Theoretical Studies on the Fixation of CO₂ with Propylene Oxide Catalyzed by Squaramide/TEAB

Abstract

The chemical fixation of carbon dioxide (CO 2 ) with propylene oxide catalyzed by a binary squaramide/TEAB catalyst, leading to a five-membered cyclic propylene carbonate, has been fully investigated by density functional theory (DFT). Our results reveal that the overall reaction involves three elementary steps: the epoxide ring opening, CO 2 electrophilic attack, and intramolecular cyclization. The rate-determining step of the catalytic reaction is found to be the ring-opening step with an energy barrier of 28.4 kcal/mol in mesitylene solvent, which is kinetically more favourable than that of non-catalytic CO 2 fixation with a relatively higher barrier of 68.8 kcal/mol. The remarkable catalytic activity originates from the stabilization effect of squaramide on the oxyanionic transition states with multiple hydrogen bonding.

Squaramide–Quaternary Ammonium Salt as an Effective Binary Organocatalytic System for Oxazolidinone Synthesis from Isocyanates and Epoxides

“…We have inferred a mechanism similar to the previously proposed ones in which a hydrogen‐bond donor entity could participate in anion‐binding processes during the course of the reaction;[20k], modulating the reactivity of the anion while stabilizing the reaction intermediates (Schemeà). First, the squaramide 1 binds reversibly to the iodide anion of the n ‐TBAI which is present in an equimolar ratio with respect to the host (Schemeà, I ) while epoxide, that is present in excess, is unlikely to compete for binding to 1 in the presence of iodide.…”

“…The lower yields by electron‐deficient isocyanates strongly suggests that the rate of the whole process is determined by the ring‐closing step, which corroborates the previously reported studies. [21b], To broaden the scope of this binary catalytic system, a variety of more challenging isocyanates including alkyl isocyanates 3g–h and tosyl isocyanate 3i were tested under the optimized reaction conditions. As shown in Table , entries 7, and 14–16, the reaction was not limited to only aromatic isocyanates whereas tosyl and aliphatic isocyanates were also tolerated.…”

“…Second, an iodide‐mediated ring‐opening reaction of epoxide furnishes a hydrogen‐bond‐stabilized iodoohydrin intermediate (Schemeà, II ). The in situ formed oxo‐anions via epoxide ring‐opening could easily displace the iodide nucleophile in the 1a /I – complex,[20j], [20k], providing 1a ‐stabilized oxo‐anion complex. Subsequent inclusion of isocyanate into the iodoohydrin entity results in a squaramide‐stabilized carbamate intermediate (Schemeà, IV ).…”

Recent developments in organocatalysed transformations of epoxides and carbon dioxide into cyclic carbonates

“…In 2018, Wang et al studied the same catalytic system via DFT calculations, considering the reaction of carbon dioxide with propylene oxide to form propylene carbonate. 96 Their results supported the reaction mechanism proposed by Kleij et al…”

Computational predictions on Brønsted acidic ionic liquid-catalyzed carbon dioxide conversion to five-membered heterocyclic carbonyl derivatives

“…CO 2 fixation was generally conducted with propylene oxide [43][44][45] or ethylene oxide [46][47][48] to produce cyclic carbonates. Our computational studies showed that CO 2 sequestration goes through smaller energy barriers when we utilize other small molecules as a substrate.…”