Aryl Nitrene Rearrangements: Spectroscopic Observation of a Benzazirine and Its Ring Expansion to a Ketenimine by Heavy-Atom Tunneling

• Published in: Journal of the American Chemical Society Vol. 135; no. 28; pp. 10246 - 10249
• Main Authors: Inui, Hiroshi, Sawada, Kazuhiro, Oishi, Shigero, Ushida, Kiminori, McMahon, Robert J
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 17.07.2013; Amer Chemical Soc
• Subjects: Aziridines - chemistry; Chemistry; Chemistry, Multidisciplinary; Ketones - chemistry; Molecular Structure; Physical Sciences; Quantum Theory; Science & Technology; Spectrophotometry, Infrared; PHENYLNITRENE; PHOTOCHEMISTRY; MATRIX-ISOLATION
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja404172s

In the photodecompositions of 4-methoxyphenyl azide (1) and 4-methylthiophenyl azide (5) in argon matrixes at cryogenic temperatures, benzazirine intermediates were identified on the basis of IR spectra. As expected, the benzazirines photochemically rearranged to the corresponding ketenimines and triplet nitrenes. Interestingly, with the methylthio substituent, the rearrangement of benzazirine 8 to ketenimine 7 occurred at 1.49 × 10–5 s–1 even in the dark at 10 K, despite a computed activation barrier of 3.4 kcal mol–1. Because this rate is 1057 times higher than that calculated for passing over the barrier and because it shows no temperature dependence, the rearrangement mechanism is interpreted in terms of heavy-atom tunneling.