Aerobic oxidation of alkenes

Figure 6.13. Fluorinated porphyrin ligand used in the cobalt catalysed aerobic oxidation of alkenes.[50]...

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. 

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]. 

Boualleg, M., Guillois, K., Istria, B., etal. (2010). Highly Efficient Aerobic Oxidation of Alkenes over Unsupported Nanogold, Chem. Commun., 46, pp. 5361-5363. 

Pd -catalyzed aerobic oxidations of alkenes with nucleophiles other than water have met with limited success. Most successful reactions utilize nucleophiles that also serve as solvent such as alcohols and acetic acid. In contrast, the use of a heteroatom or carbon nucleophile generally requires stoichiometric quantities of palladium or the secondary oxidant, often Cu salts. Oxidation of alkenes in alcohols with Pd salts in the presence of a base can afford an acetal or a vinyl ether. Alkoxypalladation, which is the first step in both cases, can be followed by 1,2-hydride shift and attack of alkoxide anion on the resulting oxonium cation, affording the corresponding acetal (eq 5, path a). Formation of the vinyl ether can be understood by p elimination of the palladation intermediate (path b). The acetal of acetaldehyde is the main product in the oxidation of ethylene (R = H) while j8-H ehmination is the main path with higher alkenes. 

The supported Co2+-substituted Wells-Dawson POM, Cs6H2[P2W17061Co(OH2)], on silica was stable up to 773 K and catalyzed the heterogeneous oxidation of various aldehydes to the corresponding carboxylic acids with 02 as a sole oxidant [116], The H5PV2Mo10O40 POM, impregnated onto meso-porous MCM-41, catalyzed the aerobic oxidation of alkanes and alkenes using isobutyraldehyde as a... 

Co-containing POMs have been found to be among the most efficient catalysts for homogeneous aerobic oxidation and co-oxidation processes [91-93]. This prompted many researchers to design solid Co-POM-containing materials [78,94-100]. Thus, various Co-POMs have been deposited on cotton cloth [94] and silica [100], datively [95] or electrostatically [96,97] bonded to NH2-modified silica surfaces (vide infra) as well as intercalated in LDHs [78,98,99]. The resulting materials were successfully used for aerobic oxidation of aldehydes, alkenes, alkanes, alcohols and some other organic substrates. 

In new studies heteropoly acids as cocatalysts were found to be very effective in combination with oxygen in the oxidation of ethylene.1311 Addition of phosphomo-lybdic acid to a chloride ion-free Pd(II)-Cu(II) catalyst system results in a great increase in catalytic activity and selectivity.1312 Aerobic oxidation of terminal alkenes to methy ketones can be performed with Pd(OAc)21313 or soluble palladium complexes. Modified cyclodextrins accelerates reaction rates and enhance selectivities in two-phase systems under mild conditions.1315 1316... 

Aqueous biphasic catalysis is also used in homogeneous hydrogenations.117-119 In new examples Ru clusters with the widely used TPPTN [tris(3-sulfonatophenyl) phosphine] ligand120 and Rh complexes with novel carboxylated phosphines121 were applied in alkene hydrogenation, whereas Ru catalysts were used in the hydro-genation of aromatics. Aerobic oxidation of terminal alkenes to methyl ketones was carried out in a biphasic liquid-liquid system by stable, recyclable, water-soluble Pd(II) complexes with sulfonated bidentate diamine ligands.124... 

Gold NPs deposited on carbons are active and selective for mild oxidations in liquid phase although they exhibit almost no catalytic activity in the gas phase. Examples are aerobic oxidation of mono-alcohols, diols, glycerol, glucose, alkenes and alkanes. 

Textbook chemistry (297,298) teaches that palladium is the preferred catalyst for aerobic oxidation of olefins. When water is the solvent, nucleophilic water addition to coordinated olefins is the key step in the so-called Wacker cycle. Wacker oxidation occurs regiospecifically because a carbonyl group is formed at that carbon atom of the double bond where the nucleophile in a Markovnikov-like addition would enter. The Wacker reaction thus yields methylketones from primary alkenes ... 

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). 

It has recently been found42 that Au/C and Au/graphite catalysts are effective for the aerobic oxidation of other alkenes at 353 K small amounts of either hydrogen peroxide or tert-butylhydroperoxide ( BuOOH) are needed as oxygen chain initiators. The products of the reactions of cyclohexene,... 

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)]. 

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),... 

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