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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 (T1K) with an (n,π*) configuration, which selectively yields 2-phenyl-1-pyrroline derivative 2a. However, 1b reacts through its less reactive T1K, 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 (TA) 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.
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This content will become publicly available on April 26, 2027
Photofracking-Assisted Enhancement of Solid-State Photochemical Reactivity: α-Azido-5-phenyl-2,4-dienoate Derivatives
Solid-state photoreactions are unmatched by their liquid or gaseous counterparts for being spatially and temporally controllable and selective; however, these assets usually come with compromised yields―localized surface reactions are favored that generate a passivating layer of photoproducts and shield the crystal interior from reacting. Here, we demonstrate that this inherent limitation could be circumvented by capitalizing on the disintegrative effects of photodynamic crystals, such as cracking and fracturing, which can expose freshly cleaved surfaces and effectively increase the overall conversion. Upon irradiation with natural or artificial visible light, crystals of two 2-azido-5-phenyl-2,4-dienoate derivatives (1a and 1b) are converted into the corresponding 5-phenyl-1H-pyrrole-2-carboxylate derivatives (2a and 2b). Crystals were observed to crack and fracture throughout the reaction due to the release of dinitrogen, and the cracking occurs preferentially along planes defined by the weakest intermolecular interactions. Laser-flash photolysis in conjunction with density functional theory calculations confirmed that in both solution and in the solid, the triplet excited states of 1a and 1b rearrange to biradicals 3Br1a and 3Br1b, which then release dinitrogen to afford biradicals 3Br2a and 3Br2b; however, in the solid state, the crystal lattice rigidity extends the triplet lifetimes of both 1 and 3Br1. This work highlights the benefits of the “photofracking” that stands for photoinduced crystal disintegration, as a convenient strategy that naturally increases the efficiency of the solid-state reactions without external intervention.
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- PAR ID:
- 10679433
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Journal of the American Chemical Society
- ISSN:
- 0002-7863
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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