Chemical catalysis affects our lives in innumerable ways. Catalysis provides a means of changing the rates at which chemical bonds are formed and broken and of controlling the yields of chemical reactions to increase the amounts of desirable products from these reactions and reduce the amounts of undesirable ones. Thus, it lies at the heart of our quality of life: the reduced emissions of cars, the abundance of fresh food at our stores, and the new pharmaceuticals that improve our health are made possible by chemical reactions controlled by catalysts. The petrochemical, chemical, and pharmaceutical industries depend on catalysts to produce everything from fuels to drugs, to paints and cosmetics. Today, we, as humans and chemists, face a variety of challenges in developing alternative energy sources, reducing dangerous by-products in manufacturing, cleaning up the environment and preventing future pollution, protecting citizens from the release of toxic substances and infectious agents, and creating safe pharmaceuticals. Catalysts are needed to meet these challenges, but their complexity and diversity demand a revolution in the way catalysts are designed and used. This revolution can become reality through the application of new methods for synthesizing and characterizing molecular and material systems. Organic chemistry can make a significant contribution in this change. In fact, in the last years green chemical processes started playing a crucial role in sustainable development, and efficient environmental catalysts, that can be conveniently applied in various chemical reactions, are the key elements for the improvement of sustainable synthetic processes. Many organic transformations rely on Lewis acid catalysts, and such compounds were widely studied in organic synthesis. The rare earth metal compounds come to play a significant role in these studies, because the majority of rare earth metal catalyst show characteristic properties of sustainable catalysts, such as low cost, aqueous/air/thermal stability, recyclability, and last but not the least high catalytic efficiency. With this idea in mind we studied new bond forming reactions using cerium trichloride in combination with sodium iodide. As reported in the first chapters of this work, we described new synthetic methods promoted by cerium(III) such as hydroiodination reaction of alkynes, synthesis of polysubstituted furans and oxazole derivatives. Furthermore we showed an example of the excellent catalytic activity of cerium trichloride in synergistic catalysis with other metal cocatalyst such as copper iodide, in the synthesis of 2-substituted benzimidazoles. We also demonstrated that these procedures are useful tools for the construction of more complex molecules having biological activity. In addition, we should recall that during the second half of the 20th century, transition metals began playing an important role in organic chemistry and this led to the development of a large number of transition metal-catalyzed reactions for creating organic molecules. In fact, transition metals have a unique ability to activate various organic compounds and through this activation they can catalyze the formation of new bonds. Recent years have witnessed tremendous growth in the number of gold-catalyzed highly selective chemical reactions. Gold was considered as an inert metal for a long time, and its ability to behave as Lewis-acid was recently recognized as a source of inspiration for organic chemists. Gold catalysts can activate unsaturated functionalities and therefore allow the creation of carbon-carbon and carbon-heteroatom bonds by addition of various nucleophiles, under mild conditions. As part of our ongoing studies on metal use in organic chemistry, in the last chapter we reported progress made on Cu-Au cocatalyzed one-pot synthesis of 2,5-dihydrofurans derivates from terminal alkynes, which could be very attractive and sustainable method, due to the reduction of work-up and purification steps and reduction of costs, materials and time.
|Titolo:||Importance of metals in organic synthesis: development of new methodologies for bond formation mediated by Lewis acid|
|Data di pubblicazione:||19-mar-2013|
|Appare nelle tipologie:||Tesi di dottorato (Pregresso)|