Carbamate

Summary

In organic chemistry, a carbamate is a category of organic compounds with the general formula and structure , which are formally derived from carbamic acid (). The term includes organic compounds (e.g., the ester ethyl carbamate), formally obtained by replacing one or more of the hydrogen atoms by other organic functional groups; as well as salts with the carbamate anion (e.g. ammonium carbamate). Polymers whose repeat units are joined by carbamate like groups are an important family of plastics, the polyurethanes. See Etymology for clarification. Properties While carbamic acids are unstable, many carbamate esters and salts are stable and well known. Equilibrium with carbonate and bicarbonate In water solutions, the carbamate anion slowly equilibrates with the ammonium NH4+ cation and the carbonate CO32- or bicarbonate HCO3- anions: : : Calcium carbamate is soluble in water, whereas calcium carbonate is not. Adding a calcium salt to an ammonium carbamate/

Official source

https://en.wikipedia.org/wiki/Carbamate

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Le cours se focalisera sur les composés carbonyles: leur structures, réactivités, et leurs transformations; la réactivité des énols/énolates et leurs réactions fondamentales. L'importance de la compréhension des mécanismes réactionnels, de la chimio-, regio- and stéréo-sélectivité sera soulignée.

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The thesis describes a simple approach for N-formylation of amines with CO2 and hydrosilane reducing agents, the use of organic salts as N-formylation catalysts, their physical properties required for high catalytic activity and their role in the catalytic cycle. The investigation is then extended to the synthesis of quinazoline-2,4(1H,3H)-dione from 2-aminobenzonitrile and CO2 as well as other C-N bond forming reactions. Basicity, as measured by pKb, is identified as the key property required for high catalytic activity of organic salt catalysts. Minor variations in catalytic activity among salts of equal basicity are assigned to the ion pairing energy, hydrogen bonding ability and steric factors. The N-formylation reaction catalysed by organic salts proceeds by the activation of the hydrosilane reducing agent. Although, the catalytic activity was originally assigned to the nucleophilicity of the anion, a detailed investigation of the reaction mechanism and physical parameters of numerous salt catalysts revealed that they are more active as bases. Extensive substituent exchange around the hydrosilane silicon centre confirms its activation and supports the proposed reaction mechanism, where in-situ formed carbamate salt acts as the reaction nucleophile. Accounting for small detrimental effect of ion paring, a linear relationship between the pKa of the salt and catalytic activity is observed. The onset of the base catalysed reaction mechanism is substrate dependent with further variations for amines of high basicity. Nevertheless, highly basic salt catalysts, such as tetra-n-butylammonium fluoride ([TBA]F), promote rapid N-formylation of all types of amines under extremely mild conditions. A linear relationship between the basicity of salt catalysts and their catalytic activity was also observed in the synthesis of quinazoline-2,4(1H,3H)-dione. Similarly, the reaction proceeds by in-situ formation of a carbamate salt, which requires a base catalyst. However, here the reaction mechanism is limited by the acidity of the quinazoline-2,4(1H,3H)-dione product. Quinazoline-2,4(1H,3H)-dione is deprotonated by more basic catalysts (pKa of conjugate acid > 14.7 in DMSO) leading to the neutralization of the base catalyst and the formation of the quinazolide anion, which then acts as the reaction catalyst. Analysis of the reported literature reveals that the findings are directly applicable to the vast majority of C-N bond forming reactions of amines with CO2 and to almost all types of organocatalysts. The reactions proceed by carbamate salt formation in the form [BaseH][RR'NCOO]. The anion of the carbamate salt then acts as a nucleophile in hydrosilane reductions of CO2 towards formamides, N-methylamines and aminals or internal cyclization reactions to quinazoline-2,4-diones, oxazolidinones and oxazin-2-ons or after dehydration as an electrophile in the synthesis of urea derivatives. The role of organocatalysts in the reactions indicates that all bases of sufficient strength should be able to catalyse the reactions.

EPFL2019

Pivotal Role of the Basic Character of Organic and Salt Catalysts in C-N Bond Forming Reactions of Amines with CO2

Paul Joseph Dyson, Martin Hulla

Organocatalysts promote a range of C-N bond forming reactions of amines with CO2. Herein, we review these reactions and attempt to identify the unifying features of the catalysts that allows them to promote a multitude of seemingly unrelated reactions. Analysis of the literature shows that these reactions predominantly proceed by carbamate salt formation in the form [BaseH][RR ' NCOO]. The anion of the carbamate salt acts as a nucleophile in hydrosilane reductions of CO2, internal cyclization reactions or after dehydration as an electrophile in the synthesis of urea derivatives. The reactions are enhanced by polar aprotic solvents and can be either promoted or hindered by H-bonding interactions. The predominant role of all types of organic and salt catalysts (including ionic liquids, ILs) is the stabilization of the carbamate salt, mostly by acting as a base. Catalytic enhancement depends on the combination of the amine, the base strength, the solvent, steric factors, ion pairing and H-bonding. A linear relationship between the base strength and the reaction yield has been demonstrated with IL catalysts in the synthesis of formamides and quinazoline-2,4-diones. The role of organocatalysts in the reactions indicates that all bases of sufficient strength should be able to catalyze the reactions. However, a physical limit to the extent of a purely base catalyzed reaction mechanism should exist, which needs to be identified, understood and overcome by synergistic or alternative methods.

2019

N-Terminal Selective C-H Azidation of Proline-Containing Peptides: a Platform for Late-Stage Diversification

Emmanuelle Madeline Dominique Allouche, Raphaël Michel Henri Simonet-Davin, Jérôme Waser

A methodology for the C-H azidation of N-terminal proline-containing peptides was developed employing only commercially available reagents. Peptides bearing a broad range of functionalities and containing up to 6 amino acids were selectively azidated at the carbamate-protected N-terminal residue in presence of the numerous other functional groups present on the molecules. Post-functionalizations of the obtained aminal compounds were achieved: cycloaddition reactions or C-C bond formations via a sequence of imine formation/nucleophilic addition were performed, offering an easy access to diversified peptides.

WILEY-V C H VERLAG GMBH2022

EPFL2019