CO2 binding by urea/thiourea compounds via the formation of carbamide–carboxylate (–CONH2–CO2) and carbamimidothiocarbonate (–NH–CS–CO2) sodium adducts.
The development of an ideal sorbent/catalyst
for CO2 capturing and fixation into cyclic carbonates under
mild
conditions
is still ongoing. We report on furnishing l-histidine ester
dihydrochloride (His-OMe) into a functionalized urea, 5,6,7,8-tetrahydro-7-(methoxycarbonyl)-5-oxoimidazo[1,5-c]pyrimidine (His-Urea, 2). The latter is prepared
via a microwave and a modified sonochemical approach using propylene
carbonate and N,N′-carbonyldiimidazole,
which is further functionalized by different mono- or disubstituted
alkyl halides with acceptable yields. Upon activation of 2 or its hydroxylated version 4b with NaH, the CO2 capturing in dimethyl sulfoxide is proven to be a dicarboxylated
species (carbamide and alkyl carboxylates, 2·2CO2Na) or alkyl carbonate adduct in the case of 4b, as verified by 1H/13C NMR and ATR-FTIR spectroscopies.
A first-time preparation of the dimeric ([DiHis-Urea-Pr]Br, 6) is reported among the prepared bio-based materials. Density
functional theory (DFT) calculations confirm the most active reaction
site and verify the CO2-sequestrated adducts. Furthermore,
the synthesized substrates (2, 4a–b, and 6) are tested for the cycloaddition reaction of
epichlorohydrin with CO2 under mild reaction conditions,
with good-to-excellent catalytic activity up to quantitative conversions
under arbitrary conditions (3.0 mol% catalyst loading, 90 °C,
8 h, 1 atm CO2). The suggested reaction mechanism is verified
via DFT calculations, in which the ring closure is the rate-determining
step.
Herein, an organocatalyzed synthetic pathway for the preparation of (a)cyclic ureas from their parent primary aliphatic or aromatic mono‐/diamines with propylene carbonate (PC) as a carbonylating agent (CAT) obtaining reasonable to very good yields with high selectivity. Our method is considered green as nine out of twelve green chemistry principles (GCPs) are fulfilled. Most importantly, the absence of solvent, energy‐efficient pathway, in addition to the ease of synthesis and separation, under fast reaction times down to a few minutes together with the straightforward workup with minimum use of organic solvents. Our method was successful in preparing 1,3‐diphenylurea from aniline giving 8 % yield in 10 minutes, to our knowledge, this was not previously reported using aromatic amines with carbonate esters. The prepared compounds were fully characterized by spectroscopic and thermal techniques. The method is applicable for primary rather than secondary amines, which implies high chemoselectivity of the former for the synthesis of urea compounds.
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