Remote trimethylsilyl groups interfering with the ortho deprotonation of fluoroarenes and chloroarenes

(2-Fluorophenyl)trimethylsilane (2-F) and (2-chlorophenyl)trimethylsilane (2-Cl) react with sec-butyllithium or lithium 2,2,6,6-tetramethylpiperidide under permutational hydrogen/metal interconversion (metalation) more slowly than, resp., the corresponding 4-fluoro- and 4-chloro isomers (1-F and 1-Cl, resp.) or, on the other hand, fluorobenzene and chlorobenzene themselves. The bulky silyl substituent impedes the neighboring halogen to get out of the way when the base attacks the ortho position. For the same reason, (2,4-difluorophenyl)- and (2,4-dichlorophenyl)trimethylsilane (7-F and 7-Cl) are less reactive than their 3,5-isomers (6-F and 6-Cl) and metalation occurs more rapidly at the 4-rather than 3-position of (2,5-difluorophenyl)- and (2,5-dichlorophenyl)trimethylsilane (5-F and 5-Cl, resp.). The decrease in kinetic acidity due to buttressing is most pronounced with the congested substrates (2,4,6-trihalophenyl)trimethylsilanes (8-F and 8-Cl) and, particularly (2,4,6-trihalophenylene)-1,3-bis(trimethylsilanes) (9-F and 9-Cl). Fluorine being far less effective than chlorine as a transmitter of steric pressure, (3,4- and 2,3-difluorophenyl)trimethylsilane (3-F and 4-F) exhibit comparable reactivity whereas (3,4-dichlorophenyl)trimethylsilane (3-Cl) clearly surpasses its 2,3-dichloro isomer (4-Cl). [on SciFinder (R)]


Published in:
European Journal of Organic Chemistry, 4, 609-675
Year:
2007
Note:
29
Organometallic and Organometalloidal Compounds
Institute of Chemical Sciences and Engineering,Ecole Polytechnique Federale, BCh,Lausanne,Switz.
Journal
1434-193X
written in English.
Other identifiers:
Laboratories:




 Record created 2007-03-07, last modified 2018-12-03


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