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The cycloaddition of carbon dioxide to epoxide (CCE) reaction stands out as one of the few feasible chemical processes for catalytically fixing CO2. Traditionally, halide anions were frequently used as cocatalysts for catalysing the CCE reaction owing to their excellent nucleophilic and leaving abilities. However, halides are known for their corrosive effects on process reactors, and their treatment is challenging. Consequently, there is a substantial increase in the demand for halide-free catalysis in the CCE reaction. Herein, a series of bifunctional catalysts were designed for CCE reactions. The reactions were assessed under conditions of 120 °C and 1 MPa employing a catalyst loading of 0.5 mol% over 24 hours. The dual activation mechanism of the catalyst was confirmed through NMR titration and control experiments. As a bifunctional catalyst, the ion pair polarized the oxygen atom of epoxide via H-bonding with N+–H while the phenolate anion activated carbon dioxide simultaneously. This study introduces a pyridinium base binary organocatalyst as a halide-free hydrogen bond donor (HBD) anion organocatalyst with broad applicability for CO2 utilization. [ABSTRACT FROM AUTHOR]

Halogen bonding, parallel to hydrogen bonding, was introduced into the catalytic cycloaddition of carbon dioxide into epoxide (CCE) reactions. A series of halogen‐bond donor (XBD) catalysts of N‐iodopyridinium halide featured with N−I bond were synthesized and evaluated in CCE reactions. The optimal XBD catalyst, 4‐(dimethylamino)‐N‐iodopyridinium bromide ([DMAPI]Br), under screened conditions at 100 °C, ambient pressure, and 1 mol % catalyst loading, realized 93 % conversion of styrene oxide into cyclic carbonate in 6 h. The substrate scope was successfully extended with excellent yields (mostly ≥93 %) and quantitative selectivity (more than 99 %). 1H NMR spectroscopy of the catalyst [DMAPI]Br on substrate epoxide certified that the N−I bond directly coordinated with the epoxide oxygen. A plausible mechanism of halogen‐bonding catalysis was proposed, in which the DMAPI cation functioned as halogen‐bond donor to activate the epoxide, and the counter anion bromide attacked the methylene carbon to initiate the ring‐opening of the epoxide. CCE reactions promoted by N‐iodopyridinium halide, exemplify a first case of halogen‐bonding catalysis in epoxide activation and CO2 transformation. [ABSTRACT FROM AUTHOR]

The cycloaddition of CO 2 to epoxide (CCE) reactions produce valuable cyclic carbonates useful in the electrolytes of lithium-ion batteries, as organic solvents, and in polymeric materials. However, halide-containing catalysts are predominantly used in these reactions, despite halides being notoriously corrosive to steel processing equipment and residual halides also having harmful effects. To eliminate the reliance on halides as cocatalyst in most CCE reactions, halide-free catalysts are highly desirable. Herein, a series of halide-free organocatalysts composed of stoichiometric base-acid binary adducts of super strong nitrogen bases and natural α-hydroxy acids was designed. The adducts were virtually biobased ionic liquids composed of a protonated base cation and an α-hydroxy carboxylate anion. Ionic liquids from four super strong bases (two amidines and two guanidines) and four α-hydroxy acids were evaluated as halide-free organocatalysts in the CCE reactions. Combining amidine 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and mandelic acid (HMAc) generated an optimal ion pair catalyst [DBUH][MAc], which showed good catalytic activity in the cycloaddition of CO 2 to styrene oxide with a 92% yield and 99% selectivity by 2.5 mol% catalyst loading, under conditions of 120 °C, 1.0 MPa, and 12 h. A reasonable bifunctional activation mechanism was proposed in which the protonated base DBUH coordinated to the epoxide by H-bonding, and the carboxylate interacted with CO 2 facilitating the formation of the acyl carbonate intermediate that attacked the epoxide. The mechanism was validated by 1 H NMR titrations, by the intermediate capture, and by controlled experiments by cation and/or anion switches.

Halogen bonding, parallel to hydrogen bonding, was introduced into the catalytic cycloaddition of carbon dioxide into epoxide (CCE) reactions. A series of halogen-bond donor (XBD) catalysts of N-iodopyridinium halide featured with N-I bond were synthesized and evaluated in CCE reactions. The optimal XBD catalyst, 4-(dimethylamino)-N-iodopyridinium bromide ([DMAPI]Br), under screened conditions at 100 °C, ambient pressure, and 1 mol % catalyst loading, realized 93 % conversion of styrene oxide into cyclic carbonate in 6 h. The substrate scope was successfully extended with excellent yields (mostly ≥93 %) and quantitative selectivity (more than 99 %). 1 H NMR spectroscopy of the catalyst [DMAPI]Br on substrate epoxide certified that the N-I bond directly coordinated with the epoxide oxygen. A plausible mechanism of halogen-bonding catalysis was proposed, in which the DMAPI cation functioned as halogen-bond donor to activate the epoxide, and the counter anion bromide attacked the methylene carbon to initiate the ring-opening of the epoxide. CCE reactions promoted by N-iodopyridinium halide, exemplify a first case of halogen-bonding catalysis in epoxide activation and CO 2 transformation., (© 2020 Wiley-VCH GmbH.)