Unsymmetrical Thienopentalenes: Synthesis, Optoelectronic Properties, and (Anti)aromaticity Analysis

The synthesis and
properties of a series of unsymmetrical thienopentalenes
are explored, including both monoareno and diareno derivatives. For
the synthesis of monoareno pentalenes, a carbopalladation cascade
reaction between alkynes and gem-dibromoolefins was
applied. Diareno pentalene derivatives were accessed via gold-catalyzed
cyclization of diynes. Thiophene was fused to pentalene in two different
geometries via its 2,3 and 3,4 bonds. 2,3-Fusion resulted in increased
antiaromaticity of the pentalene unit compared to the 3,4-fusion both
in the monoareno and diareno framework. Monothienopentalenes that
contained the destabilizing 2,3-fusion could not be isolated. For
diareno derivatives, the aromatic character of the different aryl
groups fused to the pentalene was not independent. Destabilizing fusion
on one side resulted in alleviated aromaticity on the other side and
vice versa. The synthesized molecules were characterized experimentally
by 1H NMR and UV–vis spectroscopies, cyclic voltammetry,
and X-ray crystallography, and their aromatic character was assessed
using magnetic (NICS and ACID) and electronic indices (MCI and FLU).

本研究系统探索了一系列不对称噻吩并戊搭烯（thienopentalenes）的合成方法与理化性质，涵盖单芳基取代与双芳基取代两类衍生物。针对单芳基取代戊搭烯的合成，本研究采用了炔烃与偕二溴烯烃（gem-dibromoolefins）之间的碳钯化级联反应（carbopalladation cascade reaction）。双芳基取代戊搭烯衍生物则经由金催化的二炔环化反应（gold-catalyzed cyclization of diynes）合成。噻吩可通过2,3位与3,4位两种不同的成键几何方式与戊搭烯稠合。相较于3,4位稠合，无论是单芳基还是双芳基骨架中，2,3位稠合都会使戊搭烯单元的反芳香性（antiaromaticity）显著增强。带有不稳定2,3位稠合结构的单噻吩并戊搭烯（monothienopentalenes）无法被分离提纯。对于双芳基取代衍生物而言，与戊搭烯稠合的不同芳基的芳香性并非独立存在：一侧带有不稳定稠合结构时，会缓解另一侧的芳香性，反之亦然。所合成的目标分子通过氢核磁共振波谱（1H NMR）、紫外-可见（UV–vis）光谱、循环伏安法（cyclic voltammetry）以及X射线晶体衍射（X-ray crystallography）完成了全面的实验表征，并借助磁学指标（核独立化学位移NICS与感应电流密度各向异性ACID）和电子学指标（磁流指数MCI与FLU）对其芳香性特征进行了系统性评估。