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
作者:Giuseppe Bellucci、Roberto Bianchini、Cinzia Chiappe、Dieter Lenoir、Abdul Attar
DOI:10.1021/ja00128a010
日期:1995.6
(E)-2,2,3,4,5,5-Hexamethylhex-3-ene (HMH, 2) reacts with Br-2 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 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 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 degrees 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 degrees 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 Br-2. 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) mol(-1). This is mainly attributable to steric strain present in the olefin HMH, 2,which is relieved in in the rate-determining transition state.
(E)-2,2,3,4,5,5-六甲基己烯 (HMH, 2)与Br-2在氯代烷烃溶剂中反应,得到3-溴-2-tert-丁基-3,4,4-三甲基戊烯(4),随后被脱溴生成2,3-二tert-丁基-1,3-丁二烯(5)。该烯烃与Br-2在1,2-二氯乙烷的反应首产物为1:1的比例循环转化物(CTC),其稳定自由能为 K-f = 84(0.3) M⁻¹,ΔH = -5.7(0.4) kcal mol⁻¹,ΔS = -10.3(1.3) eu。这种情况下,可能转化为1:2 的第二步产物,该产物直接形成溴化溴onium-三角溴离子对。通过加入反离子消除溴onium离子中的H+,从而得到邻位溴化物 4。通过使用2-H6及其3,4-二(D-TMS)甲基取代的溴化物 2-D6来进行动力学研究,发现在25°C时,一级动压效应 k(H)/k(D) = 2.17(0.25),并且随着温度的升高而减小,这表明解离步骤至少部分影响了反应速率。在酸性条件下的情况类似,观察到 k(H)/k(D) = 2.05(0.1)。由此可知,溴离子中间体的形成过程应为可逆的,例如对于以正丁基丙烯为模型的类似反应。在酸性条件下,2-H6的动压能 E(a(obsd)) = 1.93(0.06) kcal mol⁻¹较差于 TIB.[Hm,这里的 E(a(obsd)) = 15.65(1.22) kcal mol⁻¹,这主要归因于烯烃 HMH, 2中的立体阻碍,这部分障碍在确定步的反应速率特征中被缓解。