Cationic Intermediates for Electrophilic Reactions from 9,9-Dimethyl-9H-9- silafluorene

Author(s): Takao Okazaki, Yuki Yamamura, Takeshi Futemma, Toshikazu Kitagawa.

Journal Name: Current Organic Chemistry

Volume 20 , Issue 28 , 2016

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Abstract:

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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Article Details

VOLUME: 20
ISSUE: 28
Year: 2016
Page: [3014 - 3021]
Pages: 8
DOI: 10.2174/1385272820666160916123656
Price: $58

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