Zwitterionic phosphirenium borates derived from bis(alkynyl)mesityl phosphanes and tris(pentafluorophenyl)borane

From a series of bis(alkynyl)mesityl phosphanes 5, we prepared phosphirenium borate compounds 6 in high yields by reaction with B(C6F5)3 at room temperature. The zwitterionic compounds 6 are conveniently accessible and can be obtained with unique substitution patterns by this route. For two examples, we show the conversion of 6 to the respective 3-borylated phosphole derivatives 7 through multiple 1,1-carboboration reactions. In a useful one-pot methodology, the phosphirenium borates 6 are converted to air-stable 3-arylated phospholes 8 by a sequential 1,1-carboboration/ Suzuki-Miyaura type cross-coupling reaction.