The Thermal Rearrangement of an NHC-Ligated 3-Benzoborepin to an NHC-Boranorcaradiene

An N-heterocyclic-carbene-ligated 3-benzoborepin with a bridged structure has been synthesized by double radical trans-hydroboration of benzo[3,4]cycloundec-3-ene-1,5-diyne with an N-heterocyclic carbene borane. The thermal reaction of the NHC-ligated borepin at 150 degrees C gives an isolable NHC-boranorcaradiene. Experiments and density functional theory calculations support a mechanism whereby the borepin initially rearranges to a boranorcaradiene by a thermal 6 pi-electrocyclic reaction. This is followed by 1,5-boron shift to give a rearranged boranorcaradiene. This shift occurs with stereoinversion at boron through a transition state with open-shell diradical character. This is the first example of the isolation of a boranorcaradiene from a thermal reaction of a borepin.