Synthesis and properties of a kinetically stabilized 9-silaphenanthrene

Reaction of 9-bromo-9,10-dihydro-9-silaphenanthrene (6) with lithium diisopropylamide (LDA) in THF at room temperature afforded kinetically stabilized 9-silaphenanthrene la bearing an efficient steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt). 9-Silaphenanthrene la was isolated as pale yellow crystals, and the structure of la was completely determined by spectroscopic and X-ray crystallographic analyses. The ¹H and ¹³C NMR chemical shifts corresponding to the 9-silaphenanthrene ring of la were observed in a typical aromatic region, while the central silicon atom showed the characteristic signal at 86.9 ppm in the ²⁹Si NMR in C₆D₆. The 9-silaphenanthrene ring of la showed completely planar geometry, indicating that 9-silaphenanthrene has a delocalized 14π-electron system as an aromatic compound. In spite of its high thermal stability, la still has high reactivity at the 9,10position toward small reagents such as H_2O.