Understanding and Expanding Zinc Cation/Amine Frustrated Lewis Pair Catalyzed C–H Borylation

Matthew E. Grundy, Lia Sotorrios, Milan Kumar Bisai, Kang Yuan, Stuart A. Macgregor, Michael J. Ingleson

Research output: Contribution to journalArticlepeer-review

5 Citations (Scopus)
10 Downloads (Pure)

Abstract

[(NacNac)Zn(DMT)][B(C6F5)4], 1, (NacNac = {(2,6-iPr2H3C6)N(CH3)C}2CH), DMT = N,N-dimethyl-4-toluidine), was synthesized via two routes starting from either (NacNac)ZnEt or (NacNac)ZnH. Complex 1 is an effective (pre)catalyst for the C–H borylation of (hetero)arenes using catecholborane (CatBH) with H2 the only byproduct. The scope included weakly activated substrates such as 2-bromothiophene and benzothiophene. Computational studies elucidated a plausible reaction mechanism that has an overall free energy span of 22.4 kcal/mol (for N-methylindole borylation), consistent with experimental observations. The calculated mechanism starting from 1 proceeds via the displacement of DMT by CatBH to form [(NacNac)Zn(CatBH)]+D, in which CatBH binds via an oxygen to zinc which makes the boron center much more electrophilic based on the energy of the CatB-based LUMO. Combinations of D and DMT act as a frustrated Lewis pair (FLP) to effect C–H borylation in a stepwise process via an arenium cation that is deprotonated by DMT. Subsequent B–H/[H-DMT]+ dehydrocoupling and displacement from the coordination sphere of zinc of CatBAr by CatBH closes the cycle. The calculations also revealed a possible catalyst decomposition pathway involving hydride transfer from boron to zinc to form (NacNac)ZnH which reacts with CatBH to ultimately form Zn(0). In addition, the key rate-limiting transition states all involve the base, thus fine-tuning of the steric and electronic parameters of the base enabled a further minor enhancement in the C–H borylation activity of the system. Outlining the mechanism for all steps of this FLP-mediated process will facilitate the development of other main group FLP catalysts for C–H borylation and other transformations.

Original languageEnglish
Pages (from-to)2286-2294
Number of pages9
JournalACS Catalysis
Volume13
Issue number4
Early online date30 Jan 2023
DOIs
Publication statusPublished - 17 Feb 2023

Keywords

  • Borylation
  • DFT Calculations
  • Frustrated Lewis Pairs
  • electrophilic substitution
  • zinc

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

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