Given the faster formation of trans-haloboration products observed for some substrates by use of [BCl4] -alongside BCl3 an alternative mechanism, [BCl4] -mediated formation of the chlorinated benzoxaborinine, was calculated for the 1a and 1j derived systems (Fig. left and right, respectively). This proceeds by intramolecular activation of the alkyne by the ArOBCl2 Lewis acid and concomitant addition of chloride from [BCl4] -to the opposite face of the alkyne (TS6A and B). For both, this concerted trans-haloboration process proceeds with a barrier of ca. 25 kcal mol -1 . This forms the anionic orate intermediates I5A/B and BCl3, which then react by transfer of chloride from I5A/B to BCl3 via a low energy transition state to form the products (PD) and regenerate the [BCl4] -anion. For 1a, the "BCl3 only" mechanism (shown in Fig. ) has a lower overall barrier (by ca. 2 kcal mol -1 ) than the [BCl4] -mediated mechanism (Fig. ). However, for the CF3 substituted 1j the two mechanisms have effectively identical highest barriers (G = 24.9 and 25.0 kcal mol -1 ). This is consistent with the acceleration observed in the trans-haloboration of 1j when one equiv. of [BCl4] -is added alongside two equiv. BCl3, relative to just using two equiv. BCl3. Presumably, in the presence of BCl3 and [BCl4] -both mechanisms to form 3j are operating in solution, or due to the approximations inherent in these calculations the barrier for the [BCl4] -process is actually lower than that for the BCl3 only process, leading to the faster conversion.