A Study of the Diastereoselectivity of Diels—Alder Reactions on the Ce—SiO2 as Support.

Diels-Alder (DA) reaction, leading to six membered ring products with controlled regioselectivity and stereochemistry, is a powerful tool in organic synthesis. Lewis acid catalysts, such as ZnCl2, AlCl3, FeCl3, Yb(OTF)3, and UI3, can have a significant effect on the rate and endo selectivity of DA reactions, especially when they are coordinated to the heteroatom of dienophile, such as the oxygen of carbonyl group. The carbonyl group become more electron-withdrawing when there is the interaction of metal ions as Lewis acid sites with carbonyl group of dienophile. This effect decreases the energy level of the LUMO of dienophiles and allows more efficiently overlapping with HOMO of the diene. Therefore, the reaction proceeds with great ease. According to the Pearson’s HSAB classification, trivalent state of rare earths which located between Mg and Ti are hard acids. The rare earths salts of strong acids are sometimes able to work as effective Lewis acid catalysts in the presence of water, alcohols, or even amines, although most traditional Lewis acids lose their activities under such conditions. The rare earth Lewis acid could be recovered after reactions and reused without any loss of activity because of their stability. Another important aspect of rare earths is their high coordination numbers. These properties are highly advantageous for coordinating various chiral and achiral ligands around the metal ions and strong affinities to the oxygen of carbonyl group as a hard ligand. Recently, it was reported that chiral rare earth organophosphates-cerium complexes were evaluated as a homogeneous Lewis acid catalyst for the asymmetric hetero-DA reaction. Homogeneous Lewis acids have serious drawbacks such as the use of more than one molar equivalent, laborious work-up procedures and problems with environmentally hazardous waste-streams. It was reported that several inorganic solid acids, such as zeolite, clays, mesoporous molecular sieves (Al-HMS and La-SBA-15), alumina, and metal functionalized silica (ZnCl2, AlCl3, FeCl3) are efficient heterogeneous catalysts in DA reactions of Cyclopentadiene. It was proved that silica gel in comparing to the other inorganic material, is a suitable support for the heterogenization of several kinds of catalysts, because it is an abundant and cheap material, with good adsorption properties due to their high surface area, nanoporousity, and physico-chemical stabilities during the reaction process. The silica surface has two types of functional groups such as siloxane (Si-OSi) and silanol group (Si-OH). Silanol groups on silica surface behave like a weak acid (pKa = 9.4), thus they have both strong hydrogen bonding and dipole-dipole interactions as the well known Brønsted acid. The acidity of surface was obtained and changed by adding a metal ion on the surface by increasing metal charge and concentration. In this work, the various concentrations of Lewis acid on silica gel surface were obtained by using ion exchange method of cerium ion (III) and (IV) in alkali conditions. Then, the diastereoselectivity of DA reactions of cyclopentadiene and methylacrylate with the resulting solid acids in the absence of solvent were studied. The high endo/exo diastereoselectivity was observed for more charge and high concentration of cerium ion on silica surface.