Facile Benzene Reduction by a Ca /Al Lewis Acid/Base Combination

Attempted synthesis of the donor-acceptor complex (BDI)Ca ←Al (BDI) complex by reaction of (BDI)Ca in the form of its B(C F ) salt with (BDI)Al in benzene led to dearomatization of the solvent and formation of (BDI)Ca (C H )Al (BDI) (BDI=CH[C(CH )N-Dipp] , Dipp=2,6-diisopropylphenyl). The C H anion is strongly puckered and its boat form features four long (ca. 1.50 Å) and two short (ca. 1.34 Å) C-C bond distances. The flagpole positions of the C H anion chelate an Al cation giving a norbornadiene-like fragment with Al in the 7-position. The C=C double bonds of this alumina-norbornadiene strongly coordinate to the Ca metal ion. The complex is stable in solution up to 80 °C. Several mechanisms for its formation are discussed including a highly likely frustrated Lewis pair type mechanism in which benzene is activated by the Lewis acid (BDI)Ca followed by nucleophilic attack by the Lewis base (BDI)Al .