Organocatalytic Strategy for the Fixation of CO2 via Carboxylation of Terminal Alkynes

Guanidine-Based Covalent Organic Frameworks: Cooperation between Cores and Linkers for Chromic Sensing and Efficient CO2 Conversion

C(sp)-H carboxylation with CO₂ is an attractive route of CO₂ utilization and is traditionally promoted by transition metal catalysts, and organocatalysis for the conversion remains rarely explored and challenging. In this article, triaminoguanidine-derived covalent organic frameworks (COFs) were used as platforms to develop heterogeneous organocatalysts for the reaction. We demonstrated that the COFs with guanidine cores and pyrazine linkers show high catalytic performance as a result of the cooperation between cores and linkers. The core is vitally important, which is deprotonated to the guanidinato group that binds and activates CO₂. The pyrazine linker collaborates with the core to activate the C(sp)-H bond through hydrogen bonding. In addition, the COFs show acid- and base-responsive chromic behaviors thanks to the amphoteric nature of the core and the auxochromic effect of the pyrazine linker. The work opens up new avenues to organocatalysts for C-H carboxylation and chromic materials for sensing and switching applications.

Carboxylation of terminal alkynes with CO2 catalyzed by imidazolium-bridged bis(phenolato) rare-earth metal complexes

An imidazolium-bridged bis(phenolato) yttrium complex shows excellent catalytic reactivity in the direct carboxylation of terminal alkynes and further one-pot three-component esterification.

Cg10062 Catalysis Forges a Link between Acetylenecarboxylic Acid and Bacterial Metabolism

The reliance of biocatalysis on plant-derived carbon for the synthesis of fuels and chemicals places it in direct competition with food production for resources. A potential solution to this problem is development of a metabolic link between alternative carbon sources and bacterial metabolism. Acetylenecarboxylic acid, which can be synthesized from methane and carbon dioxide, could enable this connection. It was previously shown that the enzyme Cg10062 catalyzes hydration of acetylenecarboxylate to afford malonate semialdehyde. Subsequent hydration-dependent decarboxylation to form acetaldehyde (81%), which was also observed, limits its biocatalytic usefulness. Several Cg10062 variants including E114Q and E114D do not catalyze decarboxylation and provide malonate semialdehyde as the sole product, albeit with substantially reduced catalytic activity. To identify an efficient enzyme capable of catalyzing acetylenecarboxylate hydration without decarboxylation, we undertook a mechanistic investigation of Cg10062 using mutagenesis, kinetic characterization, and X-ray crystallography. Cg10062 is a member of the tautomerase superfamily of enzymes, characterized by their β-α-β protein fold and an N-terminal proline residue situated at the center of the enzyme active site. Along with Pro-1, five additional active site residues (His-28, Arg-70, Arg-73, Tyr-103, and Glu-114) are required for Cg10062 activity. Incubation of crystals of four catalytically slow variants of Cg10062 with acetylenecarboxylate resulted in atomic resolution structures of Pro-1 bound to a complete set of intermediates, fully elaborating the detailed mechanism of the enzyme and establishing the process to involve covalent catalysis. Further, the intermediate-bound E114D structure explains the mechanism governing decarboxylation suppression. Together, these studies provide the most detailed picture of the catalytic mechanism of a tautomerase enzyme to date.