FREE-BASE PORPHYRIN FOR CO2 ACTIVATION

Chapter I: Introduction The human influence in the raise of the atmospheric concentration of carbon dioxide registered in the last decades up to the actual alarming situation is clear. The most evident consequence is the global warming, but this is only one of the problems correlated to emissions of greenhouse gases. Unfortunately, almost all the economic sectors play a role in the CO2 production and the complete elimination of the anthropogenic emissions of carbon dioxide is not possible in the short term. Thus, one of the major scientific challenges of this century is to find a solution to this problem. In addition to the reduction of the emissions, three ways has emerged as the most promising and interesting tools to face the carbon dioxide problem: Carbon Capture and Storage (CCS), Bioenergy from Carbon Capture and Storage (BECCS) and Carbon Capture and Usage (CCU). The carbon dioxide cycloaddition to three membered heterocycles is an example of CCU that can be exploited to mitigate the human influence on the climate change. Moreover, this reaction is very promising because makes possible the synthesis of very useful fine chemicals such as cyclic carbonates and oxazolidinones from a waste -as CO2- with a 100% of atom economy. However, carbon dioxide is a very stable molecule and an efficient catalytic species is needed to make the reaction proceeds by avoiding drastic conditions. Even if the reaction between epoxides and CO2 has been deeply studied in the last 20 years, the analogous carbon dioxide cycloaddition to aziridines has been much less studied and the conversion of N aryl aziridines into corresponding oxazolidinones resulted challenging and only few examples of efficient catalysts have been reported up to now. This work is devoted to the study of porphyrins as organic catalysts of the carbon dioxide cycloaddition both under homogeneous and heterogeneous conditions with the final the goal to improve the sustainability of the production of cyclic carbonates and oxazolidinones. Chapter II: Homogeneous catalysts for the CO2 cycloaddition reaction In this chapter the full organic catalytic meso-tetraphenyl porphyrin (TPPH2)/TBACl combination has been studied in the synthesis of oxazolidinones and cyclic carbonates from CO2 and aziridines or epoxides, respectively. After the initial optimization of the experimental conditions, this catalytic system was applied for converting a plethora of different substrates into corresponding five-membered products. The gathered data made possible to present the first general methodology for the conversion of N-aryl aziridines and CO2 into N-aryl oxazolidinones. Then, a tandem methodology for the production of these useful molecules from aryl azides, styrenes and carbon dioxide has been presented. This methodology resulted effective also in the synthesis of the pharmacologically active oxazalidin-2-one 41. Encouraged by the good results obtained in these studies, the proposed catalytic procedure was successfully extended to the reaction of CO2 with N alkyl aziridines and epoxides. Moreover, different catalysts have been tested in combination with TBACl in order to shed some light on the influences of the steric and electronic characteristic of the porphyrins on the reaction productivity. Moreover, a first mechanistic study has been presented in collaboration with Dr. Gabriele Manca of the ICCCOM-CNR of Sesto Fiorentino. Chapter III: Heterogeneous catalysts for the CO2 cycloaddition reaction The heterogenization of the porphyrin onto a SBA-15 silica material has been proposed in this chapter. To improve the reaction sustainability, the synthesis of an active heterogeneous catalyst represents a crucial step to simplify the catalyst recovery and the product purification. The possibility to combine the characteristics of a heterogeneous catalyst with the high activity demonstrated by the TPPH2/TBACl homogeneous catalytic system propelled the preparation of a hybrid material by supporting a free porphyrin onto mesoporous SBA-15 silica trough covalent bondings. This solid support was selected because it is easy to functionalize and the presence of silanols group onto its surface foster the reactivity of epoxides towards CO2. The so-prepared catalyst, in combination with tetrabutyl ammonium iodide (TBAI) resulted very active in the preparation of cyclic carbonates from the reaction of epoxides with CO2. Then, the same catalytic system was successfully tested for the carbon dioxide cycloaddition to N alkyl aziridines. Finally, the supported porphyrin, in combination with TBACl, represents the first heterogeneous catalyst active in the conversion of N-aryl aziridines into N-aryl oxazolidinones. The good results obtained with the three different classes of substrates, in combination with the recyclability of the heterogeneous catalyst and the possibility to use it with up to 1.00 g of substrate, make this hybrid material an interesting candidate for further industrial applications.