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.