Design of simple salophen chromium complexes for a two-steps oxidative carboxylation of styrene with CO2 and O2

Abstract

In this work, chromium(III) Salophen complexes bearing tertiary dimethyl or diethyl amino substituents were synthesized and tested as homogeneous co-catalysts associated with n-Bu4NBr for the cycloaddition of CO2 on styrene epoxide under 11 bar of CO2. Full conversion of styrene oxide into styrene carbonate was obtained after 23 h reaction at 50 °C using Salophen-Me2N-Cr. These results emphasized that tertiary amine groups born by the aromatic cycles of Salophen clearly enhanced the co-catalytic activity of the corresponding Cr(III) complex. The direct conversion of styrene into styrene carbonate was then conducted at 80 °C with O2 and CO2 either under a single set of conditions (reactants and catalysts together) or using a two-steps transformation. The latter was based on the prior epoxidation of styrene in the presence of isobutyraldehyde and O2 (3.5 bar) with a delayed addition of the cycloaddition catalysts (n-Bu4NBr and Salophen-Me2N-Cr) and CO2 (11 bar). Using these conditions, the maximum yield of styrene carbonate, obtained after 3 h of epoxidation and 20 h of cycloaddition (with Salophen-Me2N-Cr), was 31 % at 80 °C. This work demonstrates that the key-point to afford a high styrene carbonate global yield with that system was related to the implementation of a two-step protocol, since it allowed to minimize the amounts of undesired by-products.