Reactions of Boroles Formed by 1,1-Carboboration

The borole 2, obtained by 1,1-carboboration of phenylbis(trimethylsilylethynyl)borane with B(C 6 F 5 ) 3, undergoes [4 + 2] cycloaddition reactions with 1-pentyne and with ethene to give the 7-borabicyclo[2.2.1]heptadiene and -heptene derivatives, respectively. The borole 2 reacts with pyridine at room temperature to give a pyridine-stabilized borolium ion. The reaction proceeds by phenyl migration and Me 3 SiF elimination. Carbon monoxide reacts rapidly with the borole 2 at room temperature to give the unusual ketene derivative 19 with Me 3 Si migration across the ring system.