meta- rather than ortho-directed metalations: Buttressing effects prejudicing the proton abstraction from (2,6-dihalophenyl)silanes

Article abstract of DOI:10.1002/ejoc.200500553

(2,6-Dichlorophenyl)trimethyl- and -triethylsilane exhibit untypical reactivity patterns toward strong bases. When treated at -100°C with lithium 2,2,6,6-tetramethylpiperidide, they give rise to two, with sec- or tert-butyllithium even three, different organometallic intermediates which can be intercepted with standard electrophiles. The trialkylsilyl group intervenes in two respects. Exerting steric pressure on the neighboring chlorine atoms, it facilitates the permutational displacement of one of them against lithium, a side reaction which occurs whenever alkyllithium compounds are employed. At the same time it impedes the attack of hydrogen atoms by the base in the immediate vicinity of the halogens due to relay transmission of steric hindrance. As a consequence, deprotonation occurs preferentially or exclusively at the 5-("meta") rather than the 4-("ortho")position. 4-Lithiated species, generated as by-products or separately by permutational halogen/metal interconversion, eliminate lithium chloride at -75°C to set free didehydrobenzenes ("arynes") which are trapped by their aryllithium precursors. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.