Synthesis of Boriranes by Double Hydroboration Reactions of N-Heterocyclic Carbene Boranes and Dimethyl Acetylenedicarboxylate

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Reaction of bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene borane with dimethyl acetylenedicarboxylate gives 80% yield of a stable borirane (boracyclopropane) formed by formal double hydroboration along with 5% of the (E)-alkenylborane. DFT calculations suggest a mechanism where divergence to the two products occurs after a common initial stage of hydride transfer from the NHC-borane to the acetylenedicarboxylate.