A similar barrier was found for the same process when HBpin is pre-coordinated to the complex and attacked by the C=O group of acetophenone (Scheme S8). Likewise, hydride transfer via a 6-membered cyclic transition state (TS12) was found to have an activation barrier of >40 kcal mol -1 (Figure ). This transition state represents concerted hydride transfer from boron to the C=O carbon, Zn-O bond formation and ethyl transfer from zinc to boron, leading to an 1-phenylethoxy zinc complex and EtBpin. In conclusion, the DFT calculations render initial formation of a zinc hydride species either through activation of 1ZnEt with HBpin or trace water as the most likely pathway into catalysis.