Carbenoid chain reactions through proton, deuteron, or bromine transfer from unactivated 1-bromo-1-alkenes to organolithium compounds

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 PhCtriple bond]CLi leading to Alk2C=CBr-Ctriple 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.