Multiple Cycloaddition Reactions of Ketones with a beta-Diketiminate Al Compound

A beta-diketiminate Al compound (1) with an exocyclic double bond reacts with two equivalents each of benzophenone and 2-benzoylpyridine in a [4+2] cycloaddition to generate bicyclic and tricyclic compounds 2 and 3, respectively. Compound 2 consists of six-and eight-membered aluminium rings, whereas 3 has two five-and one eight-membered ring. Compounds 2 and 3 were characterized by a number of analytical tools including single-crystal X-ray diffraction. The quantum mechanical calculations suggest that the dissociation of the solvent molecule from 1 would lead to an active species 1A having two 1,4-dipolar 4 pi electron moieties, in which the electrophilic site is the Al atom and the nucleophilic positions are polarized exocyclic and endocyclic C=C pi bonds. The detailed mechanistic study shows that the dipolarophiles, benzophenone, and 2-benzoylpyridine undergo double cycloaddition with two 1,4-dipolar 4 pi electron moieties of 1A. Herein, the addition of one molecule of the dipolarophile promotes the addition of the second one.