Epoxides aerobic alkene oxidation

Khenkin, A.M. and Neumann, R. (2000). Aerobic photochemical oxidation in meso-porous Ti-MCM-41 epoxidation of alkenes and oxidation of sulfides. Catal. Lett. 68(1-2), 109-111... 

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]. 

These results suggest the possibility of using the aerobic oxidation of aldehydes, catalyzed by NHPI, for the epoxidations of alkenes by peracids generated "in situ" under mild conditions [Eq. (6.11)]. 

Aerobic oxidation of alkanes is also possible, using dioxygen as the terminal oxidant. In these cases, Ru-porphyrin and RuCla systems have been shown to oxidize cyclohexane to cyclohexanone in the presence of acetaldehyde, with a fairly high turnover number (TON = 14,100 moles/(mole catalyst-h)). The mechanism for alkane oxidation remains largely unexplored but is suspected to be similar to the oxo-transfer mechanism that governs epoxidation of alkenes (44). 

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

Coenzyme M was shown to function as the central cofactor of aliphatic epoxide carboxylation in Xanthobacter strain Py2, an aerobe from the Bacteria domain (AUen et al. 1999). The organism metabolizes short-chain aliphatic alkenes via oxidation to epoxyalkanes, followed by carboxylation to p-ketoacids. An enzyme in the pathway catalyzes the addition of coenzyme M to epoxypropane to form 2-(2-hydroxypropylthio)ethanesulfonate. This intermediate is oxidized to 2-(2-ketopropylthio)ethanesulfonate, followed by a NADPH-dependent cleavage and carboxylation of the P-ketothioether to form acetoacetate and coenzyme M. This is the only known function for coenzyme M outside the methanoarchaea. 

Owing to their relevance in steroid chemistry Okamoto et al. investigated aerobic allylic hydroxylations of octahydronaphthalene derivatives such as lc in the presence of Fe(III) picolinate complexes Fe(PA)3H20 (Scheme 3.21) [108]. The combination of electrolysis and the Fe(PA)3-02-MeCN system suppressed epoxidation almost completely, leading exclusively to the oxidation products 2c and 3c, albeit with low yields. In contrast, when alkene lc was submitted to chemical oxidation using the... 

Another argument against the oxo-transfer mechanism in our catalytic aerobic oxidation protocol is the lack of formation of sulfoxides from sulfides, N-oxydes from amines and phosphine oxydes from phosphines. Alkenes also proved to be inert towards oxidation no epoxide formation could be detected under our reaction conditions. 

The possibility of asymmetric induction under the fluorous biphase conditions was first speculated upon by Horvath and Rabai [10], and this year has seen the first report of asymmetric catalysis in a fluorous biphase [69]. Two, C2 symmetric salen ligands (29a, b) with four C8Fi7 ponytails have been prepared (Scheme 5) and their Mn(II) complexes evaluated as chiral catalysts for the aerobic oxidation of alkenes under FBS-modified Mukaiyama conditions. Both complexes are active catalysts (isolated yields of epoxides up to 85%) under unusually low catalyst loadings (1.5% cf. the usual 12%). Although catalyst recovery and re-use was demonstrated, low enantioselectivities were observed in most cases. 

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. 

Kinetic curves of alkene consumption ( ) and epoxide accumulation ( ) for aerobic oxidation of fra 5-stilbene (0.3 mmol) in the presence of IB A (1.14 mmol),... 

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... 

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