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 pKa. 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 SE2(open) protonolysis transition state is forced to proceed through a sub-optimal non-linear Zn⋯Cα⋯HNR3 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.