Borole/Borapyramidane Relationship.
Qiu SunConstantin-Gabriel DaniliucXiaoye YuChristian Mück-LichtenfeldGerald KehrGerhard ErkerPublished in: Journal of the American Chemical Society (2022)
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(C 6 F 5 ) SMe 2 adduct 3 ·SMe 2 with HB(C 6 F 5 ) 2 gave borole 1· SMe 2 , 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· SMe 2 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 SMe 2 at 60 °C re-formed borole 1· SMe 2 , which converted at 100 °C to 2,3-bis-silyl-substituted borole isomer 8· SMe 2 . 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 C 4 -pyramidal base. The borole and borapyramidane isomers were characterized by X-ray diffraction, and the system was analyzed by density functional theory (DFT) calculations.