Catalytic transfer zincation using ammonium cations

Abstract

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.