Carboxylation alkane/carbon monoxides

Carboxylic acids can also be formed by a reaction of small alkanes, carbon monoxide, and water on solid acid catalysts (93,94). By in situ C MAS NMR spectroscopy (93), the activation of propane and isobutane on acidic zeolite HZSM-5 was investigated in the presence of carbon monoxide and water. Propane was converted to isobutyric acid at 373 73 K, while isobutane was transformed into pivalic acid with a simultaneous production of hydrogen. On SZA, methyl isopropyl ketone was observed as evidence for the carbonylation of isobutane with carbon monoxide after the sample was held at 343 K for 1 h (94). When the reaction of isobutane and carbon monoxide was carried out in the presence of water, pivalic acid was identified as the main reaction product (94). These observations are rationalized by the existence of a small number of sites capable of generating carbenium ions, which can be further trapped by carbon monoxide (93). 

Carbocations generated from alkanes using superacids react with carbon monoxide under mild conditions to form carboxyUc acid (188). In this process isomeric carboxyUc acids are produced as a mixture. However, when the reaction is mn with catalytic amounts of bromine (0.3 mmol eq) in HF-SbF solution, regio-selective carboxylation is obtained. / -Propane was converted almost exclusively to isobutyric acid under these conditions. 

The Fischer-Tropsch synthesis, which may be broadly defined as the reductive polymerization of carbon monoxide, can be schematically represented as shown in Eq. (1). The CHO products in Eq. (1) are any organic molecules containing carbon, hydrogen, and oxygen which are stable under the reaction conditions employed in the synthesis. With most heterogeneous catalysts the primary products of the reaction are straight-chain alkanes, while the secondary products include branched-chain hydrocarbons, alkenes, alcohols, aldehydes, and carboxylic acids. The distribution of the various products depends on both the type of catalyst and the reaction conditions employed (4). 

Introduction.—The oxidative dehydrogenation of alcohols to aldehydes and ketones over various catalysts, including copper and particularly silver, is a well-established industrial process. The conversion of methanol to formaldehyde over silver catalysts is the most common process, with reaction at 750—900 K under conditions of excess methanol and at high oxygen conversion selectivities are in the region 80—95%. Isopropanol and isobutanol are also oxidized commercially in a similar manner. By-products from these reactions include carbon dioxide, carbon monoxide, hydrogen, carboxylic acids, alkenes, and alkanes. 

Alkyl radicals that formed from alkanes under the action of various radical-like species can react rapidly not only with molecular oxygen, but can also be trapped by carbon monoxide. In this case, carboxylic acids can be obtained. Thus adamantane has been recently converted into 1-adamantanecarboxylic acid as the main product (Scheme II.7) [50]. N-Hydroxyphthalimide was used for this transformation as an efficient radical catalyst. The reaction occurs under mild conditions (CO pressure up to 15 atm and temperature below 100 °C) and gives l-adamantanecarboxyUc acid with selectivity 56% and conversion 75%. 

Superacid DF-SbFs induces protium-deuterium exchange in isobutane [54b]. Strong acids (BF3, BFj-HjO, HF-BFj, CF3SO3H)catalyze carbonylation of alkanes including methane by carbon monoxide [54c], whereas sulfuric acid can induce carbonylation of iso- and cycloalkanes [54e,d]. In both cases, carboxylic acids are obtained. The elimination of molecular hydrogen from alkyl can occur (see a recent theoretical study of the Hz elimination from CzHs [54f]). 

Carbon monoxide at latm has no effect on the voltammetry of cyclohexane in fluorosulfonic acid. In preparative scale electrolysis in fluorosulfonic acid containing 3.0 M water the hydrocarbon cyclohexane affords in 56% current efficiency cyclohexane-carboxylic acid. n-Pentane forms at optimum conditions (latm CO, — 25°C) 74% carboxylic acids in which the alkane skeleton is largely rearranged. The product distribution is 1% 2-methylpentanoic acid, 4% 2-ethylbutanoic acid and 95% 2,2-dimethylbutanoic acid. At higher CO pressure and room temperature only 15% rearranged product is obtained, 54% 2-ethylbutanoic acid and 31% 2-methylpentanoic acid are being formed in a somewhat lower overall yield of 57%. Cyclopentane affords 80% cyclopentanecarboxylic acid. 

Carboxylation of Alkanes and Alkenes. When alkenes react with carbon monoxide and water in the presence of strong mineral acids at elevated temperature and pressure, carboxylic acids are formed (87,88). The transformation is called the Koch reaction and may also be considered as hydrocarboxylation (eq. 61). Neocarboxylic acids with high selectivities are manufactured industrially with this process applying mixed Bronsted and Lewis acid catalysts (H2SO4, H3PO4, HF, and SbFg, BFg). 

It is apparent from the above general mechanism (eqs. 62-64) that any compound able to produce carbocations can be carboxylated. Consequently, not only alkenes but also other hydrocarbons (alkanes, dienes) and alcohols are able to react with carbon monoxide. Furthermore, the intermediate can be quenched by other proton sources such as alcohols, amines, and acids to form carboxylic acid derivatives. 

Saturated hydrocarbons, including branched and unbranched chain alkanes as well as cycloalkanes, react with carbon monoxide in the presence of copper(I) oxide in HSOsF-SbFs to afford ter tiary and secondary carboxylic adds in high yield (eq 20). The reaction proceeds at 0 °C under 1 atm CO. In some cases the reaction involves cleavage of C C bonds and isomerization of the intermediate carbocatlons. 

Recently, we have developed a versatile aqueous medium self-assembly method for the generation of diverse multicopper(II) complexes and coordination polymers derived from cheap and commercially available ligands such as aminoalcohols and benzenecarboxylates [6-15]. The obtained compounds were applied as highly efficient and versatile catalysts or catalyst precursors in two different alkane functionalization reactions. These include the mild oxidation of alkanes (typically cyclohexane as a model substrate) by hydrogen peroxide into alkyl hydroperoxides, alcohols, and ketones [6-9, 11, 16, 17], as well as the hydrocarboxylation of gaseous and liquid C ( = 2 - 9) alkanes, by carbon monoxide, water, and potassium peroxodisulfate into the corresponding carboxylic acids [12-15, 18-22]. 

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