Piers' Borane-Catalyzed Dimerization of Arylallenes via Transborylation: A Synthetic and Mechanistic Study

Herein, we report a Piers' borane-catalyzed dimerization of arylallenes. Catalytic turnover is achieved by a transborylation with pinacolborane (HBpin) that releases Piers' borane and promotes the catalytic reaction, thereby giving convenient access to a range of synthetically versatile bench-stable pinacolborane-substituted 1,5-dienes. The synthetic utility of these pinacolborane-substituted 1,5-dienes was demonstrated by Suzuki-Miyaura coupling, the synthesis of azide and halogen-substituted 1,5-dienes, and the oxidation to γ,δ-unsaturated aryl ketones. The mechanism of the reaction was investigated experimentally as well as by dispersion-corrected double-hybrid density functional theory (DFT) computations. These investigations indicate that Piers' borane mediates the allene dimerization and a subsequent Cope rearrangement to yield a B(C₆F₅)₂-substituted 1,5-diene. This diene then undergoes a transborylation via a concerted σ-bond metathesis at the final stage of the catalytic reaction.