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ABSTRACT Triplet arylnitrenes may provide direct access to aryl azo‐dimers, which have broad commercial applicability. Herein, the photolysis ofp‐azidostilbene (1) in argon‐saturated methanol yielded stilbene azo‐dimer (2) through the dimerization of tripletp‐nitrenostilbene (31N). The formation of31Nwas verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λmax ~ 375 nm) in cryogenic 2‐methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of1in methanol formed31N(λmax ~ 370 nm, 2.85 × 107 s−1). On shorter timescales, a transient absorption (λmax ~ 390 nm) that decayed with a similar rate constant (3.11 × 107 s−1) was assigned to a triplet excited state (T) of1. Density functional theory calculations yielded three configurations for T of1, with the unpaired electrons on the azido (TA) or stilbene moiety (TTw, twisted and TFl, flat). The transient was assigned to TTwbased on its calculated spectrum. CASPT2 calculations gave a singlet–triplet energy gap of 16.6 kcal mol−1for1 N; thus, intersystem crossing of11Nto31Nis unlikely at ambient temperature, supporting the formation of31Nfrom T of1. Thus, sustainable synthetic methods for aryl azo‐dimers can be developed using the visible‐light irradiation of aryl azides to form triplet arylnitrenes.
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This content will become publicly available on October 1, 2026
Aromaticity and Photoreactivity of 4‐ and 3‐Nitrenopyridine 1‐Oxides and Phenylnitrene
ABSTRACT The pursuit of sustainable organic synthesis has renewed interest in photochemistry, as sunlight‐driven reactions provide eco‐friendly alternative methods. Although the relationships among structure, properties, and reactivity are well established for ground‐state molecules, the understanding of excited states and reactive intermediates, such as triplet and singlet arylnitrenes, remains limited. Herein, we investigated the properties of triplet and singlet 4‐nitrenopyridine‐1‐pyridine oxide (1N), 3‐nitrenopyridine‐1‐pyridine oxide (2N), and phenylnitrene (PhN) using density functional theory (DFT), complete active space self‐consistent field (CASSCF(10,9)), and complete active space second‐order perturbation theory (CASPT2(10,9)) calculations. Bond length analysis demonstrated that31Nand11N, as well as12Nand1PhN, exhibit significant imine biradical character, whereas the structures of32Nand3PhNare better described as benzene‐like. Nucleus‐independent chemical shift (NICS(0), NICS(1.7)ZZ) and anisotropy of induced current density (ACID) calculations were performed to compare the induced magnetic currents in these molecules. These analyses demonstrated that31Nis weakly aromatic, whereas32Nand3PhNare best described as having Baird aromaticity. In contrast, singlet nitrenes11N,12N, and1PhNare nonaromatic. In addition, irradiation of1in argon matrices verified that31Nreacts photochemically to form corresponding ketenimine1K. Finally, the absorption difference spectrum of31Nin a frozen 2‐methyltetrahydrofuran (mTHF) matrix exhibited resolved vibrational structure, suggesting the vibrational coupling to another electronic state. These insights into the structure and aromaticity of heterocyclic nitrenes could provide new avenues for modulating the reactivity of triplet ground state and triplet excited molecules.
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- PAR ID:
- 10634540
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Journal of Physical Organic Chemistry
- Volume:
- 38
- Issue:
- 10
- ISSN:
- 0894-3230
- Subject(s) / Keyword(s):
- aromaticity, nitreniumpyridine 1-oxides, phenylnitrenes, mTHF matrices, argon matrices
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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