Attempts were made to isolate high purity CBS catalysts for further studies. This proved challenging, but the formation of several in significantly higher purity (ca. 90 -99% purity) than the commercial material was achieved. These higher purity CBS catalysts gave worse outcomes than using commercial batches of 4 in the fluorination of 6 with CsF. In addition, all > 90% purity CBS catalysts (including independently synthesised 4, termed "higher purity 4") displayed an induction period before significant fluorination occurred (Figure ). This indicated that CBS catalysts are actually pre-catalysts for phase transfer fluorination. It should be noted that 1 and BEt3 did not display induction periods during the fluorination of 6 under identical conditions. Attempts were made to elucidate the structure of the catalytically active species derived from CBS pre-catalysts under fluorination conditions, however this study was inconclusive, and these results can be found in the ESI. While this work with CBS (pre)catalysts provides proof of principle that enantioselective borane phase transfer fluorination catalysis is feasible, the ill-defined and complex mixtures produced using CBS (pre)catalysts under these conditions is a complicating factor presumably contributing to the maximum e.e. being 30%, despite using multiple CBS (pre)catalyst structures. This highlights the importance of using borane catalysts that are robust under these conditions to allow for rational control of reactivity (note under these fluorination conditions both 1 and BEt3 show no observable decomposition, e.g. by protodeboronation or pinacol hydrolysis).