REACTION OF CROWDED OLEFINS WITH BROMINE - A COMPARISON OF THE STRAINED (E)-2,2,3,4,5,5-HEXAMETHYLHEX-3-ENE WITH THE WITH STERICALLY HINDERED TETRAISOBUTYLETHYLENE

(E)-2,2,3,4,5,5-Hexamethylhex-3-ene (HMH, 2) reacts with Br-2 in chlor inated hydrocarbon solvents to give 3-bromo-2-tert-butyl-3,4,4-trimeth ylpentene (4), which is dehydrobrominated to 2,3-di-tert-butyl-1,3-but adiene (5). The first product of the interaction of the title olefin a nd Br-2 in 1,2-dichloroethane is a 1:1 CTC having K-f = 84(0.3) M(-1) at 25 degrees C, Delta H = -5.7(0.4) kcal mol(-1), and Delta S = -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 pa ir. 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-b is(trideuteriomethyl) derivative 2-D6 show a primary kinetic isotope e ffect k(H)/k(D) = 2.17(0.25) at 25 degrees C, which decreases with inc reasing temperature, showing that the deprotonation step is at least p artially rate determining. A similar conclusion holds for the reaction in acetic acid, where a k(H)/k(D) = 2.05(0.1) at 25 degrees C is obse rved. This requires that the formation of the bromonium ion intermedia te is a reversible process, as for the analogous reaction of tetraisob utylethylene (TIBE, 1) with Br-2. The apparent activation energy for t he 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) mol(-1). This is mainly attributable to steric strain pre sent in the olefin HMH, 2,which is relieved in in the rate-determining transition state.