An electron-donor–acceptor complex between two intermediates enables a N–N bond cleavage cascade process to access 2,3-difunctionalized pyridines.

Chemical transformation triggered by an electron donor–acceptor (EDA) complex without the addition of an exogenous stoichiometric electron donor/acceptor is rare. Herein, we report such a process to access 2,3-difunctionalized pyridines from readily available N-aminopyridiniums (1) and activated alkenes (2) promoted by visible light. This procedure offered multi-substituted pyridines in satisfactory yields at room temperature with broad functional group tolerance. The reaction can be easily performed on a gram scale without the loss of yield. The modification of bioactive molecules including derivatives of clinical drugs and natural products was demonstrated. Mechanistic studies and DFT calculations indicated that the formal [3 + 2] cycloaddition and aza-Michael addition between 1 and 2 generated tetrahydropyrazolo[1,5-a]pyridine and a new pyridinium salt, respectively. These two intermediates formed an EDA complex, which under visible-light illumination, triggered the following single electron transfer (SET)/N–N bond cleavage/C–N bond formation cascade process with high atom economy. [ABSTRACT FROM AUTHOR]