We questioned if the promotionary role of H-TEDA(BF 4 ) 2 came down to its Brønsted acidity and elected to examine whether other acids would serve as promotors(see the SI file for full details). Although the pK a H-TEDA(BF 4 ) 2 is not known, the pK a of analogous protonated DABCO is 9.1 (in DMSO). Thus, we examined additives with pK a s (in DMSO) that were higher, similar, and lower than 9. Brønsted acidic additives with markedly higher pK a s, such as water (pK a 31.4), 3-benzyl-1-methyl-1H-imidazol-3-ium hexafluorophosphate (BnMIM•PF 6 ) (pK a 21.6) and phenol (pK a 18.0) inhibited the reaction (Table , entries 9-11). Additives with similar pK a s -such as acetic acid (pK a 12.3), triethylammonium tetrafluoroborate (TEA-H•BF 4 ) (pK a 9.1), imidazolium tetrafluoroborate (Imid-H•BF 4 ) (pK a 6.4) -gave similar results to the unpromoted reaction. Pyridinium tetrafluoroborate (Py-H•BF 4 ) (pK a = 3.4) gave a similar yield to the unpromoted reaction, while TFA (pK a = 3.4) gave an even higher (90%) product yield (Table 1, entry 11). Although the additional promoting benefit of the additives cannot easily be distinguished from the (already efficient reactivity promoting) 4-fluorobenzoyl auxiliary, it was nonetheless intriguing that other additives could be tolerated in the reaction providing their pK a was sufficiently low. Next, we examined the effect of H-TEDA(BF 4 ) 2 on another substrate in the reported photocatalytic auxiliary fluorination method (Scheme 5). Specifically, substrate 1d when treated with SF ® under 400 nm irradiation gave a low yield (38%) of fluorinated product 2d.