Acetaldehyde metal complexes

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (76). Cyclopentadiene is also a component of supported metallocene—alumoxane polymerization catalysts in the preparation of syndiotactic polyolefins (77), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (78), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (79). 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

A group at the Academy of Sciences in Moscow 197) has synthesized chiral threonine. Derivatives of cyclic imino acids form copper complexes with glacine and carbonyl compounds. Hydroxyethylation with acetaldehyde and decomposition of the resulting complexes produced threonine with an optical purity of up to 97-100% and with threo/allo ratios of up to 19 1 197). The chiral reagents could be recovered and re-used without loss of stereoselectivity. The mechanism of this asymmetric synthesis of amino acids via glacine Schiff base/metal complexes was also discussed 197). 

The previous examples involve reduction (hydrogenation) of organic molecules, but transition metal complexes can also catalyze oxidation. For example, the Wacker process, which has been widely used to convert ethylene to acetaldehyde, depends on catalysis by palladium(II) in the presence of copper(II) in aqueous HC1. The role of the copper chloride is to provide a means of using air to reoxidize the palladium to palladium(II). Once again, Zeise-type coordination of the ethylene to the metal center is believed to be involved ... 

Woolley230 has found that a number of hydroxo metal complexes of the macrocycle (64) are active in the hvdration of C02 and acetaldehyde. Electronic spectroscopy231 and X-ray crystallography232 suggest coordination numbers in aqueous solution of five for [CoL]2+ and [ZnL]z+, and six for [NiL]2+ and [CuL]2+. For the [ZnL]2+ system, the p/Ca of the coordinated water molecule is 8.69 at 25 °C and the hydroxo complex is a reasonable catalyst for C02 hydration. The complex [Cu(glycylglycinate)OH] is also active in C02 hydration.233... 

Liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical processes,3 "16 including the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p-xylene, acetaldehyde and vinyl acetate from ethylene, propylene oxide from propylene, and many others. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. 

This section is concerned with the activation of hydrocarbon molecules by coordination to noble metals, particularly palladium.504-513 An important landmark in the development of homogeneous oxidative catalysis by noble metal complexes was the discovery in 1959 of the Wacker process for the conversion of ethylene to acetaldehyde (see below). The success of the Wacker process provided a great stimulus for further studies of the reactions of noble metal complexes, which were found to be extremely versatile in their ability to catalyze homogeneous liquid phase reaction. The following reactions of olefins, for example, are catalyzed by noble metals hydrogenation, hydroformylation, oligomerization and polymerization, hydration, and oxidation. 

The oxidative addition of methyl iodide to an unsaturated cobalt carbonyl according to Equation (27) was proposed by Wender, CO insertion gives an acetyl species (28) which is thought to be hydrogenated by cobalt carbonyl hydride or H to yield acetaldehyde [4]. Numerous examples of the oxidative addition of methyl iodide to transition metal complexes with a electron configuration (e.g. Rh Ir ) ate known from the literature [66, 67]. For the carbonytaiion of methanol, the rate has been found to be the oxidative addition of methyl iodide to rhodium(l) [68]. 

The oxidation of ethene by palladium salts in water to give acetaldehyde has been known for 100 years see Oxidation Catalysis by Transition Metal Complexes). It is often called the Wacker Process, after Wacker Chemie GmbH, which first developed the process. The key steps in this oxidation are shown in Scheme 2. Palladium catalyzes the nucleophilic addition of water to ethene, leading to the reduction of Pd to Pd°. Then the palladium is reoxidized back to Pd with Cu salts, giving Cu which in turn is oxidized by oxygen. 

Thioaldehydes have been used as ligands in metal complexes with osmium (76CC1044 77CC901 83JA5939) and rhenium (83JA1056). The first metal complexes with thio- (and seleno)-acetaldehyde as ligands were... 

The reaction is catalyzed by metal complexes, the central atoms favorably being Co or Rh. Nowadays all other routes to acetic acid (especially via acetaldehyde, cf. Section 2.4.1, and its oxidation. Section 2.4.4) are economically obsolete. 

To explain the incorporation of alcohols and aldehydes in Fischer-Tropsch products, hydroxycarbene species M=C(H)OH were invoked by several authors although not on firm organometallic grounds (Scheme 5, mechanism ). We note particularly that metal-attached hydroxycarbene readily eliminates acetaldehyde [27] while condensation steps have not yet been seen in stoichiometric reactions. However, the first C-C bond-making step of this Anderson-Emmett-Kdlbel mechanism [28] corresponds to a hydroxyacetylene HC=C-OH species, of which metal complexes were recently isolated [29]. 

To be historically correct, there were earlier examples of metal-mediated homogeneous catalysis. For example, the Hg -catalyzed hydration of acetylene to acetaldehyde became an industrial process in 1912. There is an intermediate /r-acetylene complex to activate the substrate. The lead chamber process to make sulfuric acid (NO catalysis) is even older but does not involve metals or metal complexes as catalysts [134]. 

It is generally conducted in the liquid phase, in the presence of manganese, cohalt or copper salts (acetatesX by a chain free radical mechanism involving the intermediate formation of peracetic add. This may either decompose to form acetaldehyde and oxygen, or react with the components of the reaction medium to yield a mixed metallic complex of the acetaldehyde and the peradd.. -... 

Aerobic oxidation of alkanes.1 Various metal complexes arc known to catalyze air oxidation of unactivatcd C-—H bonds. Murahashi et al. have found that both ruthenium and iron complexes arc useful catalysts for aerobic oxidation in combination with an aldehyde and an acid. Iron powder is the most effective catalyst, but FeCl3 6H2(), RuCI3 H20, and RuCI2[P(C6H5)3]3 can be used. Useful aldehydes arc hcptanal, 2-mcthylpropanal, and even acetaldehyde. A weak acid is suitable thus acetic acid is preferred to chloroacetic acid. By using the most satisfactory conditions, cyclohexane... 

Using the combination of molecular oxygen, acetaldehyde and N-hydroxyphthalimide (the last compounds is a component - in addition to a metal complex - of the catalyst in the Ishii oxidation reaction), it is possible to oxidize alkylaromatic hydrocarbons and cycloalkanes in acetonitrile at room temperature [40a]. The reaction occurs via a radical mechanism depicted in Scheme II.5. 

Absolute configuration amine metal complexes, 26 Acetaldehyde oximes... 

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. ... 

Isolable transition metal complexes containing hydride and terminal oxo ligands are rare however, Tp Re( = 0)(H)X (X = Cl, H or OTf) and TpRe( = 0)(H)Cl have been synthesized, isolated and characterized. Reactions of Tp Re( = 0)(H) OTf (12) with unsaturated substrates (e.g., ethene, propene or acetaldehyde) result in insertion of C = C or C = 0 bonds into the Re-H bond to yield Tp Re( = 0)(R) (OTf) (R = ethyl or propyl) or Tp Re( = 0)(0Et)(0Tf) (Scheme 6). Oxidation of 12 with pyridine-iV-oxide or DMSO produces Tp Re( = 0)3, acid and free pyridine or dimethylsulfide, respectively. A likely mechanism involves initial oxidation of 12 to produce [Tp Re( = 0)2H][0Tf] (13) followed by the formation of Tp Re( = 0) (OH)(OTf) (14) via a 1,2-migration of the hydride to an oxo ligand (Scheme 6). Reaction of 14 with a second equivalent of oxidant in the presence of base yields Tp Re( = 0)3 (15). Direct deprotonation of 13 is noted as less likely than the pathway shown in Scheme 6 due to the lack of precedent for acidity of related rhenium hydride systems. 

Methyl ethyl ketone, for example is oxidized under mild conditions in the presence of a number of metal complexes in aqueous solution [271-274]. The products of reaction are acetaldehyde and acetic acid. Komissarov and Denisov [272-274] have shown that an iron(III)-o-phenanthroUne complex [272] and a copper(II) pyridine complex [274] catalyze this reaction. In the proposed reaction mechanisms [272, 274] it is suggested that the enolate ion from the ketone is incorporated into the coordination sphere of the metal complex where electron transfer occurs to yield a radical which is attacked by dioxygen, equation (188). In the absence of molecular oxygen, aqueous iron(III) is capable of further oxidizing the radical to form butane 23-dione, equation (189) [271]. 

Azo 11) refers to the azo complex prepared by addition of 2-anisidine4-sulfone-diethyl-amine and 6,8-disulfonaphthol(2) acid to the corresponding metal complex. Selectivities may be enhanced in these cases because small amounts of acetaldehyde were added. 

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