Regioselective and Stereospecific Dehydrogenative Annulation Utilizing Silylium Ion-Activated Alkenes

Treatment of dialkylbenzylsilanes
(1) with trityl
tetrakis­(pentafluorophenyl)­borate (TPFPB) afforded the corresponding
silylium ions in equilibrium with their intra- or intermolecular π-complexes,
which underwent dehydrogenative annulation with various alkenes to
form 1,2,3,4-tetrahydro-2-silanaphthalenes (4) in up
to 82% isolated yield. Sterically bulkier substituents on the silicon
atom tended to increase the yield of cyclic products 4. The annulation products retained the stereochemistry in cases of
the reactions using internal alkenes. The use of diisopropyl­(1-naphthyl)­silane
(2) instead of 1 also resulted in annulation
to obtain the 2,3-dihydro-1-sila-1H-phenalene derivatives 6. Electrophilic aromatic substitution at the 8-position was
predominant, despite the two potentially reactive positions on the
naphthyl group. The steric hindrance of the naphthyl group prevented
addition of the cis-alkene to the silylium ion, which
would considerably decrease yields of the desired products from 2 compared to those from 1.