Fluorinated 9-borafluorenes vs. conventional perfluoroaryl boranes — Comparative Lewis acidity

Perfluorinated 9-phenyl-9-borafluorene, 1 , is an antiaromatic analog of the well-known tris(pentafluorophenyl)borane. Spectroscopic, structural, and electrochemical studies have been performed on 1 and its Lewis base adducts with MeCN, THF, and PMe 3 with a view to assessing its comparative Lewis acid strength relative to B(C 6 F 5 ) 3 . For the sterically undemanding Lewis base MeCN, 1 and B(C 6 F 5 ) 3 have comparable LA strengths, while for more sterically prominent THF, 1 is clearly the stronger Lewis acid (LA) based on competition experiments. We conclude that steric factors, rather than antiaromaticity, are the most important determinants in the LA strength differences between 1 and B(C 6 F 5 ) 3 .Key words: boranes, Lewis acids, fluorinated compounds, heterocycles.