US EPA

1036315 

Journal Article 

Mechanism of formation of organic carbonates from aliphatic alcohols and carbon dioxide under mild conditions promoted by carbodiimides. DFT calculation and experimental study 

Aresta, M; Dibenedetto, A; Fracchiolla, E; Giannoccaro, P; Pastore, C; Pápai, I; Schubert, G 

2005 

Yes 

Journal of Organic Chemistry
ISSN: 0022-3263
EISSN: 1520-6904 

American Chemical Society 

70 

16 

6177-6186 

English 

16050675 

10.1021/jo050392y 

WOS:000230909100006 

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. 

Addition reactions; Alcohols; Amines; Carbon dioxide; Catalysts; Nitrogen compounds; Nuclear magnetic resonance spectroscopy; Probability density function; Reaction kinetics; Aliphatic alcohols; Conversion yield; Organic carbonates; Selectivity; Carbonates; alcohol; alkanol; allyl alcohol; amine; carbon dioxide; carbonic acid derivative; cyanamide; dicyclohexylcarbodiimide; Lewis acid; Lewis base; methanol; phenol; urea derivative; article; carbon dioxide tension; catalyst; chemical interaction; chemical reaction kinetics; density functional theory; hydrogen bond; low temperature; synthesis