The trans-haloboration of the important (for accessing the core structure in BLIs) precursor to the C8 ester substituted benzoxaborinine (e.g. 3l) proved more challenging and required further optimization. Under a range of conditions using just BCl3 1l only was converted to desired product 5 (Fig. ) in low to moderate conversion (< 50%). In-situ NMR spectroscopy revealed one other major product is formed which contained two boron centers, one three and one four coordinate (11B = 52.3 and 8.7 ppm, Figs. and). We assign this as compound E (based on multinuclear NMR spectroscopy), with cis-haloboration based on an NOE experiment showing interaction between the vinylC-H and an aromatic C-H, with no other (trans-haloboration or 1,1-haloboration) isomers observed. Previously, we have demonstrated that the addition of [BCl4] -salts can enhance the amount of the alkyne haloboration product formed in reactions where haloboration proceeds in competition with another transformation. Presumably, the presence of [BCl4] -as an exogenous chloride source facilitates conversion of a vinyl cation to a vinyl chloride by chloride transfer. Reoptimizing the trans-haloboration of 1l using three equivalents of BCl3 and one equivalent of dry [nBu4N]Cl (this combination forms [BCl4] -in-situ by B NMR spectroscopy) led to the formation of 5 as the major product (> 95% by in-situ NMR spectroscopy, Fig. ). A comparable outcome occurred using one equiv. of preformed [nBu4N][BCl4] and two equiv. BCl3 to form 5, indicating that the halide source is indeed [BCl4] -and not [Cl] -. This modified procedure enabled 3l to be accessed in good isolated yield (77 %). Therefore the trans-haloboration of o-alkynyl phenols can be applied to access a chlorinated analogue of key intermediates used to form broad spectrum bicyclic boronate BLIs (e.g. A, which also has a C8-ester moiety common in the bicyclic boronate BLIs). Finally, the trans-haloboration of an internal alkyne, compound 6 (Fig. ), was explored. In this case the five membered boracycle 7 was the major product produced using BCl3 under a range of conditions, with the desired benzoxaborinine (8) only a minor product (by in-situ NMR spectroscopy). Presumably, the pentyl group affords sufficient stabilization to the vinyl cation intermediate (Fig. ) to enable a low energy pathway to the five membered boracycles 7. The addition of [nBu4N][BCl4] in the trans-haloboration of 6 did not significantly alter the 7 : 8 product ratio. While 7 proved challenging to isolate, an analogue where pentyl is replaced with cyclopropyl led to the formation of the analogous (to 7) five membered haloboration product, 9 (Fig. ), as effectively the only product formed (>95% by in-situ NMR spectroscopy, Fig. ) enabling the full characterization of 9. Compounds 7 and 9 have closely comparable NMR data (excluding the alkyl resonances), supporting the formulation of 7 as the five membered boracyclic isomer. Attempts to isolate 7 and 9 as B-OH derivatives failed due to their sensitivity to protodeboronation.