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Low-Coordinate NHC–Zinc Hydride Complexes Catalyze Alkyne C–H Borylation and Hydroboration Using Pinacolborane

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posted on 15.05.2019 by Richard J. Procter, Marina Uzelac, Jessica Cid, Philip J. Rushworth, Michael J. Ingleson
Organozinc compounds containing sp, sp2, and sp3 C–Zn moieties undergo transmetalation with pinacolborane (HBPin) to produce Zn–H species and organoboronate esters (RBPin). This Zn–C/H–B metathesis step is key to enabling zinc-catalyzed borylation reactions, and it is used in this work to develop both terminal alkyne C–H borylation and internal alkyne hydroboration. These two conversions can be combined in one pot to achieve the zinc-catalyzed conversion of terminal alkynes to 1,1-diborylated alkenes without isolation of the sensitive (to protodeboronation) alkynyl boronate ester intermediates. Mechanistic studies involving the isolation of intermediates, stoichiometric experiments, and DFT calculations all support mechanisms involving organozinc species that undergo metathesis with HBPin. Furthermore, zinc-catalyzed hydroboration can proceed via a hydrozincation step, which does not require any exogenous catalyst in contrast to all previously reported alkyne hydrozincations. Bulky N-heterocyclic carbenes (NHCs) are key for effective catalysis as the NHC steric bulk enhances the stability of the NHC–Zn species present during catalysis and provides access to low-coordinate (NHC)­Zn–H cations that are electrophilic yet Brønsted basic. This work provides an alternative approach to access synthetically desirable pinacol–organoboronate esters using earth-abundant metal-based borylation catalysts.