Borole/Borapyramidane Relationship

Boroles and borapyramidanes are classical and nonclassical constitutional isomers, respectively. It is here shown that they can indeed be interconverted. Treatment of the bis(alkynyl)B(C₆F₅) SMe₂ adduct 3·SMe₂ with HB(C₆F₅)₂ gave borole 1·SMe₂, featuring trimethylsilyl substituents in both α positions to boron, by means of a 1,1-hydroboration/alkenylboration sequence. Photolysis of the classical borole adduct 1·SMe₂ resulted in rearrangement to its nonclassical structural isomer, borapyramidane 2, in high yield, which exhibits a vicinal pair of trimethylsilyl substituents at the square pyramidane base. Neutral borapyramidane 2 is a rare example of an isoster of the (CH)₅⁺ pyramidane cation. Thermolysis of borapyramidane 2 in the presence of SMe₂ at 60 °C re-formed borole 1·SMe₂, which converted at 100 °C to 2,3-bis-silyl-substituted borole isomer 8·SMe₂. Its photolysis also gave borapyramidane 2. Prolonged photolysis of 2 at elevated temperatures converted this to borapyramidane isomer 10 containing a pair of trimethylsilyl groups in 1,3-position at its square C₄-pyramidal base. The borole and borapyramidane isomers were characterized by X-ray diffraction, and the system was analyzed by density functional theory (DFT) calculations.