The importance of conformational effects on the carbon-carbon bond cleavage of beta-phenethyl ether radical cations
The importance of conformational effects on the carbon-carbon bond cleavage of beta-phenethyl ether radical cations
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DOI：
10.1139/v97-044
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发表时间：
1997-04-01
期刊：
影响因子：
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通讯作者：
Arnold, DR
中科院分区：
文献类型：
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作者：
Perrott, AL;deLijser, HJP;Arnold, DR
本文研究了β-苯乙基醚自由基阳离子的光敏（电子转移）键断裂。在以前的研究中，键断裂的可行性被认为取决于键离解能（BDE）。然而，这个简单的假设导致了几个不正确的预测，因此额外的标准，构象效应，被添加到假设。这项研究现在已被扩展，并提供了额外的例子的构象上的自由基阳离子的碳-碳键断裂的重要性。所研究的四种β-苯乙基醚是2-甲氧基-3-苯基丁烷（9，均为非对映异构体），以及顺式和反式-2-甲基-3-苯基四氢吡喃（10 c，10 t）。一般来说，如果自由基阳离子中的溴化二苯醚（计算值）低于55千焦/摩尔，并且如果单占据分子轨道（SOMO）和易受攻击的（C-C或C-H）键之间存在显著重叠，则会发生键断裂。在β-苯乙基醚基阳离子的情况下，烷氧基也必须定向，使得氧孤对电子可以与C-C σ反键（σ *）轨道重叠。计算的BDE值的脆弱的C-C键在这里研究的四个醚的自由基阳离子远低于阈值，55千焦/摩尔，和C-C裂解，因此将由构象效应。分子力学（MM 3）计算被用来确定最稳定的中性分子的构象。基于计算的角度和轨道之间的重叠，预测醚9和10 c的全局最小构象不会产生任何显著程度的C-C键断裂产物或去质子化。醚10 t的全局最小值对于C-C裂解是很好的取向，但对于去质子化不是。在醚的存在下，电子接受光敏剂1，4-二氰基苯（2）的辐照表明醚9没有有效裂解;没有观察到去质子化或异构化。这与基于MM 3计算的预测是一致的。醚10 c和10 t都给出了合理的C-C裂解产物产率;事实上，醚10 c比10 t裂解更有效。这可以解释的事实是，构象10 c，只有4.35 kJ/mol的能量高于全球最低，是完美的分裂对齐。醚10 t没有显示出任何去质子化的证据，而10 c显示出。这也与计算结果吻合得很好。
The photosensitized (electron transfer) bond cleavage of some beta-phenylethyl ether radical cations has been investigated. In previous studies the feasibility of the bond cleavage was thought to depend on the bond dissociation energy (BDE). However, this simple hypothesis led to several incorrect predictions and therefore additional criteria, conformational effects, were added to the hypothesis. This study has now been extended and additional examples of the importance of the conformation on the carbon-carbon bond cleavage of radical cations are provided. The four beta-phenylethyl ethers studied are 2-methoxy-3-phenylbutane (9, both diastereomers), and cis- and trans-2-methyl-3-phenyltetrahydropyran (10c, 10t). Generally, bond cleavage will occur if the (calculated) BDE in the radical cation is less than 55 kJ/mol, and if there is significant overlap between the singly occupied molecular orbital (SOMO) and the vulnerable (C-C or C-H) bond. In the case of a beta-phenylethyl ether radical cation, the alkoxy group must also be oriented so that an oxygen lone pair of electrons can overlap with the C-C sigma antibonding (sigma*) orbital. The calculated BDE values of the vulnerable C-C bond in the radical cations of the four ethers studied here are well below the threshold value, 55 kJ/mol, and C-C cleavage will therefore be governed by conformational effects. Molecular mechanics (MM3) calculations were used to identify the most stable conformers of the neutral molecules. Based on the calculated angles and overlap between orbitals it was predicted that the global-minimum conformers of the ethers 9 and 10c would not give C-C bond cleavage products or deprotonation to any significant extent. The global minimum of ether 10t is well oriented for C-C cleavage but not for deprotonation. Irradiation of an electron-accepting photosensitizer, 1,4-dicyanobenzene (2), in the presence of the ethers showed that the ethers 9 did not cleave efficiently; no deprotonation or isomerization was observed. This is in good agreement with the predictions based on the MM3 calculations. Both ethers 10c and 10t gave reasonable yields of the C-C cleavage products; in fact, ether 10c cleaved more efficiently than 10t. This can be explained by the fact that a conformer of 10c, only 4.35 kJ/mol higher in energy than the global minimum, is perfectly aligned for cleavage. Ether 10t did not show any evidence for deprotonation whereas 10c did. This is also in good agreement with the calculations.