Metal-Free Regio- and Chemoselective Hydroboration of Pyridines Catalyzed by 1,3,2-Diazaphosphenium Triflate

N-Heterocyclic
phosphenium triflates (NHP-OTf) 1 serve
as efficient catalysts for the regio- and chemoselective hydroboration
of pyridines under ambient condition with good functional group tolerance.
Mechanistic studies indicate that a boronium salt, [(Py)2·Bpin]­OTf 4, is generated concomitant with NHP-H 5 via hydride abstraction from HBpin by 1 in
the initial reaction step. Hydride reduction of the activated pyridine
in [(Py)2·Bpin]­OTf 4 by NHP-H 5 affords the 1,4-hydroboration product selectively. Thus, the phosphenium
species act as a hydrogen transfer reagent in the catalytic cycle.