Bifunctional silanol-based HBD catalysts for CO2 fixation into cyclic carbonates

Abstract

First examples of unprecedented silanol-based bifunctional HBD catalysts [(^tBuO)₂{(N(CH₂CH₂)₃N)CH₂CH₂O}Si(OH)]⁺I⁻ and (Rac)- and (R)-[(^tBuO)₂{(N(CH₂CH₂)₃N)CH₂(Et)CHO)}Si(OH)]⁺I⁻ with tetraalkylammonium units directly incorporated into their structures were prepared from tailor-made silanols. These bifunctional silanols were used together with other mixed alkoxysilanols of general formula (^tBuO)₂(RO)SiOH (R = Me, Et, ⁱPr, –CH₂CH₂I and –CH(Et)CH₂I) in a systematic study of their catalytic properties for the preparation of cyclic carbonates using a library of epoxides and industrial-grade carbon dioxide. With 4 mol% catalyst loading in the absence of a solvent and an external nucleophile source, the bifunctional catalysts showed good to very good conversion of epoxides to the corresponding cyclic carbonates within 10 h at 70 °C and 75 psi of CO₂. Furthermore, the developed synthetic approach used in the preparation of these mixed alkyl silanols via the hydrolysis of the corresponding acetoxysilyl alkoxides (^tBuO)₂(RO)Si(OAc) (R = organic moiety) allowed a straightforward route to the modification of the steric bulk around the silicon atom and the introduction of functional groups for further derivatization.