Unsymmetrical, Cyclic Diborenes and Thermal Rearrangement to a Borylborylene.

Cyclic diboranes(4) based on a chelating monoanionic benzylphosphine linker were prepared through boron-silicon exchange between arylsilanes and B2 Br4 . Coordination of Lewis bases to the remaining sp2 boron atom yielded unsymmetrical sp3 -sp3 diboranes, which were reduced with KC8 to their corresponding trans-diborenes. These compounds were studied with a combination of spectroscopic methods, X-ray diffraction, and DFT calculations. PMe3 -stabilized diborene 6 was found to undergo thermal rearrangement to gem-diborene 8. DFT calculations on 8 reveal a polar boron-boron bond, and indicate that the compound is best described as a borylborylene.