An obvious strategy to try and increase the reactivity of the borate ester was to replace 2,2,2trifluoroethanol with more acidic analogues such as 1,1,1,3,3,3-hexafluoroisopropanol or perfluorotert-butanol, which should lead to a more Lewis-acidic borate with potentially higher reactivity. Our initial screen revealed, however, that these borates offered little improvement in reactivity (60% and 51% yield respectively) with the catalytic reactivity showing little correlation to the Lewis acidity of the borates (as measured by the Gutmann-Beckett acceptor number ). It is likely that in many cases the increased Lewis acidity of the borate is balanced by increased steric hindrance or increased volatility of the alcohol (and therefore a reduced catalyst stability). A borate derived from a larger polyfluorinated alcohol offered a moderate improvement in yield (65%), perhaps indicating that alcohol volatility was a significant factor affecting the performance of these catalysts. Similarly, the borate derived from 2,2,2-trichloroethanol gave a slightly improved yield (64%). We then switched to phenol-derived borates, and although the unsubstituted phenyl borate offered little advantage, the 3,4,5-trifluorophenol derivative B [B(OAr F )3] showed a significantly improved reactivity (71%).