Design of simple salophen chromium complexes for a two-steps oxidative carboxylation of styrene with CO2 and O2
Matthieu Balas, Richard Villanneau, Franck Launay
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.