Nitro-Substituted Stilbeneboronate Pinacol Esters and Their Fluoro-Adducts. Fluoride Ion Induced Polarity Enhancement of Arylboronate Esters

A series of stilbeneboronate pinacol cyclic esters, containing none to three nitro groups, have been
synthesized by various olefination reactions and characterized by X-ray single-crystal structure analysis.
A stilbeneboronate ester bearing electron-acceptor groups experiences transition to a push−pull π-electron
system upon complexation with one fluoride ion at the boron atom. The UV−vis absorption maxima of
the presented nitro-substituted stilbeneboronate esters are red-shifted upon addition of fluoride ions,
indicating this binding event. The enhancement of the polarity of the investigated compounds and the
changes in the electronic system were investigated by UV−vis absorption spectroscopy and solvatochromism. Additionally, studies were performed by natural bond orbital (NBO) analysis and RI-CC2
calculations of the vertical excitation energies. The synergism of fluoride ion complexation and solvation
upon the UV−vis band shift is interpreted in terms of linear solvation energy relationships (LSERs)
using the Kamlet−Taft solvent parameter set. It is found that the UV−vis absorption of the fluoro-boronates is strongly dependent on the solvents hydrogen-bond donating ability.

本研究通过多种烯烃化反应合成了一系列含0至3个硝基的频哪醇环状芪硼酸酯，并借助X射线单晶结构分析对其进行了表征。带有吸电子基团的芪硼酸酯在硼原子与1个氟离子配位后，其电子体系将转变为推拉型π电子体系。所制备的硝基取代芪硼酸酯的紫外-可见吸收极大值在加入氟离子后发生红移，该现象印证了其与氟离子的配位作用。本研究通过紫外-可见吸收光谱与溶剂变色效应，对目标化合物的极性增强过程及其电子体系的变化进行了考察。此外，本研究还通过自然键轨道（natural bond orbital, NBO）分析以及垂直激发能的RI-CC2计算开展了相关研究。针对氟离子配位与溶剂化共同作用引发的紫外-可见吸收峰位移现象，本研究采用卡姆莱特-塔夫特溶剂参数集（Kamlet−Taft solvent parameter set），基于线性溶剂化能关系（linear solvation energy relationships, LSERs）进行了解释。研究表明，氟代硼酸酯的紫外-可见吸收行为强烈依赖于溶剂的氢键供体能力。