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 that ³1N and ¹1N, as well as ¹2N and ¹PhN, exhibit significant imine biradical character, whereas the structures of ³2N and ³PhN are 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 that ³1N is weakly aromatic, whereas ³2N and ³PhN are best described as having Baird aromaticity. In contrast, singlet nitrenes ¹1N, ¹2N, and ¹PhN are nonaromatic. In addition, irradiation of 1 in argon matrices verified that ³1N reacts photochemically to form corresponding ketenimine 1K. Finally, the absorption difference spectrum of ³1N in 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.