Ring-Opening Reactions of NHC-Boriranes with In Situ Generated HCl: Synthesis of a New Class of NHC-Boralactones

The first ring-opening reactions of ligated boriranes (boracyclopropanes) are described. Treatment of readily available NHC-boriranes bearing ester substituents on the borirane ring with HCl provides stable γ-NHC-bora-γ-lactones in isolated yields ranging from 40% to 73%. The reactions occur through 1,3-addition of HCl across a B–C bond of the NHC-borirane to form a ring-opened NHC-boryl chloride, followed by lactonization with chloride displacement. Experimental evidence suggests that both the borirane ring-opening reaction and the boralactonization reaction occur with inversion at boron.