N‑Heterocyclic Carbene Boranes are Hydrogen Donors in Masamune–Bergman Reactions of Benzo[3,4]­cyclodec-3-ene-1,5-diynes

Thermal reactions of benzo­[3,4]­cyclodec-3-ene-1,5-diyne with N-heterocyclic carbene boranes (NHC-boranes) provided mixtures of 9-borylated 1,2,3,4-tetrahydro­anthracenes along with 1,2,3,4-tetrahydro­anthracene. These products indicate that NHC-boranes serve as hydrogen donors to a p-benzyne intermediate formed by the Masamune–Bergman reaction. Experimental results support a radical mechanism in nonpolar solvents, but suggest that ionic mechanisms compete in the production of 1,2,3,4-tetra­hydro­anthracene when the reaction is performed in a polar solvent.