Facile Benzene Reduction by a Ca2+/Al-I Lewis Acid/Base Combination

Attempted synthesis of the donor-acceptor complex (BDI)(Ca+AlI)(BDI) complex by reaction of (BDI)Ca+ in the form of its B(C6F5)(4)(-) salt with (BDI)Al-I in benzene led to dearomatization of the solvent and formation of (BDI)Ca+(C6H6)Al-III(BDI) (BDI=CH[C(CH3)N-Dipp](2), Dipp=2,6-diisopropylphenyl). The C6H62- anion is strongly puckered and its boat form features four long (ca. 1.50 angstrom) and two short (ca. 1.34 angstrom) C-C bond distances. The flagpole positions of the C6H62- anion chelate an Al-III 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 Ca2+ metal ion. The complex is stable in solution up to 80 degrees 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-I.