Acetals metal catalysis

Not all C-H activation chemistry is mediated by transition metal catalysts. Many of the research groups involved in transition metal catalysis for C-H activation have opted for alternative means of catalysis. The activation of methane and ethane in water by the hexaoxo-/i-peroxodisulfate(2—) ion (S2O82) was studied and proceeds by hydrogen abstraction via an oxo radical. Methane gave rise to acetic acid in the absence of external carbon monoxide, suggesting a reaction of a methyl radical with CO formed in situ. Moreover, the addition of (external) CO to the reaction mixture led to an increase in yield of the acid product (Equation (ll)).20... 

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. 

Several mechanistic proposals have been advanced, ranging from metal catalysis to redox cycles with Pd clusters or isolated Pd/PdO species (325-327). It is fair to assume that activation of ethylene occurs by hydrogen abstraction by palladium clusters, whereas H abstraction from acetic acid occurs on PdO (oxidized clusters) ... 

At even higher pH values, metal catalysis is required for chlorination to proceed. Thus, at pH =11, nickel(salen) catalyzes the chlorination of adamantane, cyclohexane and toluene785 and manganese porphyrin promotes the chlorination of cyclohexane786. Selective side-chain chlorination by sodium hypochlorite under PTCs at pH = 8.5 has been used for benzylic chlorinations787,788 and for the functionalization of poly(4-methyl-styrene)789,790. Similarly, with calcium hypochlorite in acetic acid, ring chlorination is reported for toluene, xylenes, anisole and other activated aromatics791. 

A long-standing success in transition metal catalysis is the carbonylation reaction [66], in particular the synthesis of acetic acid [67]. Formally this is the insertion of CO into another bond, in particular into a carbon-halogen bond. After the oxidative addition to the transition metal (the breaking of the carbon-halogen bond), a reaction with a CO ligand takes place. This reaction is often called an insertion. Mechanistic studies have, however, shown that the actual reaction... 

Catalytic reductions over platinum or palladium, which are usually quantitative methods for the identification of organic peroxides, are problematic. Little of the expected 1,2-diol is obtained because the dioxetane fragments into its carbonyl products due to metal catalysis." However, lithium aluminium hydride reduction under subambient conditions affords the expected 1,2-diol quantitatively. Again, the sterically hindered dioxetane (9) is an exception. Here zinc in acetic acid proved successful. ... 

Metathesis of olefins, through the use of transition metal catalysis, is an important application in the petrochemical as well as in the polymer chemical industry for the production of special olefins and polymers (cf. Section 2.3.3). This chemistry is also applicable to unsaturated fatty acid esters, such as acetic acid methyl ester. However, the high price and unsustainability of the catalysts, compared with the fatty acids as substrates, have made commercial utilization not yet possible nevertheless they are of great interest for researchers in this field. 

Activated alkanes, such as cyclohexanone, acetone, and aliphatic nitro compounds, can react with carbon dioxide even without metal catalysis. A typical example is the phenolate-catalyzed reaction of acetophenone with C02 to the corresponding carbon add. A catalytic conversion of the non-activated methane with C02 to give acetic acid was reported by Fujiwara and co-workers [72], The reaction was carried out in the presence of palladium and copper acetate and also stoichiometric amounts of the oxidant K2S208. 

Formation of pyrazines and quinoxalines has been a focal point of many years of study. The most common method to make these rings relies on the condensation of 1,2-diamines with 1,2-dicarbonyls in ethanol or acetic acid in 35-85% yields. The mechanism proceeds through a double imine formation. Recently improved methods have been reported, employing transition-metal catalysis and microwaves. ... 

The authors emphasize the point that they cannot exclude even in their conditions that trace metal catalysis may still be active. Indeed the addition of sequestering agents like EDTA (ethylenediamine tetra-acetic... 

The acceleration of the rate of alkaline hydrolysis of esters by phase-transfer catalysts is greatest for lipophilic carboxylate anions. Organometallic salts can act as phase-transfer catalysts for the alkaline hydrolysis of 4-nitrophenyi acetate and catalysis improves with increasing metal-ion radius. ... 

Industrial Synthesis of Acetic Acid—Transition Metal Catalysis... 

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