Low coordinate NHC-Zinc-Hydride Complexes Catalyze Alkyne C-H Borylation and Hydroboration using Pinacolborane

Richard J. Procter, Marina Uzelac, Jessica Cid, Philip J. Rushworth, Michael J. Ingleson

Research output: Contribution to journalArticlepeer-review

Abstract

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, with it 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 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. Keywords: borylation; zinc; hydrides; N-heterocyclic carbene; homogeneous catalysis
Original languageEnglish
JournalACS Catalysis
Early online date15 May 2019
DOIs
Publication statusE-pub ahead of print - 15 May 2019

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