Carbamates from dialkyl carbonates

In the synthesis of carbamates, R NH.C02R, from A iV -dialkylureas, (R NH)2CO, and dialkyl carbonates, (RO)2CO, dibutyltin oxide, Bu2SnO, acted as an efficient catalyst. The proposed mechanism (Scheme 13) involves addition of the dialkyl carbonate to Bu2SnO to give an adduct (43), which is attacked by the urea to yield a new tin complex (44) and one molecule of carbamate. Attack by dialkyl carbonate upon this complex (44) yields a further molecule of carbamate and regenerates the original tin complex (43), which can continue the catalytic cycle.42... 

CsX is useful for the simple Knoevenagel reaction of benzaldehyde with ethyl cyanoacetate even a simple NaY is sufficiently basic to form carbamates starting from primary aromatic amines and dialkyl carbonates (35, 36). At contrast CsjO-MCM-41 can also be used for the addition of C02 to epoxides, or for Michael addition of one or two molecules of diethyl malonate on neopentylglycol diacrylate (37, 38) ... 

This method has been applied to a monomer synthesis (2.20).61 The quaternary ammonium salt is used as a phase transfer catalyst. The potassium iodide converts the starting chloride to a more reactive iodide in situ. The alcohol traps the intermediate isocyanate before it can react with the small amount of water that has to be present. If no methanol is present, the corresponding urea is formed in up to 87% yield. If only 0.3 equivalent of water is used, the product is the iso-cyanurate, the cyclic trimer of the isocyanate. Where n = 1, the isocyanatoethyl methacrylate is a useful monomer now made with phosgene (2.21).62 Dow has patented a route from ethanolamine plus a dialkyl carbonate, followed transesterification to a methacrylate carbamate, which is py-rolyzed to the isocyanate (2.22) in 50% yield.63... 

Rapid addition of a pre-formed carbamate anion solution (generated from dialkyl amine, N-cyclohexyl-N, N, N",N"-tetramethylguanidine (1 equiv.), and pyridine (1 equiv.) under a pressure of carbon dioxide) to a solution of thionyl chloride (1 equiv.) at —10 °C led to an exothermic reaction. Rapid aqueous extraction (to remove salts and SO2 ) of the crude reaction mixture and distillation of the residue gave good isolated yields of the carbamoyl chloride (40-80%). The results obtained using various reagents in this reaction are shown in the table below. It is not yet clear as to why added pyridine improves the yield of carbamoyl chloride (from 72 to 85%). It is possible that pyridine acts as a trap for the sulfur dioxide that is likely to be formed during the course of the reaction. 

Using the palladium(II) chloride system, the reaction involving secondary amines was found to give mixtures of carbamate ester and an oxamate ester resulting from double carbonylation. Analogous carbonylations of alcohols can lead to a range of products, including dialkyl carbonates, oxalates, and succinates. ... 

On account of their chemical, biological and engineering significance, the slow processes involved in dissolution of carbon dioxide in water have been investigated repeatedly, from many points of view. References given here are doubtless an incomplete set, as are all the other compilations I have examined. Methods employed vary over acid-base titration , formation of dialkyl carbamates pH and other potentiometric methods , conductivity ... 

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