A one-pot organocatalytic process for the synthesis of cyclic carbonates from CO2 and alkenes using cumene hydroperoxide as a green oxidant

Abstract

Cyclic carbonates are usually obtained from coupling of carbon dioxide and epoxides. The latter are generally prepared through the selective oxidation of alkenes or other compounds containing a double bond. However, a one-pot route in which an alkene is directly converted into a cyclic carbonate would be preferable as it would circumvent the handling of generally toxic epoxides and would increase process efficiency in terms of energy, solvent and reagents usage. Here, we present an attractive strategy combining a recyclable oxidant (cumene hydroperoxide, CHP) with an inexpensive, metal-free organic halide salt as catalyst. These components act cooperatively promoting the oxidation of the chosen model substrate (styrene) and the cycloaddition of CO2 to the generated epoxide intermediate. Tetrabutylammonium bromide exhibited the best catalytic performance, providing a 55% styrene carbonate yield after 6 h at 10 barg of CO2 and 80 °C using 1.5 equivalents of oxidant; and 67% in the presence of 4 equivalents of oxidant. These cyclic carbonate yields are significantly higher than those obtained with other oxidants (tert-butyl hydroperoxide and hydrogen peroxide). A scope of substrates was converted into their respective cyclic carbonates including a new bio-based methylisoeugenol-derived product and a cyclic carbonate attained from bio-based methyl oleate (having a disubstituted double bond). From mechanistic control experiments, we determined that the oxidation step proceeds through a radical mechanism, with an active involvement of CHP in epoxide activation via hydrogen-bonding, demonstrating a dual role of the oxidant. Our strategy offers a practical proof of concept of a direct approach to cyclic carbonates with a simple organocatalyst that could be reused in four consecutive runs with a similar performance, and using a recyclable oxidant.