Mechanisms of Elimination Reactions. 36. Stereochemistry and Transition-State Structure in Eliminations from Primary Alkyltrimethylammonium Salts

Article abstract of DOI:10.1021/ja00348a037

A study of stereochemistry of elimination in E2 reactions of R₁R₂CHCHDNMe3⁺ reveals that syn elimination can become the major reaction path when R₁ and R₂ are both bulky groups such as aryl or branched alkyl.With OH^-/50percent Me2SO-H2O at 80 deg C, the percent of syn is 68.5 for R₁ = Ph, R₂ = i-Pr; 61.9 for R₁ = Ph, R₂ = p-MeOPh ; 26,5 for R₁ = Ph, R₂ = CH3.With n-BuO^-/50percent Me2SO-n-BuOH, the percent of syn runs 61.5 for R₁ = Ph, R₂ = i-Pr; 12 for R₁ = n-Bu, R₂ = Me; and < 5 for R₁ = n-Bu, R₂ = D.The results can be rationalized by a simple conformational argument in which steric interactions between bulky β-substituents and the leaving trimethylammonio group destabilize the trasition state for anti elimination.Primary β-tritium, secondary α-tritium, and primary α-14C isotope effects were determined on the (2,2-diphenylethyl)trimethylammonium ion and compared with similar data on the (2-phenylethyl)trimethylammonium ion, which eliminates by an exclusively anti mechanism.The extent of proton transfer in the transition state seems not to differ widely between the two systems, but the extent of C-N cleavage appears less in the 2,2-diphenylethyl system.Hammett ρ values are smaller in the 2,2-diphenylethyl system, though their interpretation presents ambiguities.