Intensifying organic processes: a “green” toolbox for the synthesis of benign-by-design chemicals from waste feedstocks

This Ph.D. thesis is centered on developing greener methodologies for the synthesis of bio-based compounds starting from waste or low-value feedstock. The work is divided in two main chapters based on output, one dealing with the synthesis of cyclic organic carbonates (Chapter 2), the other with the synthesis of glycerol derivatives (Chapter 3). In Chapter 2 a novel class of tungstate ionic liquids (TILs) was studied. Different synthetic routes were followed, some already published, others that exploit a green halide-free protocol developed in this thesis. The TILs were initially investigated for the CO2 fixation reaction into epoxides. Once established their potential use in this field, the TILs were investigated for the tandem direct oxidative carboxylation (DOC) of olefins to give cyclic organic carbonates. Tandem catalysis is a way to achieve process intensification by using the same catalyst for two or more sequential reaction steps having different mechanisms. TILs are demonstrated as effective tandem catalysts for the direct synthesis of COCs from olefins. In the first step, the TILs promote epoxidation of the olefin, while in the second step they catalyse insertion of CO2 into the epoxide, without any intermediate work-up. The procedure is greener than current protocols from several standpoints: H2O2 is used as oxidant, atmospheric pressure of CO2 is sufficient to achieve yields >90% in COCs and product recovery occurs by simple phase separation. Additionally, simple alkali metal halide salts (e.g., NaBr, NaI, KBr, KI) are sufficient to promote CO2 insertion into epoxides in place of traditional costlier (for their environmental burden and resource use) ammonium halides. The simple alkali metal halide salt NaBr is also used as catalysts in a new CO2 insertion process run in continuous flow. In this case, NaBr is activated by diethylene glycol (DEG) that acts as an inexpensive and largely available complexing agent for Na+ as well as a hydrogen-bond donor that promotes the ring- opening of the epoxides. An in-depth study of the continuous-flow conditions allowed to obtain COCs with high yields from terminal epoxides, and with a higher overall productivity compared to the batch process. A simple method for the recycling of the catalytic system was also developed. In Chapter 3, focus is on the synthesis of high value-added glycerol derivatives, i.e., esters, acetals and orthoesters of glycerol. Initially, the acetylation of glycerol and glycerol acetals with esters (in lieu of the commonly used acetic acid and acetic anhydride) in continuous flow was developed. A thermal, catalyst- free, continuous flow protocol allowed to reach high conversions of the substrates. Among the different esters that were tested, isopropenyl acetate (iPAc) showed the highest performance, affording quantitative yields in marketable products such as Solketal acetate and triacetin. The better performance of iPAc is due to the fact that it promotes an irreversible esterification process caused by the release of acetone. Next, tandem acetalization of glycerol with the acetone released in situ was studied. This reaction did not proceed satisfactorily under catalyst-free conditions. The direct synthesis of Solketal acetate by tandem acetalization-acetylation reactions of glycerol with isopropenyl acetate was therefore explored using Amberlyst-15 as acid catalyst. By addition of acetic acid and/or acetone as co-reactant, the selective synthesis of Solketal acetate or of a 1:1 mixture of Solketal acetate and triacetin was attained. Finally, new bio-based glycerol derivatives by reaction with orthoesters were investigated. The reactions of glycerol with this class of compounds – only scarcely explored in the 60s – yielded the first regioselective synthesis of 5-membered ring diastereoisomeric derivatives of glycerols through a catalyst-free procedure.