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 AlI (rather than reduction of AlIII), 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.

Synthesis, Isolation, and Reactivity Studies of ‘Naked’ Acyclic Gallyl and Indyl Anions

“…The HOMO energies of N,N-ligated aluminyls lie in the range À 0.89 to À 0.97 eV, whereas that of NHBO-aluminyl III is À 2.77 eV. [7,37,40] In our prior investigations, [41] and those of other research groups, [42][43][44] the utilization of the cyclopentadienyl anion as a leaving group has been successfully demonstrated. Compound III, for example, is synthesized via a substitution process at Al I , bypassing the often low-yielding and highly condition-dependent reduction of Al III precursors, and driven instead by the low solubility of KCp* and the inherent oxophilicity of aluminum.…”

“…[13] More recently a dimeric acyclic bis(amido)aluminyl compound II, K 2 [Al{N-(Dipp)SiMe 3 } 2 ] 2 , was reported by Hicks and Liptrot et al, [36] while we have also disclosed the 'naked' acyclic aluminyl anion III, [K(2.2.2-crypt)][Al{OB(NDippCH) 2 } 2 ] and its capabilities in the reversible cycloaddition of benzene. [37] However, unlike these thallyl and aluminyl examples, charge-separated ('naked') acyclic gallyl [GaX 2 ] À and indyl [InX 2 ] À compounds have yet to be reported, free from the stabilizing influence of the associated counter-cation.…”