An Electrostatically Enhanced Phenol as a Simple and Efficient Bifunctional Organocatalyst for Carbon Dioxide Fixation

Abstract

An electrostatically enhanced phenol as a simple and competent bifunctional organocatalyst for the atom‐economical conversion of epoxides to cyclic carbonates under environmentally benign conditions is described. Incorporating a positively charged center into phenols through a modular one‐step synthesis results in a bifunctional system with enhanced acidity and reactivity, capable of epoxide activation, a halide nucleophilic ring‐opening process, and CO2 incorporation in a synergistic fashion. A rational survey of the efficiency of different positively charged phenols and the influence of different parameters, such as temperature, catalyst loading, and the nature of the nucleophile, on catalytic activity was conducted. In addition, the time‐dependent conversion of epoxide into the corresponding cyclic carbonate was further explored by FTIR‐ATR and 1H NMR spectroscopy. This bifunctional catalytic platform is among one of the mildest and most efficient metal‐free systems that is capable of converting a variety of epoxides into cyclic carbonates under virtually ambient conditions. The 1H NMR titration experiment validated the bifunctional catalytic mechanism wherein both the epoxide activation and the nucleophilic ring‐opening process occur in concert en route to carbon dioxide fixation.

Toward a Neutral Single-Component Amidinate Iodide Aluminum Catalyst for the CO2 Fixation into Cyclic Carbonates

“…The reactions were carried out at 80 °C and 1 bar of CO 2 pressure for 24 h employing a combination of 1.5 mol % of complex 2 and 1.5 mol % of tetrabutylammonium iodide (TBAI) as a cocatalyst, while only 1.5 mol % of the single-component catalyst 3 were used under solvent-free conditions. Interestingly, no polycarbonates were obtained under these reaction conditions as expected, since these epoxide substrates generally showed selectivity toward cyclic carbonate formation. − In general, catalyst 2 achieved moderate to excellent yields for the preparation of cyclic carbonates 5a – h (Table , entries 1–8). It is worth noting that the mixed catalytic system ( 2 /TBAI) was reactive toward the formation of a wide variety of cyclic carbonates functionalized with alkyl, aryl, halide, and ether groups, which demonstrates that compound 2 has a broad scope and is selective for cyclic carbonate formation.…”

“…A large number of catalytic systems have been developed for the preparation of cyclic carbonates including organocatalysts − and metal complexes. − These generally act as Lewis acids in the activation of the epoxides for their transformation to cyclic carbonates. The choice of the metal center is essential, and aluminum, an abundant element in the earth’s crust, is one of the most suited metals for cyclic carbonates formation. − Moreover, regarding the use of ligands, Schiff base ligands have been widely investigated, promoting increasing reactivity of the catalysts. , Among the catalytic systems reported in the literature, the bimetallic aluminum­(salen) complex (Figure a) was found to be an excellent catalyst for the preparation of cyclic carbonates under mild reaction conditions .…”

Efficient and Easily Reusable Metal-Free Heterogeneous Catalyst Beads for the Conversion of CO2 into Cyclic Carbonates in the Presence of Water as Hydrogen-Bond Donor

“…Next, we investigated the effect of the reaction temperature on the catalytic activity of the two heterogeneous catalysts Amb-OH-I-910 and Amb-I-910 (Table and Figure S9). Most heterogeneous catalysts for the cycloaddition of CO 2 to epoxides operate at relatively high temperature, typically in the 100–150 °C range. , On the other hand, there is increasing interest for catalysts that are able to convert CO 2 into cyclic carbonate under mild conditions (e.g., T ≤ 60 °C and p ≤ 10 bar), and several homogeneous catalysts that are able to operate efficiently under such less energy-intensive conditions have been reported recently. ,,, Here, we chose to explore a relatively wide range of reaction temperatures (45–150 °C), including an evaluation of the activity at high temperature but with very low catalyst loading or at low temperature but with relatively high catalyst loading. The Amb-OH-I-910 catalyst proved to be active at a very mild temperature for a metal-free heterogeneous system (45 °C), reaching 33% yield of styrene carbonate with 98% selectivity when employing a 3 mol % loading of iodide relative to the epoxide (Table , entry 1).…”

Recent developments in organocatalysed transformations of epoxides and carbon dioxide into cyclic carbonates

“…Another pyridinium based catalyst was hydroxy functionalised pyridinium iodide 27 reported by Rostami et al in 2018. 74 The hydroxy group could act as a hydrogen bond donor during the ring-opening of the epoxide, thus improving the catalytic activity of the pyridinium iodide. The position of the hydroxyl group on the pyridine ring dramatically affected the catalytic performance, as 2-hydroxy-N-methylpyridinium iodide was much less active than 27.…”