9,9-Dimethyl-9H-9-silafluorene (1) is one of the hetero-polycyclic aromatic compounds
containing a silicon atom in their rings. NMR observation of the reaction of 1 with
superacid at low temperature showed the quantitative formation of biphenyl. The nitration of
1 with HNO3/CH2Cl2 gave 9,9-dimethyl-2-nitro-9H-9-silafluorene in 24% yield. Friedel–
Crafts alkylation with tert-BuCl/FeCl3 afforded 4,4’-di-tert-butylbiphenyl in 98% yield.
Bromination with Br2/Fe produced 2,2’-dibromobiphenyl in 18% yield. These electrophilic
reagents were able to react through silylated arenium ions. DFT calculations suggested that 1
is likely to be protonated at the quaternary sp2 carbon C(8a) attached to the silicon atom to
give a silylated arenium intermediate (1aH+), which is a σ- rather than a π-complex. Positive
charge delocalization mapping based on computed Δδ13C and Δδ29Si values indicated delocalization
into the limited sp2 carbons and the Si atom. A reaction of 1 with H+ to form a silolyl cation (4+) and
CH4 by cleavage of the Si-CH3 bond was estimated to be comparable in energy to the formation of 1aH+.
NICS(1)zz values suggested that the five-membered silolyl ring in 4+ is significantly anti-aromatic.
Keywords: Carbocations, polycyclic aromatic compound, DFT calculations, NMR, aromaticity.
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