Modular Organoboron Catalysts Enable Transformations with Unprecedented Reactivity

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Electron-deficient boron-based catalysts with metalfree but metallomimetic characteristics provide a versatile platform for chemical transformations. However, their catalytic performance is usually lower than that of the corresponding metal-based catalysts. Furthermore, many elaborate organoboron compounds are produced via time-consuming multistep syntheses with low yields, presenting a formidable challenge for large-scale applications of these catalysts. Given this context, the development of organoboron catalysts with the combined advantages of high efficiency and easy preparation is of critical importance. Therefore, we envisioned that the construction of a dynamic Lewis multicore system (DLMCS) by integrating the Lewis acidic boron center(s) and a Lewis basic ammonium salt in one molecule would be particularly efficient for on-demand applications because of the intramolecular synergistic effect. This Account summarizes our recent efforts in developing modular organoboron catalysts with unprecedented activities for several chemical transformations. A series of mono-, di-, tri-, and tetranuclear organoboron catalysts was readily designed and prepared in nearly quantitative yields over two steps using commercially available feedstocks. Notably, these catalysts can be modularly tailored by fine control over the electrophilic property of the Lewis acidic boron center(s), electronic and steric effects of the electropositive ammonium cation, linker length between the boron center and the ammonium cation, the number of boron centers, and the nucleophilic anion. This modular design allows systematic manipulation of the reactivity and efficacy of the catalysts, thus optimizing suitable catalysts for versatile chemical transformations. These include the coupling of CO 2 and epoxides, copolymerization of CO 2 and epoxides, ring-opening polymerization (ROP) of epoxides, and ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides. The utilization of mononuclear organoboron catalysts provided a turnover frequency of 11050 h −1 for the CO 2 /propylene oxide coupling reaction, an unprecedented efficiency of 5.0 kg of polymer/g of catalyst for the copolymerization of CO 2 and cyclohexene oxide, and a record-breaking catalytic efficiency of 7.4 kg of polymer/g of catalyst for the ROCOP of epoxides with cyclic anhydrides. A turnover number of 56500 was observed at a catalyst loading of 10 ppm for the ROP of epoxides using the dinuclear catalysts. The tetranuclear organoboron catalysts realized the previously intractable task of the copolymerization of CO 2 and epichlorohydrin, producing poly(chloropropylene carbonate) with the highest molecular weight of 36.5 kg/mol reported to date. Furthermore, the study revealed that the interaction between the dynamic Lewis multicore, that is, the intramolecular synergistic effect between the boron center(s) and the quaternary ammonium salt, plays a key role in mediating the catalytic activity and selectivity. This was based o...

Sulfur-containing polymers derived from SO2: synthesis, properties, and applications

“…Homopolymerization of epoxides 148,149 or copolymerizations with other oxa-monomers [150][151][152][153][154] (such as anhydrides or carbon dioxide) in the presence of coordination catalysts proceeded smoothly with high reactivity and polymer selectivity. The improved polymerization behavior could be ascribed to the coordination interaction between the catalysts and the oxygen atom in epoxides that greatly reduces the energy barrier for the ring-opening.…”

Chiral diboranes as catalysts for the stereoselective organopolymerization of epoxides

“…24 What deserves particular attention is the proposed mechanism, which necessitates two equivalents of Et 3 B per organobase, a stoichiometry inherently linked to the simultaneous monomer activation and chain-end deactivation/coordination, which is also well-known from Lewis-pair polymerization (Scheme 1). 25 The viability of using multi-centered, onium-bridged (achiral) boranes for polymerization catalysis has been impressively demonstrated by Wu and others, 26–28 while Du's work highlighted the potential of in situ -formed chiral diboranes for, i.e. , asymmetric imine hydrogenation.…”

[OSSO]‐Type Chromium(III) Complexes for the Reaction of CO2 with Epoxides

“…[11] The second family includes reactions with cyclic molecules such as aziridines to produce oxazolidinones, [12] or reactions with epoxides for the production of cyclic organic carbonates (COCs) and aliphatic polycarbonates (APCs). [13][14][15][16][17][18] The selectivity between the cyclic carbonate and the polycarbonate in the reaction between CO 2 and epoxides relies on many factors such as the nature of the substrate, the reaction conditions, and the type of the employed catalytic system. The most active catalytic systems are based on homogeneous metal complexes; hence the choice of the metal center and the design of the ancillary ligand are of utmost importance in determining the final product selectivity.…”

“…The second family includes reactions with cyclic molecules such as aziridines to produce oxazolidinones, [12] or reactions with epoxides for the production of cyclic organic carbonates (COCs) and aliphatic polycarbonates (APCs) [13–18] …”