Synthesis of symmetrical and unsymmetrical cyclic diborenes <i>via</i> NHC-directed C–H borylation

Cyclic diborenes are recognized for their diverse reactivity and properties arising partly due to their ring strain. However, progress in this field has been relatively slow due to the unavailability of suitable synthons for their synthesis. In this study, we describe the synthesis of both symmetrical and unsymmetrical cyclic diborenes using an N-heterocyclic carbene (NHC) as a chelating agent. The synthesis involves various haloboranes and diboranes that induce spontaneous and reductive C-H borylation of an aryl substituent of the NHC. Interestingly, the 1,1-(geminal) chelated isomer of the symmetrical diborane undergoes isomerization to form its 1,2-(vicinal) chelated isomer. Furthermore, the 1,1-(geminal) isomer exhibits a dihedral angle of 44°, making it the most twisted example of all established diborenes. Furthermore, the unsymmetrical cyclic diborene showed a polar B[double bond, length as m-dash]B double bond.