Electronic effects in iridium C-H borylations: insights from unencumbered substrates and variation of boryl ligand substituents

Abstract

Experiment and theory favour a model of C-H borylation where significant proton transfer character exists in the transition state.

Fig. 1

Transition structures for C–H activation of anisole by 1a at the ortho, meta, and para-positions. Distances are in Å and barriers are mPW1K/BS2//B3LYP/BS1 at 150 °C.

Fig. 2

Limiting descriptions of the C–H activation transition states.

Fig. 3

Structures of veratrole (3) and benzodioxole (4) highlighting the relative accessibilities of the ortho-C–H bonds.

Fig. 4

Transition structures for borylation of benzodioxole by 1a (left) and 6a (right) (distances in Å). The barriers are calculated at 25 °C.

Fig. 5

(a) Correlation of calculated barriers (ΔE‡) for C–H activation (E + zpe, B3LYP/BS1) vs. the energy of the resulting intermediates (ΔE). (b) Correlation of ΔE with the charge developed on the aryl group with linear fits for 6- and 5-membered substrates. Sterically hindered arenes (blue) were omitted from the arene fit.

Scheme 1

Transition states (9) and intermediates (10) for C–H activation.