Air and Moisture Stable para- and ortho-Quinodimethane Derivatives Derived from bis-N-Heterocyclic Olefins

Herein we report the development of a new methodology for the synthesis of various quinodimethane derivatives under two-electron oxidation of bis-Nheterocyclic olefins linked by different pi-conjugated aromatic spacers. In case of paraand ortho-phenylene bridge, we obtained air and moisture stable diimidazolium paraand ortho-quinodimethane derivatives. Analogues of the para-phenylene spacer such as tetrafluoro-p-phenylene and p-anthracene also led to the corresponding air and moisture stable quinodimethane derivatives. This emphasizes the influence of imidazolium substituents which facilitate the air and moisture stability of the quinodimethane derivatives. Differences were observed for the electron transfer processes: two one-electron vs one two-electron redox transitions between bis-N-heterocyclic olefins and diimidazolium-quinodimethanes depending on the employed pi-conjugated aromatic spacer. The formation of the pi-conjugated radical-cations, transient redox intermediates between bis-N-heterocyclic olefins and diimidazolium-quinodimethanes, was addressed by an EPR investigation.

Automated summary

We developed a new method for synthesizing different quinodimethane derivatives by oxidizing bis-N-heterocyclic olefins linked by pi-conjugated aromatic spacers. Stable diimidazolium para- and ortho-quinodimethane derivatives were obtained with para- and ortho-phenylene bridges, as well as with tetrafluoro-p-phenylene and p-anthracene spacers. The influence of imidazolium substituents on the air and moisture stability of the quinodimethane derivatives was emphasized. Different electron transfer processes were observed depending on the pi-conjugated aromatic spacer used, with two one-electron or one two-electron redox transitions between bis-N-heterocyclic olefins and diimidazolium-quinodimethanes. The formation of pi-conjugated radical-cations, which are transient redox intermediates, was confirmed by EPR investigation.