Bifunctional organoboron–phosphonium catalysts for coupling reactions of CO2 and epoxides. Issue 50 (14th November 2022)

• Record Type: Journal Article
• Authors: Sengoden, Mani; Bhat, Gulzar A.; Darensbourg, Donald J.
• Is Part Of: RSC advances. Volume 12:Issue 50(2022)
• Page Start: 32440
• Page End: 32447
• Publication Date: 2022-11-14
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/d2ra06358a
• Languages: English
• ISSNs: 2046-2069

Abstract

Abstract : A series of organoborane phosphonium salts have been shown to be very effective catalysts for the copolymerization of alicyclic epoxides with CO2 or the cycloaddition of aliphatic epoxides and CO2 to afford polycarbonates or cyclic carbonates, respectively. Abstract : Recent years have witnessed intensive research activity in exploring novel metal-free organocatalysts for catalyzing the coupling reactions of CO2 and epoxides to afford cyclic or polymeric carbonates. In this direction, herein we report a series of boron–phosphonium organocatalysts for catalyzing the coupling reactions of CO2 and epoxides. These organophosphonium catalysts were synthesized in high yields by following a two step protocol involving Menschutkin and hydroboration reactions in succession. The purity of these organocatalysts was confirmed by spectroscopic techniques like 1 H, 13 C and 31 P NMR, and molecular structures were confirmed by single crystal X-ray diffraction studies. We have also demonstrated that these bifunctional organoboron–phosphonium catalysts are comparatively much less hygroscopic compared to the analogus ammonium catalysts. These phosphonium organocatalysts were shown to catalyze the copolymerization of CO2 and cyclohexene oxide or vinyl cyclohexene oxide to provide polycarbonates with >99% polymer selectivity and carbonate linkages. The coupling reactions of aliphatic epoxides such as PO, having lower energy barrier to cycloaddition formation compared to alicyclic epoxides, preferentially provided cyclic carbonates in good yields. It was demonstrated that these organoboron–phosphonium catalysts are sensitive to chain transfer agents like water, and hence are deactivated in its presence. This is opposite to what is observed for metal based catalysts for these transformations, where water serves as a precursor to the chain-transfer agent diols.