Knorr Rudolf, Pires Claudio, Freudenreich Johannes
Department of Chemistry and Biochemistry, Ludwig-Maximilians-Universität, Butenandtstrasse 5-13, 81377 Munich, Germany.
J Org Chem. 2007 Aug 3;72(16):6084-90. doi: 10.1021/jo070623w. Epub 2007 Jul 11.
The deceptively simple vinylic substitution reactions Alk2C=CA-Br + RLi --> Alk2C=CA-R + LiBr (A = H, D, or Br) occur via an alkylidenecarbenoid chain mechanism (three steps) without transition metal catalysis. 2-(Bromomethylidene)-1,1,3,3-tetramethylindan (Alk2C=CH-Br, 2a) is deprotonated (step 1) by phenyllithium (PhLi) to give the Br,Li-alkylidenecarbenoid Alk2C=CLi-Br (3). In the ensuing chain cycle, 3 and PhLi (step 2) form the observable alkenyllithium intermediate Alk2C=CLi-Ph that characterizes the carbenoid mechanism in Et2O and is able to propagate the chain (step 3) through deprotonation of 2a, furnishing carbenoid 3 and the product Alk2C=CH-Ph. The related 2-(dibromomethylidene)-1,1,3,3-tetramethylindan (Alk2C=CBr2, 2c) and methyllithium (MeLi) generate carbenoid 3 (step 1), which incorporates MeLi (step 2) to give Alk2C=CLi-CH3, which reacts with 2c by bromine transfer producing Alk2C=CBr-CH3 and carbenoid 3 (step 3). PhCCLi cannot carry out step 1, but MeLi can initiate (step 1) the carbenoid chain cycle (steps 2 and 3) of 2c with PhC[triple bond]CLi leading to Alk2C=CBr-C[triple bond]C-Ph. Reagent 2a may perform both proton and bromine transfer toward Alk2C=CLi-CH3, feeding two coupled carbenoid chain processes in a ratio that depends on the solvent and on a primary kinetic H/D isotope effect.
看似简单的乙烯基取代反应Alk2C=CA-Br + RLi --> Alk2C=CA-R + LiBr（A = H、D或Br）通过亚烷基类卡宾链机制（三步）发生，无需过渡金属催化。2-(溴亚甲基)-1,1,3,3-四甲基茚（Alk2C=CH-Br，2a）被苯基锂（PhLi）去质子化（步骤1），生成Br,Li-亚烷基类卡宾Alk2C=CLi-Br（3）。在随后的链循环中，3和PhLi（步骤2）形成可观察到的烯基锂中间体Alk2C=CLi-Ph，它在Et2O中表征了类卡宾机制，并且能够通过使2a去质子化来传播链（步骤3），生成类卡宾3和产物Alk2C=CH-Ph。相关的2-(二溴亚甲基)-1,1,3,3-四甲基茚（Alk2C=CBr2，2c）与甲基锂（MeLi）生成类卡宾3（步骤1），其与MeLi结合（步骤2）得到Alk2C=CLi-CH3，后者通过溴转移与2c反应生成Alk2C=CBr-CH3和类卡宾3（步骤3）。PhCCLi不能进行步骤1，但MeLi可以引发（步骤1）2c与PhC≡CLi的类卡宾链循环（步骤2和3），生成Alk2C=CBr-C≡C-Ph。试剂2a可能同时向Alk2C=CLi-CH3进行质子和溴转移，以取决于溶剂和一级动力学H/D同位素效应的比例提供两个耦合的类卡宾链过程。
