Carboxylation of alkanes

The thermally induced carboxylation of alkanes has been thoroughly investigated by the same group, who have developed a range catalysts, based on vanadium (VO(acac)2) or palladium analogs.22,22a,22b Photochemically induced carbonylation of linear alkanes, to afford aldehydes, is also known (Equation (15)).23,23a... 

Although the activation and functionalization of C-H bonds of alkanes are the important, promising routes for synthesis of functionalized materials, it is difficult to achieve the functionalization of alkanes because they are unreactive due to the low reactivity of alkane C-H bonds. Carboxylation of alkanes to carboxylic acids is one of the interesting and important functionalization processes. 

Alkanes also undergo carboxylation with CO, as shown in Scheme 2. Carboxylation of alkanes to carboxylic acids is one of the interesting and important functionalization processes. The first example of carboxylation of alkanes was performed on cyclohexane using a Pd(0Ac)2/K2S208/TFA catalyst system [2]. This carboxylation reaction was extended to gaseous alkanes, for example methane, ethane, and propane. Thus, acetic acid (AcOH) was synthesized from methane and CO in the presence of a Pd catalyst but with a low conversion yield [3]. Recently, the acetic acid synthesis was much improved by using vanadium catalysts such as VO(acac)2 and afforded AcOH almost quantitatively [4],... 

As illustrated in Scheme 3 [la], palladium-catalyzed carboxylation of alkanes may proceed in a fashion similar to the carboxylation of arenes, involving electrophilic attack of cationic [Pd02CCF3]+ species on the C-H bonds of alkanes to give alkyl-Pd(II)-02CCF3 species. 

Surprisingly, CaCl2 was also found to be effective in the carboxylation of alkanes such as ethane, propane, and cycloalkanes by CO in the presence of K2S208 and TFA [6, 7]. With methane acetic acid was formed almost quantitatively by using CaCl2 as catalyst, although a long reaction time (140 h) was required [7]. In TFA, methane and CO could be converted to acetic acid by the catalytic system... 

Transition-metal Catalyzed Carboxylation of Alkanes 599 Yuzo Fujiwara and Tsugio Kitamura... 

Other electrophilic systems investigated include Pd"-acetate mediated oxidation of CH4 to CF3C02CH3 in trifluoroacetic acid by Sen, including a catalytic version of this reaction using H202 as oxidant.149 Direct carboxylation of alkanes including CH4 can be catalyzed by Rh and Pd complexes 150... 

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). 

Alkanes such as methane, ethane, propane, and cyclohexane also undergo carboxylation with CO. The first example of carboxylation of alkanes was performed on cyclohexane using a Pd(0Ac)2/K2S208/TFA catalyst system (Scheme... 

A metal-free system was already established for the carboxylation of alkanes with CO and water, operating in water-acetonitrile and under acid-free conditions. The latter feature (no added acid) is common to a few other catalytic systems active for alkane hydroxylation, namely by taking advantage of a ligand bearing an acid group ( dual-role catalyst ). 

The direct carboxylation of alkanes by CO to give carboxylic acids is a very attractive transformation [12-15, 18-22], as aliphatic carboxylic acids are important commodity chemicals [27], However, their industrial synthetic methods still represent a number of limitations [20, 27], such as the use of relatively expensive olefins and aldehydes as starting materials, the requirement of harsh reaction conditions, multistage transformations, and expensive metal catalysts. 

TABLE 22.2 Carboxylation of Alkanes Catalyzed by Scorpionate or Pyrazole V or Re Complexes ... 

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