Mechanistic Insights into Photocatalytic Reduction and Triplet Sensitization of 4-Azido-1-phenylbutan-1-ones to Selectively Form 2-Phenyl-1-pyrrolines

Organic azides are valuable precursors in synthetic chemistry, particularly for nitrogen-based functionalization through photochemical activation. In this study, the photoreactivities of 4-azido-1-phenylbutan-1-one (1a) and 4-azido-(4-methoxy)phenylbutan-1-one (1b) were investigated using visible-light photocatalysts [Ir(dF(CF₃)ppy)₂(dtbpy)]PF₆ and [Ru(bpy)₃]Cl₂ to elucidate the mechanistic differences between triplet energy transfer and photoreductive electron transfer pathways. Direct irradiation of 1a in methanol favors the formation of a biradical species via intramolecular H atom abstraction to generate its lowest triplet ketone (T_1K) with an (n,π*) configuration, which selectively yields 2-phenyl-1-pyrroline derivative 2a. However, 1b reacts through its less reactive T_1K, which has a (π,π*) configuration, to form 2-phenyl-1H-pyrrole as the major product. When sensitized by [Ir(dF(CF₃)ppy)₂(dtbpy)]PF₆, selective excitation of the triplet azido moiety (T_A) of both 1a and 1b yields the corresponding pyrroline (2a and 2b) via triplet alkylnitrene (³1aN and³1bN) formation. In contrast, photoactivation of [Ru(bpy)₃]Cl₂ in the presence of diisopropylethylamine (DIPEA) results in photoreductive electron transfer, forming azido radical anion intermediates, which cyclize to also yield 2a and 2b. Product studies, cyclic voltammetry, laser flash photolysis, and DFT calculations supported these mechanistic assignments. This work demonstrates complementary approaches to control alkyl azide photoreactivity and unlock new strategies for visible-light-induced nitrogen incorporation.