A Tin Analogue of the Cycloheptatrienyl Anion: Synthesis, Structure, and Further Reduction to Form a Dianionic Species

The
synthesis and electronic structure of a tin analogue of the
cycloheptatrienyl anion are described. The reaction of 5,5-diphenyldibenzo­[b,f]­stannepin with lithium at low temperatures
reductively cleaved a Sn–Ph bond to give a lithium salt of
the stannepinyl anion, the structure of which was established by X-ray
diffraction analysis. Elongating the reaction time resulted in further
reduction of the other Sn–Ph bond to generate a dilithium species,
the formation of which was confirmed by a trapping reaction using
MeI. Unlike general cycloheptatrienyl anions, the monoanionic species
was proved to be nonaromatic on the basis of the NMR and X-ray diffraction
studies. When the reduction was carried out at room temperature, decomposition
of the seven-membered ring followed by Z/E isomerization promoted by lithium proceeded to generate
(E)-2,2′-dilithiostilbene, which forms a dimer
in the crystalline state.