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

Abstract

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(sp)-F bond cleavage with DIBAL-H, and the catalytic setup is applied to hydrodefluorinative Friedel-Crafts alkylations. A broad range of CF-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.

Selective synthesis of spirobiindanes, alkenyl chlorides, and monofluoroalkenes from unactivated gem-difluoroalkanes controlled by aluminum-based Lewis acids

“…The abstraction of a fluoride anion from the C(sp 3 )–F bonds is facilitated with increasing strength of the Lewis acidity of the main-group promoter, as the fluorine moiety is neither a good Lewis base nor a good leaving group 13 . Indeed, species with a stronger formal positive charge such as [Ph 3 C] + , [R 3 Si] + , [R 2 Al] + , [(C 6 F 5 ) 3 PF] + , and even P(III) dications with weakly coordinating anions, have recently been used for the direct cleavage and functionalization of C(sp 3 )–F bonds 10,12,51–53 . 2.…”

Recent Developments in Radical‐Mediated Transformations of Organohalides

“…Cleavage of the C–F bonds in Ar‐CF 3 substrates was accomplished by electrochemistry, reducing metals, transition metal complexes, or main‐group Lewis acids . Perichon reported electrochemical conditions for the formation of α,α‐difluorobenzylic anion and subsequent trapping with carbonyl‐based electrophiles .…”

Activation of Saturated Fluorocarbons to Synthesize Spirobiindanes, Monofluoroalkenes, and Indane Derivatives

“…In addition, the stronger Lewis acidity of fluorophilic electrophiles is essential for the direct heterolytic cleavage of C(sp 3 )-F bonds, given the high activation barrier. The substitution of the fluoride in C(sp 3 )-F bonds to form C-H, C-C, and C-heteroatom bonds has been initiated by neutral, strong aluminum- and boron-based Lewis acids (Stahl et al., 2013, Greb, 2018, Morgan et al., 2013, Koerte et al., 2017, Ahrens et al., 2013, Jaiswal et al., 2017) or cationic species such as [CPh 3 ] + , [SiEt 3 ] + , [ i Bu 2 Al] + , [(C 6 F 5 ) 3 FP] + , and even P(III) dications such as [(bipy)PPh] 2+ bearing weakly coordinating counter anions such as [B(C 6 F 5 ) 4 ] - (Stahl et al., 2013, Klahn et al., 2007, Gu et al., 2009, Forster et al., 2017, Douvris and Ozerov, 2008, Scott et al., 2005, Großekappenberg et al., 2015, Zhu et al., 2016, Chitnis et al., 2018).…”