Study On The Applications Of Cerium Trichloride As Lewis Acid Catalyst In Organic Synthesis

Cerium is a group III B element in the periodic table. As a lanthanide, Cerium possesses a lot of special features, such as large ionic radius, high coordination number and unique electronic arrangement. China has abundant cerium reserves. Therefore, it is significant to develop new applications of cerium compounds in organic synthesis. In recent years, Lewis acid-catalyzed reactions have drawn much attention in the field of organic synthesis. Compared with the conventional Lewis acid, CeCl3 has advantages. It is water-insensitive and environmentally benign, because of its low toxicity. In view of the principles of green chemistry, this thesis focus on CeCl3-catalyzed organic reactions under mild conditions, especially the reaction to form C-N bonds.NaBH4-CeCl3·7H2O was used to reduce aromatic esters and aromatic imines. Experimental results showed that CeCl3·7H2O can effectively catalyze the reduction of aromatic esters to the corresponding alcohols with NaBH4 as reducing reagent. A wide range of functional groups such as nitro and alkene groups can be tolerated in the reaction. NaBH4-CeCl3·7H2O can also reduce the condensation products formed from aromatic aldehydes or ketones with aromatic amines. The reduction products were obtained in the yield of 62%-92%. Compared with the traditional transition metal-catalyzed reactions, CeCl3·7H2O showed high catalytic activity. The reduction proceed under mild conditions, and may become a potential method for industrial applications.The condensation of aryl sulfonamides with different aromatic aldehydes or ketones can be catalyzed with 10 mol% CeCl3 and the corresponding N-sulfonylimines were obtained. It was also found that catalytic amount of CeCl3 can promote the deprotonation of 1,3-dicarbonyl compounds. This was used in the synthesis of 1,3,3’- tricarbonyl compounds and the products can be obtained in a 63%-93% yield at room temperature.A novel method for the synthesis of 2-substituted-benzimidazoles, benzoxazoles and benzothiazoles from the condensation of aldehydes with 2-phenylenediamine, 2-aminophenol or 2-aminothiophenol in the presence of CeCl3-NaI was developed. Experimental results showed that the catalytic system of CeCl3-Nal in this kind of carbon-nitrogen bond forming process was highly efficient.2-Substituted-benzimidazoles and 2-substituted benzothiazoles were obtained in 61%-89% and 75%-92% yields respectively using 10 mol% CeCl3-NaI in dimethyl carbonate under oxygen atmosphere. 2-Substituted benzoxazoles were obtained by changing the solvent to toluene. Fukui index of the reactant molecules were calculated, it was found that the yield of terminal product was relative to the electronic capability on the aldehyde carbon atom. Aldehydes bearing electron-withdrawing groups were favorable for the formation of the desired products.A simple, and efficient CeCl3-catalyzed procedure for the synthesis of 2-aryl or alkyl substituted quinazolinones from the condensation and oxidative cyclization of 2-aminobenzamide with aldehydes was reported. The reaction was carried out in the green solvent dimethyl carbonate in the presence of 5 mol% CeCl3 under air and gave the corresponding 2-substituted-quinazolin-4(3H)-ones in 60%-90% yield. The reactions of a variety of aldehydes, isatoic anhydride with ammonium acetate in the presence of 5 mol% CeCl3 were also studied and the corresponding 2-substituted-quinazolin-4(3H)-ones were obtained in high yields.5 mol% CeCl3 can also catalyze the reaction of triethyl orthoformate, isatoic anhydride with anilines to give the corresponding 3-substituted-quinazolin-4(3H)-ones.Si02-CS-supported CeCl3·7H2O-NaI was successfully prepared and applied for deprotection reactions such as the deprotection of t-butyl ether and dithio acetals. Results showed that the supported catalyst had good stability and catalytic activity, and the catalyst could be easily recovered and reused without significant loss of catalytic activity.Thus, as a Lewis acid catalyst, CeCl3 can catalyze several reactions, such as reduction, condensation and C-N bond forming reaction, under mild conditions with high catalytic reactivity. We believe the research of new reactions with CeCl3 as catalyst is significant in both academic and industrial aspects. Advances in this field will also promote the development of green chemistry.