Synthesis of Carbamates from Amines and Carbon Dioxide
SCHEME 1
electrolyte-solvent system.2 In any case, after addition of EtI
as an alkylating agent, the carbamates were isolated from the
cathodic solutions in good or high yields. Therefore, under the
above-indicated experimental conditions (i.e., in organic solvents
containing large nonmetal cations such as tetraalkylammonium
ions), the O-alkylation reaction of the carbamate anions (yielding
organic carbamates) is strongly competitive with the N-
alkylation reaction (yielding N-ethylamines).
Room-temperature ionic liquids [RTILs; e.g., salts of qua-
ternary ammonium, phosphonium, imidazolium, or pyridinium
cations with inorganic counterions (BF4)-, (PF6)-, (CF3SO3)-,
etc.] are molten salts with melting points close to room
temperature.10 In recent years, RTILs, because of their low vapor
pressure, chemical and thermal stability, solvating ability,
nonflammability, and ability to act as catalysts, have been
frequently used as “green” reaction media in clean organic
synthetic processes as substitutes for conventional toxic and
volatile solvents.
Recently, chemical fixation of carbon dioxide with epoxides
in RTILs to form cyclic carbonates has been described (tem-
perature, 110 °C; CO2 pressure, 1.5 MP; catalyst, ZnCl2; time,
1.0 h).11 In addition, 5-methylene-1,3-oxazolidin-2-ones were
isolated via reaction of propargyl alcohols, aliphatic primary
amines, and CO2 in ionic liquids under relatively mild conditions
(temperature, 100 °C; CO2 pressure, 2.5 MPa; catalyst, CuCl;
time, 10 h).12
Nevertheless, the use of volatile and toxic solvents (CH3CN,
DMF, etc.) and of large amounts of supporting electrolytes
makes more complex the workup of the reaction mixture
targeted at the isolation of the products and the recovery of the
solvents. In addition, with the growing demand of environmental
friendly technologies, any effort should be devoted to avoid the
use of volatile and damaging solvents.
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In addition to the above-reported properties, RTILs show a
high ionic conductivity and a wide electrochemical potential
window.13 Accordingly, ionic liquids have been frequently used
as electrolytes for studies related to organic electrosynthesis and
to the evaluation of the redox behavior of electroactive
substrates,14 as well as metal deposition, batteries, photovoltaic
devices, fuel cells, and solar cells.13b,c As concerns electro-
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