Liu Kanglei, Lalancette Roger A, Jäkle Frieder
Department of Chemistry , Rutgers University-Newark , 73 Warren Street , Newark , New Jersey 07102 , United States.
J Am Chem Soc. 2019 May 8;141(18):7453-7462. doi: 10.1021/jacs.9b01958. Epub 2019 Apr 26.
We demonstrate that the modification of anthracene with B ← N Lewis pairs at their periphery serves as a highly effective tool to modify the electronic structure with important ramifications on the generation and reactivity toward singlet oxygen. A series of BN-fused dipyridylanthracenes with Me groups in different positions of the pyridyl ring have been prepared via directed electrophilic borylation. The steric and electronic effects of the substituents on the structural features and electronic properties of the isomeric borane-functionalized products have been investigated in detail, aided by experimental tools and computational studies. We find that BDPA-2Me, with Me groups adjacent to the pyridyl N, has the longest B-N distance and shows overall less structural distortions, whereas BDPA-5Me with the Me group close to the anthracene backbone experiences severe distortions that are reflected in the buckling of the anthracene framework and dislocation of the boron atoms from the planes of the phenyl rings they are attached to. The substitution pattern also has a dramatic effect on the self-sensitized reactivity of the acenes toward O and the thermal release of singlet oxygen from the respective endoperoxides. Kinetic analyses reveal that BDPA-2Me rapidly reacts with O, whereas BDPA-5Me is converted only very slowly to its endoperoxide. However, the latter serves as an effective singlet oxygen sensitizer, as demonstrated in the preferential formation of the endoperoxide of dimethylanthracene in a competition experiment. These results demonstrate that even relatively small modifications in the substitution of the pyridyl ring of BN-fused dipyridylanthracenes change the steric and electronic structure, resulting in dramatically different reactivity patterns. Our findings provide important guidelines for the design of highly effective sensitizers for singlet oxygen on one hand and the realization of materials that readily form endoperoxides in a self-sensitized manner and then thermally release singlet oxygen on demand on the other hand.
我们证明，在蒽的外围用B←N路易斯对进行修饰是一种非常有效的工具，可用于修饰电子结构，这对单线态氧的产生和反应性具有重要影响。通过定向亲电硼化反应制备了一系列在吡啶环不同位置带有甲基的BN稠合二吡啶蒽。借助实验工具和计算研究，详细研究了取代基的空间和电子效应对异构硼烷官能化产物的结构特征和电子性质的影响。我们发现，吡啶基N相邻带有甲基的BDPA-2Me具有最长的B-N距离，并且整体结构畸变较小，而甲基靠近蒽骨架的BDPA-5Me则经历了严重的畸变，这反映在蒽骨架的弯曲以及硼原子与其相连的苯环平面的错位上。取代模式对并苯对O的自敏化反应性以及相应内过氧化物中单线态氧的热释放也有显著影响。动力学分析表明，BDPA-2Me与O快速反应，而BDPA-5Me仅非常缓慢地转化为其过氧化物。然而，后者是一种有效的单线态氧敏化剂，在竞争实验中二甲蒽过氧化物的优先形成证明了这一点。这些结果表明，即使是BN稠合二吡啶蒽吡啶环取代上相对较小的修饰也会改变空间和电子结构，从而导致截然不同的反应模式。我们的研究结果一方面为设计高效的单线态氧敏化剂提供了重要指导，另一方面为实现能够以自敏化方式轻松形成内过氧化物并按需热释放单线态氧的材料提供了指导。
