2,7-Diisopropoxy-1,8-dibromonaphthalene 5 was prepared in two steps from 2,7-dihydroxynaphthalene and was coupled under Suzuki cross-coupling conditions with boronic acids 9 and 10 to provide the corresponding 1,8-diarylnaphthalene systems 12a and 13a respectively. In contrast, attempted coupling of dibromide 5 with o-tolylboronic acid 11 proved unrewarding. Single crystal X-ray structure determination of compounds 12a and 13a showed that both structures possessed a high degree of structural deformation due to high internal steric repulsions between the 1,8-diaryl rings and their substituents.
Dynamic 1H NMR experiments showed that these two systems possessed very slow phenyl–naphthalene bond rotation (ca. 2 s−1), corresponding to rotation barriers (ΔG*) of 16–18 kcal mol−1. Molecular modelling predicts that such systems have approximately similar rotation barriers and that in order to completely prevent phenyl–naphthalene bond rotation, an ortho-phenyl substituent is required, with a barrier to rotation of ca. 40 kcal mol−1.
2,7-二异丙氧基-
1,8-二溴萘5是由
2,7-二羟基萘分两步制备的,并在Suzuki交叉偶联条件下与
硼酸9和10偶联,得到相应的1,8-二芳基
萘体系12a和分别如图13a所示。相反,尝试将二
溴化物 5 与邻
甲苯基
硼酸 11 偶联被证明是没有回报的。化合物12a和13a的单晶X射线结构测定表明,由于1,8-二芳基环与其取代基之间的高内部空间排斥,两种结构都具有高度的结构变形。
动态 1H NMR 实验表明,这两个系统具有非常慢的苯基-
萘键旋转(约 2 s−1),对应于 16-18 kcal mol−1 的旋转势垒(ΔG*)。分子模型预测,此类系统具有大致相似的旋转势垒,并且为了完全防止苯基-
萘键旋转,需要邻苯基取代基,其旋转势垒约为。 40 kcal mol−1。