Steric Crowding Can Stabilize a Labile Molecule: Solving the Hexaphenylethane Riddle
Steric Crowding Can Stabilize a Labile Molecule: Solving the Hexaphenylethane Riddle
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DOI：
10.1002/anie.201103615
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发表时间：
2011-01-01
影响因子：
16.6
通讯作者：
Schreiner, Peter R.
中科院分区：
文献类型：
--
作者：
Grimme, Stefan;Schreiner, Peter R.
空间位阻被认为是六苯乙烷(1)热力学不稳定的唯一原因(方案1)。尽管尝试了很多次，所有合成方法都失败了。直到1968年，人们才认识到Gomberg S[2]三苯甲基自由基不会二聚成1[3]，而是不那么对称的亚甲基环己二烯异构体({[4-(二苯亚甲基)环己烷-2，5-二烯-1-基]甲三基}三苯)。[4]同样，三(4-叔丁基苯基)甲基自由基不会二聚成相应的乙烷衍生物(3)。[5]这些六苯乙烷衍生物的不稳定性的一般论点是苯基的空间相互排斥，尽管它们相互具有良好的局部T型苯二聚体取向。然而，空间上更密集的全间叔丁基衍生物(2)在室温下是稳定的，它的晶体结构(MP=2148℃)已经被解析。[6]由于空间位阻而解离的分子的衍生物如何通过增加空间体积而变得稳定？我们通过证明2通过非常强的伦敦分散(范德华(VDW)吸引人的部分)相互作用将2结合在一起，由于叔丁基团有许多有利的CH···CH接触[7]，这些相互作用将二聚反应从对1和3的吸热过程转变为对2的放热过程。我们不同寻常的发现与米斯洛1981年S关于“叔丁基对2的成键参数没有特殊影响”的说法形成鲜明对比，[1b]这符合普遍的预期；然而，这并不能为2的稳定性提供理论基础。对2的分子模型(见图1的空间填充模型)的观察表明，它确实是一个空间拥挤的分子，但叔丁基也不一定被放置在关键位置，因此可能不会对1中已经存在的苯环接触引入显著的Pauli排斥力。[8]叔丁基的电子效应预计很小，因为它们离中心C？C键很远。此外，即使存在通键效应，它也应该稳定解离的缺电子自由基产物；它在一定程度上做到了这一点，在3中也可以解离成持久的自由基。[5]解决这个谜团，即为什么全间叔丁基对2的稳定性比1和3有如此显著的影响是这项工作的主要目的。上一次计算-
Steric congestions has been made solely responsible for the thermodynamic instability of hexaphenylethane(1)(Scheme 1).[1] Despite many attempts, all synthetic approaches to preparing this seemingly simple molecule failed. It was not recognized until 1968 that Gomberg s [2] triphenylmethyl radical does not dimerize to give 1 [3] but instead a less symmetrical methylenecyclohexadiene isomer ({[4-(diphenylmethylene) cyclohexa-2, 5-dien-1-yl] methanetriyl} tribenzene).[4] Equally, the tri (4-tert-butylphenyl) methyl radical does not dimerize to the corresponding ethane derivative (3).[5] The generic argument for the instabilities of these hexaphenylethane derivatives is steric repulsion of the phenyl groups, despite their favorable mutual local T-shaped benzene dimer type orientations. Yet, the sterically much more crowded all-meta-tert-butyl derivative (2) is stable at room temperature, and its crystal structure (mp= 2148C) has been resolved.[6] How can the derivative of a molecule that dissociates owing to steric hindrance become stable by increasing steric bulk? We answer this question by demonstrating that 2 is being held together by extraordinarily strong London dispersions (the attractive part of van der Waals (vdW)) interactions that turn dimerization from an endothermic process for 1 and 3 into an exothermic one for 2 owing to many favorable CH··· CH contacts [7] of the tert-butyl groups. Our unusual findings are in marked contrast to Mislow s 1981 statement that “the tert-butyl groups have no special effect on the bonding parameters of 2”,[1b] which is in line with common expectations; however, this does not provide a rationale for the stability of 2.Visual inspection of molecular models (see Figure 1 for a space-filling model) of 2 suggests that it is indeed a sterically crowded molecule but also that the tert-butyl groups are not necessarily placed in critical positions and thus might not introduce significant Pauli repulsion to the phenyl ring contacts already present in 1.[8] The electronic effect of the tert-butyl groups is expected to be small because of their long distance to the central CÀC bond. Furthermore, even if a through-bond effect existed, it should stabilize the electrondeficient radical products of dissociation; it does so to some extent in 3, which also dissociates into persistent radicals.[5] Solving this riddle, that is, why the all-meta tert-butyl groups have such a dramatic effect on the stability of 2 compared to 1 and 3 is the main objective of this work. Previous computa-