Sharma Nivedita, Kour Manjinder, Gupta Raakhi, Bansal Raj K
The IIS (deemed to be University) Jaipur 302020 India
RSC Adv. 2021 Jul 21;11(41):25296-25304. doi: 10.1039/d1ra03981d. eCollection 2021 Jul 19.
Cross-conjugated mesomeric betaine (CCMB) has been defined as the dipolar species in which positive and negative charges are restricted to different parts of the molecule. In contrast to indolizine which undergoes [8+2] cycloaddition with dimethyl acetylenedicarboxylate (DMAD), its 1-aza analogue, namely imidazo[1,2-]pyridine reacts with the same reagent to afford the first representative of the CCMB isoconjugate with the odd non-alternant hydrocarbon anion. The structure of the product could be assigned on the basis of the NMR and HRMS results. Furthermore, the spectral studies indicated the presence of additional DMAD molecules in CCMB, possibly in the form of a charge-transfer (CT) complex. The whole sequence of reactions initiated by the attack of imidazo[1,2-]pyridine on DMAD could be rationalized on the basis of the computational study of a model reaction sequence at the DFT (B3LYP/6-31+G(d)) level indicating the formation of a new CCMB derivative. The electronic excited states of the product were investigated by time-dependent density functional theory (TDDFT) calculations at the wB97XD/6-311++G(d,p) level, which indicate low-lying charge transfer that is characteristic of the CCMBs.
交叉共轭的中介内盐（CCMB）被定义为正电荷和负电荷局限于分子不同部分的偶极物种。与能与二甲基乙炔二羧酸酯（DMAD）发生[8 + 2]环加成反应的中氮茚不同，其1-氮杂类似物，即咪唑并[1,2 - ]吡啶与同一试剂反应，得到了与奇数非交替烃阴离子等共轭的CCMB的首个代表物。产物的结构可根据核磁共振（NMR）和高分辨质谱（HRMS）结果确定。此外，光谱研究表明CCMB中存在额外的DMAD分子，可能是以电荷转移（CT）络合物的形式存在。基于在密度泛函理论（DFT，B3LYP/6 - 31 + G(d)）水平对模型反应序列的计算研究，合理化了由咪唑并[1,2 - ]吡啶对DMAD的进攻引发的整个反应序列，该研究表明形成了一种新的CCMB衍生物。通过在wB97XD/6 - 311++G(d,p)水平的含时密度泛函理论（TDDFT）计算研究了产物的电子激发态，结果表明存在CCMB特有的低能级电荷转移。
