One attractive feature of using boranes as CsF phase transfer fluorination catalysts is the ready accessibility of many enantioenriched boranes. 12 Herein 4 and 5 were assessed in the enantioselective fluorination of 6 and 8 (which proceed via ring opening of the meso aziridinium and thiiranium cations, respectively). While 5 was ineffective as a catalyst in halocarbon solvents, it did function in the presence of MeCN. However, the use of stoichiometric Cs[5-F] in DCM/MeCN mixtures while leading to formation of 7 and 9, resulted in no e.e. being observed by chiral HPLC analysis. Furthermore, significant amounts of the hydrodehalogenation also was observed using Cs[5-F] alongside formation of 7/9, possibly via a mechanism related to the Midland reduction (Scheme 2). 12c The use of commercially available CBS catalyst 4 (0.5 M in toluene) also was explored as it is not prone to loss of hydride. Surprisingly (given its low calculated fluoride affinity), as received 4 effectively catalysed fluorination of 6 with CsF and led to appreciable e.e. in 7 (maximum e.e. observed using commercial 4 was in CHCl3 at 20 °C = 30% e.e.). In addition to 7, ca. 5% of the β-amino-alcohol, 10, was formed at early of the reaction, attributed to the presence of low quantities of water that leads to hydroxide transfer to 6. A range of CBS catalysts were bought or made (see supporting information) and used as crude mixtures (as per CBS-catalysed hydroboration procedures). However, none gave better e.e. than commercial 4 in the catalytic fluorination of 6 with CsF. Notably, commercial CBS catalyst 11, supplied as a solid, only enabled fluorination after an induction period. Due to this disparity detailed analysis of the commercial batches of 4 and 11 was performed. This revealed a number of impurities present at significant levels (upto 30% by 11 B NMR spectroscopy), including resonances consistent with products derived from reaction of 4/11 with water as previously reported (e.g. 12/13/14; Figure ). Figure : Structures of compounds present in commercial sourced CBS catalyst.