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

Michele Aresta, Angela Dibenedetto, Elisabetta Fracchiolla, Potenzo Giannoccaro, Carlo Pastore, I. Pápai, G. Schubert

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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(ORO=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 CO 2, 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.

Original languageEnglish
Pages (from-to)6177-6186
Number of pages10
JournalJournal of Organic Chemistry
Volume70
Issue number16
DOIs
Publication statusPublished - Aug 5 2005

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Carbodiimides
Carbonates
Discrete Fourier transforms
Carbon Dioxide
Alcohols
Carbon Monoxide
Urea
Lewis Bases
Dicyclohexylcarbodiimide
Lewis Acids
Carbamates
Phenol
Temperature
Amines
Methanol
Ethanol
Nuclear magnetic resonance
Catalysts
Molecules
Kinetics

ASJC Scopus subject areas

  • Organic Chemistry

Cite this

Mechanism of formation of organic carbonates from aliphatic alcohols and carbon dioxide under mild conditions promoted by carbodiimides. DFT calculation and experimental study. / Aresta, Michele; Dibenedetto, Angela; Fracchiolla, Elisabetta; Giannoccaro, Potenzo; Pastore, Carlo; Pápai, I.; Schubert, G.

In: Journal of Organic Chemistry, Vol. 70, No. 16, 05.08.2005, p. 6177-6186.

Research output: Contribution to journalArticle

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T1 - Mechanism of formation of organic carbonates from aliphatic alcohols and carbon dioxide under mild conditions promoted by carbodiimides. DFT calculation and experimental study

AU - Aresta, Michele

AU - Dibenedetto, Angela

AU - Fracchiolla, Elisabetta

AU - Giannoccaro, Potenzo

AU - Pastore, Carlo

AU - Pápai, I.

AU - Schubert, G.

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N2 - 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(ORO=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 CO 2, 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.

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