20082-66-0Relevant articles and documents
meta- rather than ortho-directed metalations: Buttressing effects prejudicing the proton abstraction from (2,6-dihalophenyl)silanes
Heiss, Christophe,Cottet, Fabrice,Schlosser, Manfred
, p. 5236 - 5241 (2007/10/03)
(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.
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Bennetau, Bernard,Rajarison, Florent,Dunogues, Jacques,Babin, Pierre
, p. 10843 - 10854 (2007/10/02)
Functionalisation in 2 position of benzenes 1,3-disubstituted by para-directing groups has been investigated, involving 2-trimethylsilylated intermediates when the classical methods (particularly these involving the anion in 2 position) are ineffective. Thus 2-trimethylsilyl derivatives of 1,3-dihalo- or 1,3-dimethoxybenzene have been prepared in good yields. Their conversion into the corresponding 1,2,3-trisubstituted benzenes has been conveniently performed upon acetylation, iodination or sulfonation, except in the case of the sulfonation of the 1,3-dimethoxylated compound in which the ipso effect of the silyl group is surpassed by the directing effect of the methoxy substituents. In contrast, whith the same substrate, cyanation in position 2 was successfully carried out using chlorosulfonyl isocyanate: to our knowledge this reaction is the first example of substitution of a trimethylsilyl by a cyano group in the aromatic series.