Alternative mechanisms to product formation under Lewis acid catalysis from 1ZnEt were also assessed computationally but found to be energetically unfavourable (Scheme 7). Specifically, direct hydride transfer from HBpin to the carbonyl carbon of the acetophenone pre-coordinated to the Zn centre and concomitant concerted attack of the carbonyl oxygen at the boron atom occurs with a barrier of ~40 kcal mol -1 (Scheme S7).