The Organic Reactions Catalyzed By Zirconium Compounds

Lewis acids are wide used in organic reactions. Many organic reactions, which originally cannot be carried out or are difficult to operate, have be successfully performed in mild conditions in the presence of Lewis acids. The field of Lewis acid catalysts is expanding and will boost the development new synthetic methods under milder conditions, atom economies and with high yields and selectivities in 21 century. With increasing environmental concerns and the need for efficient and green Lewis acid catalysts for various useful organic transformations, the interest in zirconium and its compounds has increased in last decade. Zirconium compounds are considered to be relatively high abundance (0.016%, ranking about twentieth in the earth's crust. Most zirconium compounds are relatively low toxicity, easy to handle, low cost and can tolerate small amounts of moisture. Zirconium (IV) compounds possess a high coordinating ability that allows displaying a strong Lewis acid behavior and high catalytic activity. On the basis of that pioneering works, the new applications of Zirconium (IV) compounds as Lewis acid catalysts in organic synthesis have been expanded. Firstly, from a viewpoint of synthetic organic chemistry, the recent development and application of zirconium compounds as promoter and catalyst in organic reactions such as addition reactions, cycloadditions, reduction, cyclization, rearrangement, the formation and cleavage of common protecting groups were reviewed.Replacenment organic solvents with environmentally reaction media fits for the demand of green chemistry. But, it is unavoidable to introduce some problems such as cycle and disposal of solvent-free organic synthesis would be the first choice. For reasons of economy and pollution, solvent-free methods are of great interest in order to modernize classical procedures making them more clean, safe and easy to perform. An enhancement in kinetics can result from increasing concentrations in reactants when a diluting agent such as a solvent is avoided. A variety ofβ-enamino ketones and esters have been synthesized in high to excellent yields by reactingβ-dicarbonyl compounds with amines in the presence of zirconium oxidechloride. The reaction proceeds smoothly in short reaction time under solvent-free conditions. This method is suitable for various substrates, operational and experimental simplicity. Most of compounds were characterized by IR and 1HNMR, 13HNMR and MS.The synthesis of xanthene derivatives has been of considerable interest to pharmaceutical chemist and organic chemist because of their wide range of biological and pharmaceutical properties such as anti-dysplasia, antibacterial, anti-allergy, anticancer activities, and used to treatment of allergic trachitis and diabetes. In addition, these compounds are structural units of many natural products and important heterocyclic compounds intermediate. Therefore, the synthesis of xanthene derivatives is present research hot spot. Traditional method of synthesis of xanthene derivatives is reaction of aldehydes with carbonyl compound which containing active methylene compounds under acidic conditions or alkali conditions. The key for this reaction is to search an effective catalyst. The efficacy of various Lewis acids was tested for this conversion and zirconium oxidechloride was found to be superior in terms of yield and reaction rates. The method is applicable to a wide range of aldehydes. The reaction proceeds smoothly in short reaction time under solvent-free conditions.Pyrroles are important class of heterocyclic compounds having different biological activities and special use. They can be used in many filed such as medicine, pesticide, daily chemistry, dyestuff, food industries, polymeric medical materials, and optical information materials. They are a highly versatile class of intermediates in the synthesis of natural products as well as in heterocyclic chemistry. A number of methods have been developed for their synthesis. The most simple and significant procedure is Paal-Knorr reaction. We investigated the catalytic activities of various Lewis acids to promote this conversion and found that ZrCl4 is an efficient catalyst. We developed a modified Paal-Knorr method to synthesis of valuable substituted pyrroles under ultrasound irradiation using ZrCl4 as a catalyst. This method is bestowed with several unique merits, such as relatively short reaction times, simplicity in operation, high yields of products, mild reaction conditions, and the use of cheap and easily accessible catalyst.In conclusion, zirconium compounds as promoter and catalyst demonstrated the importance and advantages in many important organic transformations. Despite the significant progress in the application of zirconium salts as catalysts in diverse organic synthesis has been achieved, the search for some novel synthetic process involving zirconium compounds will continue to be an interesting and challenging research topic. On the basis of that pioneering works, the new applications of zirconium compounds as Lewis acid catalysts in organic synthesis have been expanded. The research of new applications of zirconium compounds will renew and expand for classical acid and base theory in organic chemistry. This will provide a platform and opens up a new path for the search for new reagents, the establishment of new methods and new theories. It will promote the development of organic synthesis chemistry.