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(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.