Ionized H-Donors as Simple and Diverse Organobase Catalysts for Selective and Versatile Ethoxylation

Le résumé fourni par la source

Organobases constitute a pivotal part of the toolbox of organocatalysis. We have created a series of ionic organobases (IOBs) by a simple dehydration reaction between weak Bro̷nsted acids (ureas, thioureas, squaramides, and carbamates) and tetra- n -butylammonium hydroxide. The IOBs exhibit higher activity than neutral organobases with similar basicity when used in combination with triethylborane for the ring-opening polymerization (ROP) of epoxides. Poly(propylene oxide)s with controlled molar mass and low dispersity are afforded due to the absence of detrimental transfer reactions. Construction of diverse α,ω-di-/bifunctional polyethers using alcohol, water (H 2 O), (thio)carboxylic acid, and phenol as initiators is fulfilled thanks to the chemoselective ROP and independence of the initiator from the catalyst. Importantly, poly(ethylene oxide) (PEO) diols synthesized by use of IOBs comprising only C, H, O, and N atoms are noncytotoxic without removal of catalysts as shown by the MTT assay, and end-group derivatization of PEO is achievable via a strict “1 + 1” reaction with functional epoxides under optimized conditions. It is unusual that the catalytic activity of IOBs increases in the absence of an NH group, which is attributed by DFT calculation to the formation of easily dissociated “Et 3 B–OH-IOB” ternary complexes. Our study showcases unexploited values of mildly acidic H-donors, with an enormous variety, as precursor IOBs which will strongly propel the continuous enrichment and refinement of organocatalysts for advancing synthetic (polymer) chemistry.

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