Carbenoid Chain Reactions through Proton, Deuteron, or Bromine Transfer from Unactivated 1-Bromo-1-alkenes to Organolithium Compounds

• Published in: Journal of organic chemistry 2007-08, Vol.72 (16), p.6084-6090
• Main Authors: Knorr, Rudolf, Pires, Claudio, Freudenreich, Johannes
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Kinetics and mechanisms; Organic chemistry; Reactivity and mechanisms
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo070623w

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). PhC⋮CLi cannot carry out step 1, but MeLi can initiate (step 1) the carbenoid chain cycle (steps 2 and 3) of 2c with PhC⋮CLi leading to Alk2CCBrC⋮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.