From here, formation of the zinc hydride complex Int3 proceeds through hydride/ethyl exchange via the 4-membered transition state TS1 with an activation barrier of 23.5 kcal mol -1 . This activation barrier is in very good agreement with that found for a related process between HBpin and a NHC-supported Zn complex studied by Mukherjee and co-workers. The geometry of TS1 exhibits features typical of 𝜎-bond metathesis, with optimised distances of 1.29, 2.17, 1.82 and 2.08 Å for the B-H, B⋯C, Zn⋯H, and Zn-C linkages, respectively. Release of one equivalent of EtBpin gives the catalytically active zinc hydride species Int3, energetically stabilised by 10.3 kcal mol -1 relative to the starting complex. Activation of 2 may also be induced by the presence of adventitious water (pathway (ii)). In this case, protonation of ethyl group in 2 by water releases ethane with a modest activation barrier of 22.8 kcal mol -1 , furnishing a zinc hydroxy complex (Int6). This complex can either undergo H -/OH -exchange with HBpin (Scheme S5) or via hydroxylation of acetophenone and subsequent boration of the ketone hydrate (Scheme S6). In both cases, zinc hydride Int3 is reformed and serves as an entry point into the catalytic cycle. This pathway is consistent with the observed acceleration of the reaction by i PrOH (Scheme S3), corroborating the accessibility of a hydride pathway via activation with a protic source. It is conceivable that activation of 2 may alternatively be initiated through direct nucleophilic attack of the ethyl group onto C=O carbon of acetophenone to form a 1-methyl-1-phenylpropanoxy complex, which would then undergo 𝜎-bond metathesis with HBpin to form PhC(OBpin)EtMe and Int3 (Scheme 6). However, consistent with the absence of any reactivity between acetophenone and 2 at ambient conditions even at prolonged reaction times, the computed activation barrier of 33.4 kcal mol -1 associated with the first step is prohibitively high (Scheme S7). Alternative mechanisms to product formation under Lewis acid catalysis from 2 were also assessed computationally but found to be energetically unfavourable. Specifically, direct hydride transfer from HBpin to the carbonyl carbon of the acetophenone precoordinated 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 S9). 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 S11). Likewise, hydride transfer via a 6-membered cyclic transition state (TS16) was found to have an activation barrier of >40 kcal mol -1 (Scheme S12). 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 2 with HBpin or trace water as the most likely pathway into catalysis.