Cyclic Carbonate Synthesis Catalysed by Bimetallic Aluminium–Salen Complexes

Abstract

The development of bimetallic aluminium-salen complexes [{Al(salen)}(2)O] as catalysts for the synthesis of cyclic carbonates (including the commercially important ethylene and propylene carbonates) from a wide range of terminal epoxides in the presence of tetrabutylammonium bromide as a cocatalyst is reported. The bimetallic structure of one complex was confirmed by X-ray crystallography. The bimetallic complexes displayed exceptionally high catalytic activity and in the presence of tetrabutylammonium bromide could catalyse cyclic carbonate synthesis at atmospheric pressure and room temperature. Catalyst-reuse experiments demonstrated that one bimetallic complex was stable for over 60 reactions, though the tetrabutylammonium bromide decomposed in situ by a retro-Menschutkin reaction to form tributylamine and had to be regularly replaced. The mild reaction conditions allowed a full analysis of the reaction kinetics to be carried out and this showed that the reaction was first order in aluminium complex concentration, first order in epoxide concentration, first order in carbon dioxide concentration (except when used in excess) and unexpectedly second order in tetrabutylammonium bromide concentration. Further kinetic experiments demonstrated that the tributylamine formed in situ was involved in the catalysis and that addition of butyl bromide to reconvert the tributylamine into tetrabutylammonium bromide resulted in inhibition of the reaction. The reaction kinetics also indicated that no kinetic resolution of racemic epoxides was possible with this class of catalysts, even when the catalyst was derived from a chiral salen ligand. However, it was shown that if enantiomerically pure styrene oxide was used as substrate, then enantiomerically pure styrene carbonate was formed. On the basis of the kinetic and other experimental data, a catalytic cycle that explains why the bimetallic complexes display such high catalytic activity has been developed.

Synthesis of cyclic carbonates from epoxides and carbon dioxide using bimetallic aluminum(salen) complexes

“…One role for the tetrabutylammonium bromide is well established, 9 it acts as a bromide source to ring-open the epoxide. The induction period is consistent with the second role of tetrabutylammonium bromide being to form tributylamine in situ by a reverse Menschutkin reaction, 18 and we postulate that the tributylamine reacts with the carbon dioxide to form a carbamate salt. The adduct between a tertiary amine and carbon dioxide would normally be very unstable, but the bimetallic aluminium(salen) complex can stabilize both this and the bromo-alkoxide and allow the rate determining carbonate bond formation to occur intramolecularly rather than intermolecularly.…”

“…The aliphatic cyclohexyl and tert-butyl groups present in catalyst 1 facilitate the solubility of the catalyst in the epoxide, allowing cyclic carbonate synthesis to be carried out under solvent free conditions. Subsequent work 18 extended the range of cyclic carbonates which could be prepared using catalyst 1 and tetrabutylammonium bromide to include the three commercially most important examples (ethylene carbonate, propylene carbonate and glycerol carbonate; Scheme 2, R = H, Me, CH 2 OH respectively). Since ethylene oxide is a gas, the synthesis of ethylene carbonate was carried out in an autoclave at 6 bar carbon dioxide pressure and the synthesis of propylene carbonate was carried out at 1 bar carbon dioxide pressure but at 0 o C due to the volatility of propylene oxide.…”

An Efficient Iron Catalyst for the Synthesis of Five‐ and Six‐Membered Organic Carbonates under Mild Conditions

“…We have chosen to use MEK as the solvent because carbonyl-containing solvents allow for a higher dissolution of carbon dioxide in the reaction mixture compared with other solvent types. [36,37] It can be seen from the results listed in Table 1 that neither the co-catalyst NBu 4 I (entry 1) nor the [FeA C H T U N G T R E N N U N G (TPhOA)] 2 catalyst alone is active (entry 2). However, the combination of both the Fe complex and the co-catalyst produces an active system giving good yields of the cyclic organic carbonate product when using the iodide nucleophile, even when ambient temperatures and carbon dioxide pressures as low as 0.2 MPa (i.e., 2 bar) are used (entry 5).…”

Recent Advances in the Catalytic Preparation of Cyclic Organic Carbonates

“…[35][36][37] Shortly hereafter, the group of Kleij reported on a mononuclear Zn(salphen) complex (Figure 1; 2) active towards CO 2 coupling with terminal epoxides under moderate CO 2 pressures [p(CO 2 ) = 0.2-1 MPa] and mild operating temperatures (T = 25−45 ºC). 38,39 The high activity of the Zn(salphen) complex was ascribed to its constrained geometry imposed by the ligand scaffold, which imparts increased Lewis-acid character to the catalytically active Zn ion.…”