Mechanism of Formation of Organic Carbonates from Aliphatic Alcohols and Carbon Dioxide under Mild Conditions Promoted by Carbodiimides. DFT Calculation and Experimental Study

Abstract

Dicyclohexylcarbodiimide (CyN=C=NCy, DCC) promotes the facile formation of organic carbonates from aliphatic alcohols and carbon dioxide at temperatures as low as 310 K and moderate pressure of CO2 (from 0.1 MPa) with an acceptable rate. The conversion yield of DCC is quantitative, and the reaction has a very high selectivity toward carbonates at 330 K; increasing the temperature increases the conversion rate, but lowers the selectivity. A detailed study has allowed us to isolate or identify the intermediates formed in the reaction of an alcohol with DCC in the presence or absence of carbon dioxide. The first step is the addition of alcohol to the cumulene (a known reaction) with formation of an O-alkyl isourea [RHNC(OR')=NR] that may interact with a second alcohol molecule via H-bond (a reaction never described thus far). Such an adduct can be detected by NMR. In alcohol, in absence of CO2, it converts into a carbamate and a secondary amine, while in the presence of CO2, the dialkyl carbonate, (RO)2CO, is formed together with urea [CyHN-CO-NHCy]. The reaction has been tested with various aliphatic alcohols such as methanol, ethanol, and allyl alcohol. It results in being a convenient route to the synthesis of diallyl carbonate, in particular. O-Methyl-N,N'-dicyclohexyl isourea also reacts with phenol in the presence of CO2 to directly afford for the very first time a mixed aliphatic-aromatic carbonate, (MeO)(PhO)CO. A DFT study has allowed us to estimate the energy of each intermediate and the relevant kinetic barriers in the described reactions, providing reasonable mechanistic details. Calculated data match very well the experimental results. The driving force of the reaction is the conversion of carbodiimide into the relevant urea, which is some 35 kcal/mol downhill with respect to the parent compound. The best operative conditions have been defined for achieving a quantitative yield of carbonate from carbodiimide. The role of temperature, pressure, and catalysts (Lewis acids and bases) has been established. As the urea can be reconverted into DCC, the reaction described in this article may further be developed for application to the synthesis of organic carbonates under selective and mild conditions.

On the Existence of the Elusive Monomethyl Ester of Carbonic Acid [CH3OC(O)OH] at 300 K: 1H‐ and 13C NMR Measurements and DFT Calculations

“…[19] In particular, we investigated the reaction of 4 with the methylating agent O-methylisourea (6) [20,21] [Equation (11)], which is obtained by treating dicyclohexylcarbodiimide (DCC) with methanol [Equation (12)]. This reagent was preferred to other classical methylating systems (e.g.…”

“…This reaction both confirms the existence of 4 in solution and underlines the role of 6 as a methylating agent, as already reported in the literature. [20,21] If the isourea is treated with NaOC(O)OMe no methylation reaction occurs, although the addition of a small amount of water allows the reaction to proceed to form urea and DMC as CH 3 OC(O)OH is formed. In order to explain the mechanism of reaction 12 we carried out DFT calculations on the reaction of 4 with a simplified model of 6 where the cyclohexyl groups have been replaced by methyl moieties (see Figure 4).…”

Chemical Technologies for Exploiting and Recycling Carbon Dioxideinto the Value Chain

“…As the reaction is equilibrium limited, [127,137] the resulting water has to be removed from the process to achieve reasonable conversions. [137][138][139][140][141][142] As yet, this has been a major obstacle for implementation of the carboxylation reaction.…”

Amino acids as selective acylating agents: regioselective N1-acylation of imidazolidin-4-one derivatives of the antimalarial drug primaquine

“…The explanation for this exception most probably lies on the fact that MeOH reacts with DIPCDI to form the corresponding O-methyl-isourea (Scheme 2). Reaction of alcohols with carbodiimides has been explored by Aresta and co-workers, 23 who investigated the factors that influence the yield and selectivity of reaction of N,N 0 -dicyclohexylcarbodiimide (DCCI) with alcohols and concluded that the alkylation is dependent on the nature and rate of addition of the alcohol to the C]N bond, and that reaction rate generally increases with temperature and the presence of a catalyst. The occurrence of such a reaction in our case simultaneously explains the extensive formation of DIU and the fact that synthesis of 6a is completely inhibited in a solvent mixture that contains 50% MeOH.…”