Improving and stabilizing fluorinated aryl borane catalysts for epoxide ring-opening

Epoxide ring-opening is a key reaction in organic chemistry. We have previously shown that B(C6F5)3, a strongly Lewis acidic arylborane, exhibited high rates and unusual selectivities for catalyzing the ring-opening of aliphatic epoxides with alcohols. Here we compare catalysts of the form B(C6H5−XFX)3 (x = 5, 4, 3, and 0) and determine that moderately Lewis acidic arylboranes have higher regioselectivity, but slower rates in this reaction. At high temperatures, these arylboranes can also hydrolyze into inactive species. However, deactivation is suppressed in the presence of co-catalytic amounts of 1,2-propanediol, and DFT calculations suggest a role for arylborane-H2O-diol complexes. Thermal stabilization and regioselectivity enhancement by diol are both more pronounced for B(C6F5)3 than for less Lewis acidic B(C6HF4)3 and B(C6H2F3)3 catalysts. These results further demonstrate the catalytic relevance of H-bound networks of arylboranes and diols and enable their use at higher temperatures and greatly increased rates.