Formation of Cyclophane Macrocycles in Carbazole-Based Biradicaloids: Impact of the Dicyanomethylene Substitution Position
Formation of Cyclophane Macrocycles in Carbazole-Based Biradicaloids: Impact of the Dicyanomethylene Substitution Position
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DOI：
10.1021/acsomega.8b03418
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发表时间：
2019-03-01
期刊：
影响因子：
4.1
通讯作者：
Ruiz Delgado, M. Carmen
中科院分区：
文献类型：
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作者：
Badia-Dominguez, Irene;Perez-Guardiola, Andres;Ruiz Delgado, M. Carmen
我们最近证明了以咔唑为基础的双二萜是动态共价化学中有前途的构建块。为了阐明其令人着迷的动态共价化学性质，有必要了解其二元性质的物理起源。为此，我们重点研究了两种通过对位（p- cz -烷基）或元位（m-Cz-ph）取代双氰胺（DCM）基团的醌类咔唑体系，它们能够通过与DCM基团相连的桥头堡碳原子之间形成长C-C键形成环烷大环。我们的目标是探索以下问题：(i) DCM基团的取代位置如何影响醌类咔唑的双根碱特性？（ii）所得环烷骨料的稳定性是如何获得的？（iii）卡唑基单体和环烷聚集体之间的动态相互转化如何受到取代模式位置的这种细微变化的影响？基于密度泛函理论的计算表明，p- cz -烷基和m-Cz-ph在基电子状态下都是开壳双基，中间位置的DCM取代使得双基特征更加明显。相反，经咔唑单元的氮的衍生化反应预计不会影响双根碱的性质。基于环烷的大环形成（即p- cz -烷基中的环四聚体和m-Cz-ph中的环三聚体和四聚体）的自发性质得到了298 K计算的负相对吉布斯自由能的支持。有趣的是，DCM基团插入元位置的环状低聚物倾向于采用折叠构象，具有吸引的pi-pi相互作用，从而产生更稳定的聚集体；相反，请注意（p- cz -烷基）(4)得到一个扩展的环状构象。此外，元取代体系中桥头堡碳原子上较大的自旋密度增强了聚集体形式的桥接C-C键，阻碍了其解离。事实上，（m-Cz-ph）(4)和（m-Cz-ph）(3)的C-C键解离在溶液状态下受到抑制，尽管在固体状态下，由于外部软刺激（即温度和研磨），C-C键解离得以实现。总之，我们报告了一项非常全面的研究，旨在阐明以咔唑为基础的双根碱系统具有挑战性的化学性质。
We have recently demonstrated that carbazole-based biradicaloids are promising building blocks in dynamic covalent chemistry. To elucidate their intriguing dynamic covalent chemical properties, it is necessary to understand the physical origin of their biradical nature. To this end, here we focus on two quinoid carbazole systems substituted with dicyanomethylene (DCM) groups via para (p-Cz-alkyl) or meta positions (m-Cz-ph), which are able to form cyclophane macrocycles by the formation of long C-C bonds between the bridgehead carbon atoms linked to the DCM groups. We aim at exploring the following questions: (i) How is the biradicaloid character of a quinoid carbazole affected by the substitution position of the DCM groups? (ii) How is the stability of the resulted cyclophane aggregate attained? (iii) How is the dynamic interconversion between the carbazole-based monomers and cyclophane aggregates affected by this subtle change in the substitution pattern position? Density functional theory-based calculations reveal that both p-Cz-alkyl and m-Cz-ph are open-shell biradicals in the ground electronic state, with the DCM substitution in the meta position resulting in a more pronounced biradical character. In contrast, the derivatization via the nitrogen of the carbazole unit is not predicted to affect the biradicaloid character. The spontaneous nature of the cyclophane-based macrocycle formation (i.e., the cyclic tetramer in p-Cz-alkyl and the cyclic trimer and the tetramer in m-Cz-ph) is supported by the negative relative Gibbs free energies calculated at 298 K. Interestingly, cyclic oligomers in which the DCM groups are inserted in the meta position tend to adopt folded conformations with attractive pi-pi interactions resulting in more stable aggregates; in contrast, note that an extended ring-shaped conformation is acquired for (p-Cz-alkyl)(4). In addition, the larger spin density on the bridgehead carbon atom in the meta-substituted system strengthens the bridging C-C bond in the aggregate forms, hampering its dissociation. In fact, the C-C bond dissociation of (m-Cz-ph)(4) and (m-Cz-ph)(3) was suppressed in solution state, although it was achieved in solid state in response to soft external stimuli (i.e., temperature and grinding). In summary, we report a very comprehensive study aiming at elucidating the challenging chemical properties of carbazole-based biradicaloid systems.