of them, the epoxides may nndergo carboxylation the enzyme... 

Pozzi and co-workers have also reported a fluorous soluble cobalt complex, which is active in the aerobic epoxidation of alkenes in a fluorous biphasic system (FBS).[50] The ligand used in this complex was a fluorinated tetraarylporphyrin, with eight perfluorooctyl chains shown in Figure 6.13. The cobalt complex was dissolved in perfluorohexane and added to a solution of the alkene with 2-methylpropanal (aldehyde substrate — 2 1) at room temperature. 

Remarkably JRu (0)2(por)] with sterically bulky ligands can catalyze the aerobic epoxidation of alkenes 332,333 example was demonstrated by [Ru (0)2(TMP)]. With the... 

Although the chiral ketoiminatomanganese(lll) complexes were reported to catalyze the asymmetric aerobic alkene epoxidations, an aldehyde such as pivalaldehyde is required as a sacrihcial reducing agent. Groves reported that the dioxo(porphyrinato)ruthenium complexes 31, prepared with m-chloroperoxyben-zoic acid, catalyzed the aerobic epoxidation without any reductant. " On the basis of these reports, Che synthesized the optically active D4-porphyrin 35 and applied it to the truly aerobic enantioselective epoxidation of alkenes catalyzed by the chiral frani-dioxo (D4-porphyrinato)ruthenium(Vl) complex. The dioxoruthenium complex catalyzed the enantioselective aerobic epoxidation of alkenes with moderate to good enantiomeric excess without any reductant. In the toluene solvent, the turnovers for the epoxidation of T-(3-methylstyrene reached 20 and the ee of the epoxide was increased to 73% ee. 

Nonstereospecific aerobic epoxidations of alkenes in the presence of aldehydes catalyzed by nickel(II),298 iron(in),299 and cobalt(II)300 complexes, and clay-supported nickel acetylacetonate301,302 have been reported. A radical mechanism has been postulated. The involvement of active copper species and peracids were suggested in a similar reaction catalyzed by copper salts.303... 

The disadvantage of this method is that only terminal alkenes give epoxides with high enantiomeric excess. For example, in microsomal aerobic epoxidation pro-pene, trans-2-butene and 2-methyl-2-butene are transformed to the corresponding epoxides with 40%, 14% and 0% ee, respectively.350 Chloroperoxidase, however, was shown to be very effective in the epoxidation of cis alkenes.353... 

Most ruthenium catalysts used in epoxidation reactions are based on bulky porphyrins or other amine ligands and require the use of PhIO and Cl2PyNO as oxidants. For examples see the reviews in Refs. [5,6,45] and some recent examples by Liu and coworkers [46,47] and Jitsukawa et al. [48]. Examples for the aerobic epoxidation of alkenes are the ruthenium mesityl porphyrin complex Ru(TMP)(0)2, where TMP is 5,10,15,20-tetramesitylporphyrinato, of Groves and Quinn [12] in 1985 (Eq. 7), the ruthenium dimethylphenanthroline complex, czs-[Ru(2,9-dimethyl-l,10-phenanthroline)(CH3CN)2]2+ published by Goldstein et al. [23] in 1994 (Eq. 8), and the ruthenium POM catalyst [WZnRu2(0H)(H20)](ZnW9034)2 n of Neumann and Dahan [49] in 1997 (Eq. 9). 

Aerobic oxidation of alkenes with a ruthenium catalyst has been explored by several groups. Groves et al. reported that Ru(TMP)(0)2 (34)-catalyzed aerobic epoxidation of alkenes proceeds under 1 atm of molecular oxygen without any reducing agent [111b]. 

Aqua(phosphine)ruthenium(II) complexes [121] are useful for activation of molecular oxygen, and catalytic oxidation of cyclohexene can be carried out with 1 atm of O2 [121a,bj. The ruthenium catalyst bearing perfluorinated 1,3-diketone ligands catalyzes the aerobic epoxidation of alkenes in a perfluorinated solvent in the presence of i-PrCHO [122]. Asymmetric epoxidations of styrene and stilbene proceed with 56-80% e.e. with ruthenium complexes 38-40 (Figure 3.2) and oxidants such as PhI(OAc)2, PhIO, 2,6-dichloropyridine N-oxide, and molecular oxygen [123-125]. 

G. Pozzi, F. Cinato, F. Montanari, S. Quid, Efficient aerobic epoxidation of alkenes in perfluori-nated solvents catalysed by chiral (salen) Mn complexes, Chem. Commun. (1998) 877. 

Actually the aerobic oxidation of acetaldehyde in acetonitrile solution at RT and atmospheric pressure in oxygen in the presence of alkenes and catalytic amounts of NHPI led to the corresponding epoxides (Table 6.3). No oxidation occurred under the same conditions in the absence of NHPI, clearly indicating that Eq. (6.12) plays a key role in the aerobic epoxidation. 

B. B. Wentzel, S. M. Leinonen, S. Thomson, D. C. Sherrington, M. C. Eeiters, R. J. M. Nolte, Aerobic epoxidation of alkenes using polymer-bound Mukaiyama catalysts, f. Chem. Soc., Perkin Trans. 1 (2000) 3428. 

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. 

We have studied efficiency of MNaY and MNaZSM-5 type zeolites with M= Co(II), Cu(II), Ni(n) and Fe(III) in aerobic epoxidation using /roras-stilbene as model substrate and isobutyraldehyde (IBA) as reductant. The results are summarized in Table 1. Trons-stilbene epoxide was found to be the main oxidation product, isobutyric add being the main product of transformation of IBA. Order of the catalytic activity of the metal ions introduced into NaY zeolites (Co > Cu Ni, Fe, NaY) is similar to that obtained previously for M-substituted heteropolytungstates [13]. Pronounced catalytic activity of CoNaY and NiNaY zeolites was earlier observed for co-oxidation of linear alkenes with acetaldehyde at 70°C [15]. The extents of ion exchange that can be attained for NaZSM-5 catalysts are less than those for NaY... 

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... 

A fluorous version of the cobalt-porphyrin-catalyzed aerobic epoxidation of alkenes was examined in the presence of 2-methylpropanal (Scheme 9). A fluorous porphyrin ring containing four 3,5-diperfluorooctylphenyl groups was prepared and complexed with Co(OAc)2 to form the cobalt catalyst 6. The reaction was carried out with vigorous stirring at room temperature in a biphasic system comprising... 

Several recent studies of Au-catalyzed aerobic epoxidation of aUsenes reported by Caps et al. [170-174], using stilbene as a model alkene and a range of oxidants, demonstrated that O2 is activated by radical species (hence, the solvent has a pronounced effect) and led to the formulation of a strategy for the development of a reference catalyst system. Reproducible and scalable fabrication of the proposed reference catalyst was achieved via direct reduction of AuPPh3Cl in the presence of a silica support functionahzed with dimethylsiloxane. After activation by heating to 200 °C in vacuum, the catalyst was shown to contain 2.9 1.2 nm Au particles. It was highhghted that the activation step (vacuum treatment at... 

Table 21.2. Aerobic epoxidation of alkenes with Fe-substituted POMs.

Group 9 metal-promoted oxidations aerobic epoxidation of alkenes... 

---