Reaction of crowded olefins with bromine. a comparison of the strained (E)-2,2,3,4,5,5-hexamethylhex-3-ene with the sterically hindered tetraisobutylethylene

Article abstract of DOI:10.1021/ja00128a010

(E)-2,2,3,4,5,5-Hexamethylhex-3-ene (HMH, 2) reacts with Bn in chlorinated hydrocarbon solvents to give 3-bromo-2-tert-butyl-3,4,4-trimethylpentene (4), which is dehydrobrominated to 2,3-di-tert-butyl-1,3-butadiene (5). The first product of the interaction of the title olefin and Br2 in 1,2-dichloroethane is a 1:1 CTC having K_f = 84(0.3) M^-1 at 25 °C, ΔH = -5.7(0.4) kcal mol^-1, and ΔS = -10.3(1.3) eu in this solvent. This probably evolves to a second CTC of 1:2 stoichiometry, which ionizes directly to a bromonium-tribromide ion pair. Removal of an H⁺ from the bromonium ion by the counteranion gives the allylic bromide 4. Kinetic investigations using 2-H6 and its 3,4-bis(trideuteriomethyl) derivative 2-D6 show a primary kinetic isotope effect k_H/k_D = 2.17(0.25) at 25 °C, which decreases with increasing temperature, showing that the deprotonation step is at least partially rate determining. A similar conclusion holds for the reaction in acetic acid, where a k_H/k_D = 2.05(0.1) at 25 °C is observed. This requires that the formation of the bromonium ion intermediate is a reversible process, as for the analogous reaction of tetraisobutylethylene (TIBE, 1) with Br₂. The apparent activation energy for the reaction of 2-H6 in acetic acid, E^a(obsd) = 1.93(0.06) kcal mol^-1, is much lower than that for the reaction of TIBE (1), E_a(obsd) = 15.65(1.22) kcal mol^-1. This is mainly attributable to steric strain present in the olefin HMH, 2, which is relieved in the rate-determining transition state.