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Zwischenstufen bei der HCl-Abspaltung aus Styrylchlorid

cf. CA 55, 23400c. Pure isomeric cis- and trans-PhCH:CHCl (I and II), sepd. by preparative gas chromatography, were treated with PhLi in Et2O at 0 Deg in the presence of LiI, LiBr, and without salt and the rates of reaction measured to give the 2nd order consts. (k * l02 mole-1 min.-1): 1.70, 3.60, 5.15 and -, 0.39, 0.52 for II and I, resp. Since the cis elimination from II was 9.9 times more rapid than trans elimination from I it was proposed that PhCH:CHCl was converted slowly by a b-elimination to PhC.tplbond.CH (III) and that this was quickly metalized to PhC.tplbond.CLi (IV). The formation of III was excluded by trial expt. Treatment of I, II, and III at 0 Deg in Et2O with 1:30 BuLi-MeLi gave consumption of 0.24, 0.18, 0.64 mole MeLi and 1.76, 1.82, 0.36 mole BuLi, resp. Accordingly III cannot be the intermediate in conversion of PhCH:CHCl to IV. The exptl. results are in agreement with the mechanism postulated by Cristol and Helmreich (CA 50, 8488d) for the analogous HBr-cleavage from PhCH:CHBr, according to which PhCH:CHO was slowly converted to the a-metalated stronger base PhCH:CClLi (V) and rapidly transformed by b-elimination to the weaker base IV by reason of the driving force of the shift in basicity. [on SciFinder (R)]

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