Firstly, the intermediacy of o-alkynyl-arylOBCl2 species was confirmed by monitoring reactions in-situ (by B NMR spectroscopy), which revealed it is formed extremely rapidly (< 5 mins). In some cases, such as with 1j or internal alkyne 6, the ArylOBCl2 species ( 11 B ca. 32 ppm, e.g. Fig. and) is present for hours at room temperature before conversion to the benzoxaborinine is complete. In the reactions using just BCl3 no [BCl4] -is observed by B NMR spectroscopy (e.g. Fig. ). However, this does not preclude some [BCl4] -being present that could catalyze trans-haloboration as BCl3 and [BCl4] -undergo fast halide exchange, thus only an average 11B is observed even at low temperature. Notably, in the trans-haloboration of 1j (which at room temperature using just BCl3 requires 12 h for high conversion to 3j), the addition of [BCl4] -as a "chloride shuttle" led to more rapid formation of 3j (complete in < 1 h). Attempts to observe intermediates by in-situ NMR spectroscopy using multiple substrates did not reveal any species in addition to ArylOBCl2 and the final haloboration product (e.g. 2-Cl). Therefore mechanisms using just BCl3 and using a combination of BCl3 and [BCl4] -were explored at the M06-2x/6-311+G(d,p) level with a PCM (CH2Cl2). This level has given accurate thermodynamic parameters for other electrophilic borylations. Note, a mechanism where the o-alkynyl-arylOBCl2 species undergoes alkyne trans-haloboration with BCl3 and then condensation (of the vinylBCl2 and O-BCl2) to Scheme 2: Uchiyama and co-workers' calculated haloboration isomerization pathway. Note, the free alkyne and BX3 are set as zero energy in this work. form 2-Cl with release of BCl3 is precluded based on the absence of any trans-haloboration of alkyne 1l (no trans isomer of E is observed under these conditions). This is consistent with BCl3 only resulting in the cis chloroboration of terminal alkynes at room temperature. The first route explored uses just BCl3, and is related to calculations from Uchiyama (Scheme 2) who found the double syn-haloboration of ethyne followed by retro-haloboration (BX3 elimination) as the most probable mechanism to form trans-haloborated alkene products. With BCl3, this process has a significant barrier (highest G ‡ = 36 kcal mol -1 , Scheme 2) partly due to the entropic penalty, as indicated by the smaller E ‡ of 21.9 kcalmol -1 for this step. In the formation of PD1A (Fig. ) via a related mechanism, one of the two haloboration steps starting from o-alkynyl-PhOBCl2 and BCl3, will be intramolecular. Thus the entropic penalty will be lower in this case potentially leading to barriers that would be consistent with an ambient temperature reaction. Starting from SM1A, BCl3 addition to the alkyne proceeds via a vinyl cation intermediate (I1A) which contains an essentially linear CCC unit (C-C-C= 173.5°). Halide transfer from boron to carbon then affords the BCl2 substituted vinyl halide I2A as the cis isomer via TS2A (G ‡ = + 22 kcal mol -1 ) in a slightly exergonic step (relative to SM1A). The second haloboration is intramolecular, proceeding via the interaction of the ArylOBCl2 unit with the vinylic unit to produce a benzyl cation type intermediate (I3A). Halide transfer from boron to the benzyl cation then occurs in a separate step to form I4A. Note the double haloboration product I4A is only 5 kcal mol -1 higher in free energy than the starting alkyne + BCl3, in contrast to the fully intermolecular process which is much more endergonic (Scheme 2). The final step is retro-haloboration to form the benzoxaborinine product 2-Cl and BCl3 which proceeds via a low barrier (TS5A) and results in an overall transformation that is significantly exergonic. The highest barrier in the process (G = 23.2 kcal mol -1 ), is consistent with the formation of 2-Cl occurring at ambient temperature, suggesting this is a feasible mechanism starting from 1a.