Remote Trimethylsilyl Groups Interfering with the <i>ortho</i> Deprotonation of Fluoroarenes and Chloroarenes

Abstract

Abstract (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, respectively, the corresponding 4‐fluoro‐ and 4‐chloro isomers ( 1‐F and 1‐Cl , respectively) 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 , respectively). 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 ). (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2007)