Bifunctional Porphyrin Catalysts for the Synthesis of Cyclic Carbonates from Epoxides and CO₂: Structural Optimization and Mechanistic Study

Abstract

We prepared bifunctional Mg(II) porphyrin catalysts 1 for the solvent-free synthesis of cyclic carbonates from epoxides and CO2. The activities of 1d, 1h, and 1i, which have Br(-), Cl(-), and I(-) counteranions, respectively, increased in the order 1i < 1h < 1d. Catalysts 1d and 1j-m, which bear four tetraalkylammonium bromide groups with different alkyl chain lengths, showed comparable but slightly different activities. Based on the excellent catalyst 1d, we synthesized Mg(II) porphyrin 1o with eight tetraalkylammonium bromide groups, which showed even higher catalytic activity (turnover number, 138,000; turnover frequency, 19,000 h(-1)). The catalytic mechanism was studied by using 1d. The yields were nearly constant at initial CO2 pressures in the 1-6 MPa range, suggesting that CO2 was not involved in the rate-determining step in this pressure range. No reaction proceeded in supercritical CO2, probably because the epoxide (into which the catalyst dissolved) dissolved in and was diluted by the supercritical CO2. Experiments with (18)O-labeled CO2 and D-labeled epoxide suggested that the catalytic cycle involved initial nucleophilic attack of Br(-) on the less hindered side of the epoxide to generate an oxyanion, which underwent CO2 insertion to afford a CO2 adduct; subsequent intramolecular ring closure formed the cyclic carbonate and regenerated the catalyst. Density functional theory calculations gave results consistent with the experimental results, revealing that the quaternary ammonium cation underwent conformational changes that stabilized various anionic species generated during the catalytic cycle. The high activity of 1d and 1o was due to the cooperative action of the Mg(II) and Br(-) and a conformational change (induced-fit) of the quaternary ammonium cation.

Recent Advances in the Catalytic Preparation of Cyclic Organic Carbonates

“…The same issue was addressed by Ema and co-workers, 51 Alternative strategies to prepare diastereo-pure COCs have also been reported, including the Pd(II) catalyzed coupling of CO with diols, as described by Dibenedetto and co-workers. 60 Although modest success was observed in terms of substrate scope and yields, this oxidative carbonylation of 1,2-and 1,3-diols catalyzed by palladium iodide in conjunction with KI gave access to diastereo-pure COCs starting from diastereoisomeric mixtures of the substrates (Scheme 12; T = 100 ºC, DMA, PdI 2 = KI = 10 mol%, p(CO 2 ) = 2.0 MPa of a 1:4 CO-air mixture).…”

“…To further enhance the catalytic activity of these bifunctional Mg(II) porphyrin systems, a Mg(II) porphyrin complex ligated to eight tetraalkylammonium bromide groups (i.e., a 1:8 LA/co-cat site ratio) was prepared. 51 Consequently, owing to the increased number of anionic nucleophilic centers tightly coordinated to the cationic catalyst molecule, higher TON were achieved after t = 24 h (TON = 138000) and an increased initial TOF of 19000 h -1 (conditions: cat = 0.0005 mol%, p(CO 2 ) = 1.7 MPa, T = 120 ºC). For these bifunctional Mg(porphyrin) based catalysts, the authors suggested a cooperative effect of embedding the nucleophilic moiety (Br) and the Lewis acidic metal center in a tight coordination sphere, leading to a simultaneous epoxide activation/nucleophile attack.…”

Experimental and theoretical insights into binary Zn-SBA-15/KI catalysts for the selective coupling of CO2and epoxides into cyclic carbonates under mild conditions

“…However, when further raising the CO2 pressure up to 6.0-7.0 MPa, an obvious negative influence appeared (entries 9, 10), which could be explained that the dissolved adjacent supercritical CO2 molecules act as a diluent, and the dilution of the epoxide and catalysts likely contributed to the drop in reaction rate and gave a negative impact. 30 The positive impact should be more significant at low CO2 pressure while the negative impact at high pressure. Under the conditions used, a low CO2 pressure of 1.0 MPa is sufficient for the synthesis of PC.…”

Activation of Heteroallenes COxS2–x (x = 0–2): Experimental and Theoretical Evidence of the Synthetic Versatility of a Bulky Guanidinato SmII Complex

“…[1] While several approaches exist to activate and reduce this inert molecule by hydrides, [2] to incorporate it into carbamides, [3] to polymerize it with epoxides to polycarbonates [4] or to form cyclic carbonates [5] with it, reduction and/or disproportionation of CO 2 to form carbon monoxide, oxide, carbonate, or oxalate appears equally attractive. In this case, the metal complexes provide the electronic platform for reduction and transformation of CO 2 in the absence of additional electron providing substrates.…”