Catalyst-Free π-Extended Conjugate Addition of Amines to Various Electron-Deficient π-Systems

Development of atom-economical, catalyst-free, and benign synthetic methods for functionalizing π-conjugated molecules is of great interest for unconventional organic materials. Here, we report catalyst-free 1,6- or more π-extended conjugate addition of amines to ethynyl groups directly attached to various electron-deficient π-systems. This hydroamination proceeded quantitatively under mild conditions to afford amino-functionalized π-systems. A kinetic study of the reaction revealed that the rate-determining step was the nucleophilic attack of an amine to the π-system, and the reaction obeyed typical second-order kinetics. We found that the hydroamination rates of the π-systems (the logarithm of the second-order rate constants) showed a good linear correlation with the chemical shifts of the terminal ethynyl proton in ¹H NMR spectrum. On the basis of the kinetic study of these π-systems, an asymmetric amine monoadduct could be obtained from the corresponding two ethynyl-substituted π-system in a designed manner, and the other ethynyl group could be functionalized through different reactions such as Glaser coupling reaction. Some of the resultant amino-functionalized π-systems exhibited characteristic near-IR emission, and the emission wavelength can be switched reversibly in response to a Brønsted acid and a base.