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Cooperative Al–H Bond Activation in DIBAL-H: Catalytic Generation of an Alumenium-Ion-Like Lewis Acid for Hydrodefluorinative Friedel–Crafts Alkylation

Forster, Francis, Metsänen, Toni T., Irran, Elisabeth, Hrobárik, Peter, Oestreich, Martin
Journal of the American Chemical Society 2017 v.139 no.45 pp. 16334-16342
Lewis acids, Lewis bases, alkylation, aluminum, aromatic hydrocarbons, catalysts, chemical bonding, chemical structure, heterolytic cleavage, hydrides, ions, ruthenium, sulfur
The Ru–S bond in Ohki–Tatsumi complexes breaks oligomeric DIBAL-H structures into their more reactive monomer. That deaggregation is coupled to heterolytic Al–H bond activation at the Ru–S bond, formally splitting the Al–H linkage into hydride and an alumenium ion. The molecular structure of these Lewis pairs was established crystallographically, revealing an additional Ru–Al interaction next to the Ru–H and Al–S bonds. That bonding situation was further analyzed by quantum-chemical calculations and is best described as a three-center–two-electron (3c2e) donor–acceptor σ(Ru–H) → Al interaction. Despite the extra stabilization of the aluminum center by the interaction with both the sulfur atom and the Ru–H bond, the hydroalane adducts are found to be stronger Lewis acids and electrophiles than the free ruthenium catalyst and DIBAL-H in its different aggregation states. Hence, the DIBAL-H molecule and its Al–H bond are activated by the Ru–S bond, but these hydroalane adducts are not to be mistaken as sulfur-stabilized alumenium ions in a strict sense. The Ohki–Tatsumi complexes catalyze C(sp³)–F bond cleavage with DIBAL-H, and the catalytic setup is applied to hydrodefluorinative Friedel–Crafts alkylations. A broad range of CF₃-substituted arenes is efficiently converted into unsymmetrical diarylmethanes with various arenes as nucleophiles. Computed fluoride-ion affinities (FIAs) of the hydroalane adducts as well as DIBAL-H in its aggregation states support this experimental finding.