We herein report the reaction of arylallenes with tris(pentafluorophenyl)borane that yields pentafluorophenyl substituted indens. The tris(pentafluorophenyl)borane induces the cyclization of the allene and transfers a pentafluorophenyl ring in the course...

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.

Here we report the in situ generation of nucleophilic allylboranes from H2 and allenes mediated by a pyridonate borane that displays frustrated‐Lewis‐pair reactivity. Experimental and computational mechanistic investigations reveal that upon H2 activation, the covalently bound pyridonate substituent becomes a datively bound pyridone ligand. Dissociation of the formed pyridone borane complex liberates Piers borane and enables a hydroboration of the allene. The allylboranes generated in this way are reactive towards nitriles. A catalytic protocol for the formation of allylboranes from H2 and allenes and the allylation of nitriles has been devised. This catalytic reaction is a conceptually new way to use molecular H2 in organic synthesis.