Reversible [4 + 1] Cycloaddition of Arenes by a “Naked” Acyclic Aluminyl Compound

Abstract

The large steric profile of the N-heterocyclic boryloxy ligand, -OB(NDippCH)₂, and its ability to stabilize the metal-centered HOMO, are exploited in the synthesis of the first example of a "naked" acyclic aluminyl complex, [K(2.2.2-crypt)][Al{OB(NDippCH)₂}₂]. This system, which is formed by substitution at Al^I (rather than reduction of Al^III), represents the first O-ligated aluminyl compound and is shown to be capable of hitherto unprecedented reversible single-site [4 + 1] cycloaddition of benzene. This chemistry and the unusual regioselectivity of the related cycloaddition of anthracene are shown to be highly dependent on the availability (or otherwise) of the K⁺ countercation.