A Three-Center Orbital Interaction in the Diels-Alder Reactions Catalyzed by Lewis Acids

Ab initio calculations were performed on title reactions between butadiene and acrolein with BCl(3), AlCl(3), GaCl(3), InCl(3), ZnCl(2), SnCl(2), and SnCl(4). A dimethyl ether molecule is explicitly considered in various reaction systems to examine solvent effects. First, the reaction path of an AlCl(3)-promoting reaction was examined thoroughly. This reaction has two channels. The first one involves a weak reactant-like complex (precursor) and a normal [4 + 2] addition. The second does three elementary processes, one-center addition, ring closing, and Claisen shift. The first channel is more favorable by 12.1 kcal/mol (B3LYP/6-311+G(2d,p) SCRF//B3LYP/6-31G SCRF) than the second one. Then the first channels with other Lewis acids were traced with and without an ether molecule. The ether molecule has an appreciable effect not on geometries but on activation energies. BCl(3) is desolvated and has extraordinarily strong catalytic ability. Even with the strongest catalyst, not a [2 + 4] but a normal [4 + 2] cycloaddition takes place. Except for BCl(3), SnCl(4) is the strongest Lewis acid with the ether molecule. The frontier orbital, LUMO, of acrolein is distorted in the course of the reaction so that the formation of two C-C covalent bonds is possible. The precursor formation and the one-center addition were discussed also by the frontier orbital theory.