Carbenoid Chain Reactions through Proton, Deuteron, or Bromine Transfer from Unactivated 1-Bromo-1-alkenes to Organolithium Compounds
作者:Rudolf Knorr、Claudio Pires、Johannes Freudenreich
DOI:10.1021/jo070623w
日期:2007.8.1
The deceptively simple vinylic substitution reactions Alk(2)CCABr + RLi -> Alk(2)CCAR + 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 (Alk(2)CCH-Br, 2a) is deprotonated (step 1) by phenyllithium (PhLi) to give the Br,Li-alkylidenecarbenoid Alk(2)CCLi-Br (3). In the ensuing chain cycle, 3 and PhLi (step 2) form the observable alkenyllithium intermediate Alk(2)CCLiPh 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 Alk(2)CCHPh. The related 2-(dibromomethylidene)-1,1,3,3-tetramethylindan (Alk(2)CCBr(2), 2c) and methyllithium (MeLi) generate carbenoid 3 (step 1), which incorporates MeLi (step 2) to give Alk(2)CCLiCH(3), which reacts with 2c by bromine transfer producing Alk(2)CCBrCH(3) 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 PhCCLi leading to Alk(2)CCBrCC-Ph. Reagent 2a may perform both proton and bromine transfer toward Alk(2)CCLiCH(3), feeding two coupled carbenoid chain processes in a ratio that depends on the solvent and on a primary kinetic H/D isotope effect.