Electrophilic aromatic substitution. Part 36. Protiodetritiation of some annelated meta-cyclophanes: effect of ring-buckling on reactivity, and the first example of electrophilic substitution through a hole
Electrophilic aromatic substitution. Part 36. Protiodetritiation of some annelated meta-cyclophanes: effect of ring-buckling on reactivity, and the first example of electrophilic substitution through a hole
复制标题
亲电芳族取代。
DOI：
--
复制
发表时间：
1987
期刊：
影响因子：
--
通讯作者：
R. Taylor
中科院分区：
文献类型：
--
作者：
A. Laws;A. Neary;R. Taylor
一系列 1,3-桥联萘，其中烷基桥含有十个、八个或七个亚甲基，已用位于 2-位或 4-位的氚制备。已测量了它们在 70 °C 无水三氟乙酸中的脱氚率。随着桥尺寸的减小，4 位交换的部分速率因子增加。 1.28×107([CH2]10) 2.71×107([CH2]8), 2.97×107([CH2]7);参见1,3-二甲基萘为 1.15 × 107，相应的 σ+ 值为 –0.812、–0.849、–0.854 和 –0.807。这与芳环变得更加弯曲时基态芳香性损失增加是一致的。在 2 位处，部分速率因子为 2.37 × 105([CH2]10)、2.92 × 105([CH2]8) 和 1.50 × 105([CH2]7)；参见1,3-二甲基萘为 2.82 × 105，相应的 σ+ 值为 –0.614、–0.625、–0.592 和 –0.623。这里的结果反映了由于基态芳香性损失而增加的反应性以及随着桥变小而导致的交换空间位阻的相反影响。由于氢交换反应路径的对称性，2位反应涉及烷基链和芳环之间形成的环的一个面上的亲电子试剂的攻击，以及离去基团（triton）从相反面的离开，即通过“孔”发生取代。这是封闭位点亲电取代的第一个例子。氢交换中试剂和产物的空间效应几乎相同这一事实被认为是反应中通常观察到的极低空间位阻的主要原因。首次确定了双环取代基稠合到芳环上的影响。对于六氢菲和六氢苊（在 1、2 和 3 位上连接有双环取代基的萘），“4”位脱氚的平均部分速率因子为 5.7 × 107，σ+ 为 –0.886。这大于基于烷基取代基效应的加和性计算的结果，并且归因于超共轭的空间促进。最近的建议（基于结构计数）认为螺旋烯的高反应性（以及包含弯曲苯环的其他芳香系统的暗示）并不是由于基态稳定性的损失而被证明是无效的。
A series of 1,3-brigded naphthalenes in which the alkyl bridge contains ten, eight, or seven methylene groups have been prepared with tritium located at either the 2- or the 4-position. Their rates of detritiation in anhydrous trifluoroacetic acid at 70 °C have been measured. The partial rate factors for exchange at the 4-position increase as the size of the bridge decreases viz. 1.28 × 107([CH2]10) 2.71 × 107([CH2]8), 2.97 × 107([CH2]7); cf. 1.15 × 107 for 1,3-dimethylnaphthalene, the corresponding σ+ values being –0.812, –0.849, –0.854, and –0.807. This is consistent with increased loss of ground-state aromaticity as the aromatic ring becomes more buckled. At the 2-positions the partial rate factors are 2.37 × 105([CH2]10), 2.92 × 105([CH2]8), and 1.50 × 105([CH2]7); cf. 2.82 × 105 for 1,3-dimethylnaphthalene, the corresponding σ+ values being –0.614, –0.625, –0.592, and –0.623. The results here reflect the opposing effects of increased reactivity due to loss of ground-state aromaticity, and steric hindrance to exchange as the bridge is made smaller. Because of the symmetry of the reaction pathway for hydrogen exchange, reaction at the 2-positions involves attack of the electrophile at one face of the loop formed between the alkyl chain and the aromatic ring, and departure of the leaving group (triton) from the opposite face, i.e. substitution occurs through a ‘hole’. This is the first example of electrophilic substitution at an enclosed site. The fact that steric effects in both reagent and product in hydrogen exchange are almost identical is postulated as a primary reason for the extremely low steric hindrance generally observed in the reaction. The effect of fusing a bicyclic substituent onto an aromatic ring has been determined for the first time. For hexahydrophenanthrylene and hexahydroaceanthrylene (naphthalenes with bicyclic substituents attached to the 1-, 2-, and 3-positions) the average partial rate factor for detritiation of the ‘4’-positions is 5.7 × 107, and σ+ is –0.886. This is greater than calculated on the basis of additivity of the alkyl group substituent effects and is attributed to steric facilitation of hyperconjugation. The recent suggestion (based on structure counts) that the exalted reactivities of helicenes (and by implication of other aromatic systems containing bent benzene rings) is not due to loss of ground-state stability is shown to be invalid.