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 (<b>1a</b>) and 4-azido-(4-methoxy)phenylbutan-1-one (<b>1b</b>) 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 <b>1a</b> 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 <b>2a</b>. However, <b>1b</b> reacts through its less reactive T_1K, which has a (π,π*) configuration, to form 2-phenyl-1<i>H</i>-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 <b>1a</b> and <b>1b</b> yields the corresponding pyrroline (<b>2a</b> and <b>2b</b>) via triplet alkylnitrene (³<b>1aN</b> and³<b>1bN</b>) 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 <b>2a</b> and <b>2b</b>. 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.