Metal-Free Intermolecular C-H Borylation of N-Heterocycles at B-B Multiple Bonds

Carbene-stabilized diborynes of the form LBBL (L=N-heterocyclic carbene (NHC) or cyclic alkyl(amino)carbene (CAAC)) induce rapid, high yielding, intermolecular ortho-C-H borylation at N-heterocycles at room temperature. A simple pyridyldiborene is formed when an NHC-stabilized diboryne is combined with pyridine, while a CAAC-stabilized diboryne leads to activation of two pyridine molecules to give a tricyclic alkylideneborane, which can be forced to undergo a further H-shift resulting in a zwitterionic, doubly benzo-fused 1,3,2,5-diazadiborinine by heating. Use of the extended N-heteroaromatic quinoline leads to a borylmethyleneborane under mild conditions via an unprecedented boron-carbon exchange process.

Automated summary

Carbene-stabilized diborynes, including N-heterocyclic carbene (NHC) and cyclic alkyl(amino)carbene (CAAC), have been found to efficiently catalyze the intermolecular ortho-C-H borylation of N-heterocycles at room temperature. The reaction of NHC-stabilized diborynes with pyridine leads to the formation of a simple pyridyldiborene, while the combination of CAAC-stabilized diboryne with pyridine results in the activation of two pyridine molecules and the formation of a tricyclic alkylideneborane. Heating the alkylideneborane causes a further H-shift reaction, leading to the formation of a zwitterionic, doubly benzo-fused 1,3,2,5-diazadiborinine. Moreover, using quinoline as a substrate led to the formation of a borylmethyleneborane via an unprecedented boron-carbon exchange process.