PROTON SHIFTS IN GASEOUS ARENIUM IONS AND THEIR ROLE IN THE GAS-PHASE AROMATIC-SUBSTITUTION BY FREE ME3C+ AND ME3SI+ CATIONS
PROTON SHIFTS IN GASEOUS ARENIUM IONS AND THEIR ROLE IN THE GAS-PHASE AROMATIC-SUBSTITUTION BY FREE ME3C+ AND ME3SI+ CATIONS
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DOI：
10.1021/ja00043a024
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发表时间：
1992-08-12
影响因子：
15
通讯作者：
FORNARINI, S
中科院分区：
文献类型：
--
作者：
CACACE, F;CRESTONI, ME;FORNARINI, S
在720托、47 - 120 ℃温度范围内，用辐解技术，用适当氘代的底物，研究了Me 3C+与苯和甲苯的气相反应。结果强调了在主要带电中间体，σ-复合物I中1，2-H（D）位移的动力学作用。当后者来源于Me 3C+进攻p-MeC 6 H4 D的帕拉时，D的位移速率相对较慢，约为1000。5 x 10（7）s-1（47 ℃），活化能E（a）= 7.6 +/- 0.2 kcal mol-1，指前因子A = 10（12.9+/-0.4）。来自Me 3C+攻击苯或甲苯Meta位的离子I的寿命短于几纳秒，并且1，2-H位移虽然在整个苯反应中仍然具有动力学意义，但不受在高达8 Torr的压力下向气体中加入强碱的影响，这确保了I在1 ns内的去质子化。由于1，2-H（D）位移与I的反解离竞争，它们的速率影响净烷基化速率，这解释了在C6 H6和C6 D 6的竞争性叔丁基化中观察到的超过2的动力学同位素效应，这是气相中热离子反应的第一个报道。涉及Me 3Si+与苯和甲苯反应的类似实验表明，H（D）从异硅烷化位置的位移非常慢，与先前的质谱和辐解研究所证明的高位点碱性一致。
The gas-phase reaction Of Me3C+ with benzene and toluene has been studied at 720 Torr, in the temperature range from 47 to 120-degrees-C with the radiolytic technique, using suitably deuterated substrates. The results underline the kinetic role of the 1,2-H(D) shifts in the primary charged intermediate, the sigma-complex I. When the latter originates from Me3C+ attack at the para position of p-MeC6H4D, the D shift rate is relatively slow, ca. 5 x 10(7) s-1 at 47-degrees-C, with an activation energy E(a) = 7.6 +/- 0.2 kcal mol-1 and a preexponential factor A = 10(12.9+/-0.4). Ions I from the Me3C+ attack on benzene or at the meta positions of toluene have lifetimes shorter than a few nanoseconds, and the 1,2-H shift, while still kinetically significant in the overall benzene reaction, is unaffected by addition of strong bases to the gas at pressures up to 8 Torr, which ensures deprotonation of I within 1 ns. Since 1,2-H(D) shifts compete with back dissociation of I, their rate influences the net alkylation rate, which accounts for the kinetic isotope effect in excess of 2 observed in the competitive tert-butylation of C6H6 and C6D6, the first one reported for a thermal ionic reaction in the gas phase. Analogous experiments involving the reaction of Me3Si+ with benzene and toluene show that H(D) shifts are remarkably slow from the ipso-silylated position, consistent with its high site basicity demonstrated by previous mass spectrometric and radiolytic studies.