Hydrocarboxylation of alkanes

OXYGENATION OF C-H BONDS WITH PEROXIDES 9 TABLE 1.1 Hydrocarboxylation of Alkanes by the l.l/CO/KjSjOj System"... 

This has been achieved in the hydrocarboxylation of alkanes in water-acetonitrile (2 1 -1 2 volume ratio range) medium, with CO and peroxydisulfate (S20g ) (Eq. 2.1). [25, 26, 28, 47-51]. 

Scheme 2.8 Main radical mechanism of the hydrocarboxylation of alkanes with peroxydisulfate, CO, and water, in aqueous (H20/MeCN) medium [51]. The minor 7 (or 7a) to 8 alternative pathway does not concern water as the hydroxylating agent. (5ee insert for color...

SELF-ASSEMBLED MULTICOPPER COMPLEXES AND COORDINATION POLYMERS FOR OXIDATION AND HYDROCARBOXYLATION OF ALKANES... 

TABLE 3.1 Selected Self-Assembled Multicopper(II) Compounds as Catalysts or Catalyst Precursors in Oxidation and Hydrocarboxylation of Alkanes... 

Scheme 3.11 Simplified mechanism for the Cu-catalyzed hydrocarboxylation of alkanes (RH) to carboxylic acids (RCOOH). Adapted from Reference 14.

Early hydrocarboxylation-hydroesterification literature deals largely with Ni and Co as activating metals, but during the last three decades the noble group VIII metals, especially Pd, Pt, Rh and Ir, have been studied. Similarly, the use of pyridine promoted Co catalysts has been optimized. This section will not include references to metals of lesser or more specialized activity, such as Fe, Ru and Cu(I), nor strong acid catalysis, nor oxidative carbonylation of alkanes. 

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). 

We also found recently [8] the first example of alkane hydrocarboxylation in aqueous acetonitrile with the C0/S20g /H20 system catalyzed by an iron complex, that is, ferrocene (Table 1.1). For example, the reaction of propane (1 atm) with CO (10 atm) at 60 °C during 4 h gave isomeric butyric acids in 60% total yield. 

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]. 

We have recently developed a new and cleaner method for the direct and highly efficient hydrocarboxylation of various C alkanes into the corresponding C , j carboxylic acids [18, 31], It consists in reacting an alkane with CO, H2O, and K2S20g, and in the presence of Cu-catalyst (Scheme 3.5). In contrast to prior alkane carboxylation methods [32, 33], this protocol does not require absolute trifluoroacetic acid as a solvent, and undergoes efficiently at mild temperatures (50-60 °C) and in aqueous acid-solvent-free medium (H20/MeCN mixed solvent), wherein water also plays a main role as a reagent, apart from being a component of the solvent system [18]. 

Interestingly, these hydrocarboxylation reactions also occur to some extent in metal-free systems, but the reaction efficiency can be improved significantly by the use of metal catalysts or promoters [18]. Among the variety of different transition metal catalysts, multicopper(II) compounds were usually the most active ones [18, 20], leading to product yields that are circa two to five times superior to those in the metal-free systems. The water-soluble tefracopper(II) complex [Cu4(/x4-0)(/u,3-tea)4 ( u,3-BOH)4][BF4]2 (6) was formerly used as a model catalyst in the hydrocarboxylations of C2-Q alkanes [18, 31]. Since then, the reactions have been optimized further [19-21] and extended to other alkanes and multicopper catalysts, namely including the dimer 2 [22], the trimer 5 [13], the tetramer 7 [14], and the polymers 11 [12], 12 [12], 13 [14], and 15 [15] (Table 3.1). Interestingly, in contrast to alkane oxidation, the hydrocarboxylation reactions do not require an acid cocatalyst. 

Although the hydrocarboxylation of methane to acetic acid has so far been unsuccessful [18, 19], other quite inert gaseous C2-C4 alkanes can be transformed into the corresponding Q+i carboxylic acids, when using the compounds 5, 6, 7, and 11-13 as catalysts or catalyst precursors. Owing to the presence of only primary carbon atoms, C2H6 is the least reactive alkane, the selective transformation of which to propanoic acid occurs with reasonable efficiency (up to 29% yield based on substrate) in the presence of 5 [13] (Scheme 3.6). 

A simplified mechanistic pathway (Scheme 3.11) was proposed [14, 15, 18] for the Cu-catalyzed hydrocarboxylation of various alkanes (RH) on the basis of experimental data, including the analysis of various selectivity parameters [14, 18-21], tests with radical traps [18-20] and 0-labeled H2O [18], DPT calculations, and other studies [18, 33]. It includes the following steps [12-15, 18-22] (1) generation of the alkyl radicals R from an alkane [formed via H-abstraction by... 

The described alkane hydrocarboxylations show a number of important features. In particular, very high product yields (up to 95% based on alkane) can be attained [12-15, 18-22], especially considering the exceptional inertness of saturated hydrocarbons and the fact that such reactions involve C-H bond cleavage, C-C bond formation, and proceed in an acid-solvent-free H20/MeCN medium and at very mild temperatures (50-60 °C). Besides, these hydrocarboxylation reactions contrast with most of the state-of-the-art processes [1, 2] for the relatively mild transformations of alkanes that require the use of strongly acidic reaction media, such as concentrated trifluoroacetic or sulfuric acid, or a superacid. 

The initial steps of the mechanism of the alkane (RH) carboxylation (Scheme 2.9b) [52, 53] are identical to those of the hydrocarboxylation, discussed above, with S20g as the source of the sulfate radical, which acts as H-abstractor from the alkane to give the alkyl radical R, and CO as the carbonylating agent of this radical to yield the acyl radical RC 0. 

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