We thus hypothesized, on the basis of these initial findings, that the catalyst could form a complex with the OH acceptor to generate an activated catalytic species. To prove this, four control reactions were carried between 2a and isopropanol 3g or deuterated isopropanol d-3g as the nucleophile, in which catalyst 1e is either added to the reaction mixture as before (A), or prestirred with 3g or d-3g for 3 h at room temperature before 2a is added (B) and the reaction rates to obtain 8 compared (Scheme 5 and SI for full details). Interestingly, under pre-stirring conditions (B) reactions were 1.28 times faster (rprestirH/rH) and the overall yield around 17% higher with less dimerization of starting materials observed. In general, reaction rates with deuterated substrates were slower than those of non-deuterated acceptors (rH/rD = 1.68), however under pre-stirring conditions, rate differences were less pronounced rprestirH/rprestirD = 1.15). These results support the in situ formation of an active catalytic intermediate between 1e and the incoming OH nucleophile and suggest that breaking the O-H bond could potentially be a key step in the reaction mechanism since pre-forming the catalytic intermediate accelerates rates and diminishes KIE.