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

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.