Diastereoselective and Chemically Reversible C-C Bond Formation Mediated by an (N-heterocyclic)boryloxy Aluminyl Compound.

We report on the reductive C-C coupling of alkenes at a single main-group-metal center. Treatment of the bis(boryloxy)aluminyl compound K[Al{OB(NDippCH)}] with ethene results in clean formation of a five-membered alumina-cyclopentane via alkene dimerization. In the case of propene, similar chemistry occurs regio- and diastereoselectively to generate the corresponding -3,4 disubstituted metallacycle. Isolation of an intermediate alumina-cyclopropane and quantum chemical analysis allows the mechanism of these transformations to be elucidated, with coupling between the most hindered carbon centers in propene being driven by the necessity to minimize steric interactions with the bulky aluminum-bound ligand scaffold. Although these transformations are thermodynamically favorable and physically irreversible (no alkene regeneration under vacuum), chemical reversibility can be effected through the addition of small unsaturated molecules (CO, CO, alkynes), which leads to C-C bond scission and the loss of one or both alkene equivalents. These findings expand the reactivity landscape of low-valent Al(I) reagents and showcase main-group-mediated pathways that parallel classical transition metal chemistry.