Catalytic transfer zincation using ammonium cations

C-H zincation is an efficient route to useful organozinc compounds. To date, most approaches effect aryl C-H zincation with selectivity controlled by substrate pK_a. Herein, we report a heteroaryl C-H zincation method that uses an easy to access (β-diketiminate)Zn-Me complex and [(R₃N)H][Anion] and proceeds under electronic control. This catalytic process is anion dependent; the less coordinating anion [B(C₆F₅)₄]^- proved superior to [CHB₁₁H₅Br₆]^-, with the latter forming the intimate ion pair [(β-diketiminate)Zn(CHB₁₁H₅Br₆)]. The Zn-Me complex and the [(R₃N)H]⁺ salt can also initiate catalytic C-H borylation. A key requirement for a viable transfer zincation is a low barrier protonolysis reaction between the zinc-alkyl and [(R₃N)H]⁺. This study found significant differences between the zinc-methyl and zinc-ethyl systems which stem from increased steric crowding in the zinc-ethyl congener. For the latter, the S_E2(open) protonolysis transition state is forced to proceed through a sub-optimal non-linear Zn⋯C^α⋯H_NR3 orientation which increases the energy of this barrier (vs. that for the Zn-Me analogue). While the scope of this initial process is limited, this work demonstrates that catalytic electrophilic transfer zincation is feasible and also highlights the sensitivity of a key transition state to small changes in the steric environment.