Boron–pyridine nitrogen cooperative catalytic conversion of carbon dioxide and epoxides to cyclic carbonates†

Abstract

A type of N-heterocyclic carbene (NHC)-diboron adduct was used as a catalyst for the coupling of CO₂ with epoxides. A multifunctional organocatalyst, which contains both boron as a Lewis acidic site and pyridine nitrogen as a Lewis basic site, offers an efficient alternative to traditional organoboron catalysts for the conversion of CO₂ and epoxides into cyclic carbonates. The NHC-diboron adduct, in combination with tetrabutylammonium iodide (TBAI) as a nucleophile, catalyzes the coupling of CO₂ and epoxides to obtain the cyclic carbonates in high yields under relatively mild conditions (50 °C and 1 MPa of CO₂). The cooperative activation of NHC-diboron adducts was elaborated by in situ infrared spectroscopy and electrospray ionization-high-resolution mass spectrometry (ESI-HRMS). These results suggested that the Lewis acid boron center in NHC-diboron adduct activates the epoxide by coordination with its oxygen atom to promote ring opening and that the pyridine nitrogen acts as a Lewis basic site to activate CO₂.