Meta- rather than ortho-directed metalations: Buttressing effects prejudicing the proton abstraction from (2,6-dihalophenyl)silanes
(2,6-Dichlorophenyl)trimethyl- and -triethylsilane exhibit untypical reactivity patterns toward strong bases. When treated at -100 DegC 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 std. 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 alkylithium compds. 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 byproducts or sep. by permutational halogen/metal interconversion, eliminate lithium chloride at -75 DegC to set free didehydrobenzenes ("arynes") which are trapped by their aryllithium precursors. [on SciFinder (R)]
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29 Organometallic and Organometalloidal Compounds Institute of Chemical Sciences and Engineering,Ecole Polytechnique Federale,Lausanne,Switz. Journal 1434-193X written in English.
Record created on 2006-03-03, modified on 2016-08-08