Reaction Mechanisms in the Direct Carboxylation of Alcohols for the Synthesis of Acyclic Carbonates

Abstract

Dialkylcarbonates, (RO)<inf>2</inf>CO, can be prepared from alcohols and CO<inf>2</inf>. Such reaction is clean (water is the co-product) but thermodynamically disfavored. In principle, the reaction mechanism of formation of carbonates requires the acid-base activation of alcohols. Existing data support that the first step is the formation of the alkoxo group RO^- that reacts with CO<inf>2</inf> to give the hemicarbonate moiety ROC(O)O^-. The latter converts into the relevant carbonate (RO)<inf>2</inf>CO following different pathways depending on the catalyst used. DFT calculations have been used in a few cases to support the reaction mechanism. Transition states relevant to various mechanistic scenarios have been identified. The results indicated that the relative energies of these transition states depend on the nature of the alkyl group and the molecularity of the reactive step. Organic catalysts, homogeneous-, heterogenized- and heterogeneous-metal systems are discussed in this paper and the known relevant mechanisms compared. Water represents a serious limitation to equilibrium shift to the right and can affect the catalysts. Techniques used to remove water are also discussed

Monomeric alkoxide and alkylcarbonate complexes of nickel and palladium stabilized with the iPrPCP pincer ligand: a model for the catalytic carboxylation of alcohols to alkyl carbonates

“…18e In brief, the trends in the K3 and K1 depend on the different intensity in which the R group influences the thermodynamic stability of the partially ionic (2) x 10 -3 1.6(3) x 10 -2 1. 4(2) x 10 -3 K3, b Pd 5.0(10) x 10 -2 4.5(6) x 10 -2 2.6(8) x 10 -2 -OCOOH). This is an intuitive concept, as it merely confirms that CO2 insertion takes place more readily into the weaker alkoxide M-OR bond than in the stronger M-OH bond of hydroxides.…”

“…3 Direct carboxylation of alcohols with CO2 (Scheme 1, up) is an attractive alternative that leads to organic carbonates directly from alcohols, and water as the only by-product. 4 This is a clean and atom-efficient process that fully complies with the main requisites of green and sustainable chemistry. 5 Unfortunately, direct combination of aliphatic alcohols with CO2 to yield alkyl carbonate and water is only weakly exothermic, and endergonic under the ambient conditions (e. g., for methanol ΔH º ≈ -4 Kcal•mol -1 and ΔG o ≈ + 6 Kcal•mol -1 at 1 atm and 298 K) largely due to the unfavorable entropy contribution of gaseous CO2.…”

“…5 Unfortunately, direct combination of aliphatic alcohols with CO2 to yield alkyl carbonate and water is only weakly exothermic, and endergonic under the ambient conditions (e. g., for methanol ΔH º ≈ -4 Kcal•mol -1 and ΔG o ≈ + 6 Kcal•mol -1 at 1 atm and 298 K) largely due to the unfavorable entropy contribution of gaseous CO2. 4,6 The equilibrium constant rapidly decreases with temperature in such a way that heating to enhance the reaction rate severely limits the maximum theoretical yield, unless the entropy effect is compensated by rising pressure. However, calculations based on standard thermodynamic values have shown that, even at moderately high temperatures, the synthesis of dimethyl carbonate from CO2 and methanol only becomes exergonic under enormous pressures.…”

“…The mechanisms associated to such catalysts were studied in detail by the groups of Ballivet-Tkatchenko, 11,13 Aresta 6a,12b,14 and others. 4,11 In both cases, the carboxylation process begins with the CO2 insertion into M-O bonds to afford species containing a methylcarbonate (or hemicarbonate) linkage, [M]-OCOOMe. However, the processes leading to the release of dimethyl carbonate and regeneration of the reactive alkoxo linkage are less well understood.…”

Conversion of CO2 to Light Olefins Over Iron-Based Catalysts Supported on Niobium Oxide

“…Methanol can also be industrially produced from CO 2 (Pérez-Fortes et al, 2016). Nevertheless, organic carbonates, that are valuable chemicals (Aresta and Dibenedetto, 2002;Sakakura and Kohno, 2009;North et al, 2010;Aresta et al, 2015;Ozorio and Mota, 2017), as well as hydrocarbons (Yang et al, 2017), appear as promising products that could be obtained from CO 2 .…”