Carbon Dioxide as a Phosgene Replacement: Synthesis and Mechanistic Studies of Urethanes from Amines, CO2, and Alkyl Chlorides

Abstract

Several carbamate esters were synthesized from amines, carbon dioxide, and alkyl chlorides. The effect of added hase on the yield and selectivity of carbamate ester formation was found to be highly important with the use of sterically hindered guanidine bases giving the best results. Relative rate studies were carried out giving the following order of reactivity between carbamate anions in acetonitrile with benzyl chloride giving carbamate esters:Rate studies were carried out with the diethyl, s-butyl, phenyl, and rc-butyl carbamates and activation parameters were determined to be Et2NC02~, AH* = 11.8 kcal/mol, AS* = -33 eu; s-BuNHC02", AH* = 12.8 kcal/mol, AS* --33 eu; PhNHC02-, AH* = 14.3 kcal/mol, AS* = -28 eu; rc-BuNHC02~, AH* = 23.4 kcal/mol, AS* = -3 eu. The unusual results obtained from the use of 7i-BuNHC02~prompted further studies which showed that the rate of reaction was inversely dependent on carbon dioxide pressure (20 psig C02, k = 4.84 x 10~4 M_1 s_1; 120 psig C02, k = 1.83 x 10-4 M-1 s_1). Nitrogen NMR spectroscopy indicated, via a labeling study with 15N amines and 13C enriched carbon dioxide, the formation of a doubly inserted product from the addition of two carbon dioxides to ethylamine in acetonitrile.

The Effects of Solvent and Added Bases on the Protection of Benzylamines with Carbon Dioxide

“…at 25 °C and 1 bar was bubbled through the amine solutions for 30 min using a stainless steel needle to ensure complete conversion of (4) to benzylcarbamic acid (5). Samples analyzed via 13 C NMR at room temperature and pressure.…”

“…It has been shown that amines readily react with CO2 to form a variety of products depending on the solvent and/or the presence of strong organic bases [4,5,9,25]. These CO2 adducts are, in principle, protected amines.…”

“…CO2 is known to react with primary and secondary amines to form the corresponding carbamic acid or ammonium carbamate ion pair [1][2][3][4][5][6][7][8][9][10][11]. The CO2-amine reaction can be reversed thermally under relatively mild conditions-a key to developing "in situ, reversible" protection schemes.…”

Preparation of Highly Stabilized Silver Nanopowders by the Thermal Reduction and Their Properties

“…Carbamate is thermally unstable and equilibrated between CO 2 and corresponding amine upon heating. 19,36,37 This can be useful for the capping of growing AgNPs with carbamic acid as a stabilizer. Therefore, 2-ethylhexylammonium 2-ethylhexylcarbmate (EHAEHC) reacted with the surfaces of Ag-NPs during thermal reduction.…”

ZIF-8 : novel catalytic material for the conversion of CO2 to cyclic carbonates.

“…Organic and inorganic bases, crown ethers, and cryptands [63][64][65][66] catalyze this reaction. However, these homogeneous catalysts will cost extra energy and effort to be separated from the product.…”