Carboxylation of alcohols

Carboxylation of alcohols or olefins with HCO2H or with CO (super-saturated solution) in cone, sulfuric acid via carbocations, usually with rearrangement. 

Reaction Mechanisms in the Direct Carboxylation of Alcohols, Polyols, Cyclic Ethers, and Cyclic Amines to Afford Monomeric Compounds and Polymeric Materials... 

Abstract This chapter deals with the utilization of CO2 in the carboxylation of alcohols, diols, polyols, and epoxides to create a variety of compounds such as linear carbonates, cyclic monomeric carbonates, and polycarbonates. Homogeneous, heterogenized, and heterogeneous catalysts are described. The problem of water elimination is considered and routes for water-trapping discussed. DPT calculations used to support the reaction mechanism are presented with the identified transition states relevant to various mechanistic scenarios. 

Reaction Mechanisms in the Direct Carboxylation of Alcohols, Polyols. 

The direct carboxylation of alcohols (6.6) and the reaction of alcohols with urea (used as an active form of CO2) (6.7) represent appealing alternatives. The former has some kinetic and thermodynamic barriers, the latter raises NH3 recovery issues and product separation problems. 

In a catalyst-free process, the direct carboxylation of alcohols requires, a priori, two distinct steps as categorized in Scheme 6.1. In (/) an alcohol molecule is activated by a base and an alkoxo-moiety is generated which interacts (//) with CO2 affording the hemi-carbonate, R0-C(0)0. The acid activation of the second molecule of alcohol Hi) produces the alkyl moiety which reacts with the hemi-carbonate and... 

Scheme 6.1 Putative reaction path fra- the carboxylation of alcohols. Adapted with permissitm from [28]. Copyright (2006) Springer Scitaice +Business...

Scheme 6.2 Nb-hemicarbonate as active species in the direct carboxylation of alcohols with Nb-alkoxo catalysts. Adapted with permission from [34]. Copyright (2014) American Chemical Society...

Niobium-Based Catalysts Other soluble alkoxides such as those of titanium (IV) [27, 35, 36] and Group 5 metals [21, 28, 37] are active catalysts in the direct carboxylation of alcohols. The reaction mechanism has been elucidated with penta-alkoxo species of Group 5 elements [21, 28, 37] combining DFT and experimental studies. The overall reaction mechanism is depicted in Scheme 6.2. 

Taking into account what has been discussed above, it appears that heterogeneous catalysts would be the best candidates to drive the carboxylation of alcohols. In this paragraph their use is discussed and the known aspects of the reaction mechanisms reported. The catalysts are divided into two classes single-metal and multi-metal catalysts. 

The benefit of a higher conversion is, thus, almost cancelled out by the loss in selectivity, which implies energy-intensive post reaction separation processes which makes the carboxylation of alcohols a net CO2 emitter, more than a CO2 user. 

B3LYP/6-311-I-I-G calculations reveal that 5 is 19.8 kcal mol more stable than 5, which is attributed to the formation of the double H-bond and also to the internal structural rearrangement of the protonated isourea moiety (Fig. 6.9). However, the data above clearly demonstrate that DCC is a promoter of the carboxylation of alcohols to dilakylcarbonates at room temperature, with a AG of ca. —38 kcal mol because of the formation of DCU. 

The unfavorable thermodynamics of the direct carboxylation of alcohols has pushed efforts to find alternative routes for the conversion of alcohols into the relevant carbonates. An interesting possibility is represented by the use of urea which is formed from CO2 and NH3. Urea can be considered an active form of CO2 (Fig. 6.12). Its formation AG is some 200 kJ mol less negative than that of CO2. However, in principle, its reactions with alcohols would have less negative thermodynamics than the reaction of CO2 and, consequently, the equilibrium positions should be shifted towards the right, reaching higher conversion at the equilibrium. 

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