Photolysis of α-Azidoacetophenones:  Direct Detection of Triplet Alkyl Nitrenes in Solution

• Published in: Journal of organic chemistry Vol. 68; no. 21; pp. 7951 - 7960
• Main Authors: Singh, Pradeep N. D, Mandel, Sarah M, Robinson, Rachel M, Zhu, Zhendong, Franz, Roberto, Ault, Bruce S, Gudmundsdóttir, Anna D
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 17-10-2003
• Subjects: Chemistry; Exact sciences and technology; Free radicals chemistry; Organic chemistry; Reactivity and mechanisms; Chemical solution; Nitrogen 15; Isotope labelling; Molecular structure; Organic azide; Triplet; Nitrene; Photochemical reaction; Ultraviolet spectrometry; Characterization; Infrared spectrometry; Organic free radical; Acetophenone derivatives; Density functional method; Argon; Flash photolysis; Laser photolysis; Energy transfer; Carbon 13; Sensitizer; Room temperature
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo034674e

We report the first detection of triplet alkyl nitrenes in fluid solution by laser flash photolysis of α-azido acetophenone derivatives, 1. Αzides 1 contain an intramolecular triplet sensitizer, which ensures formation of the triplet alkyl nitrene by bypassing the singlet nitrene intermediate. At room temperature, azides 1 cleave to form benzoyl and methyl azide radicals in competition with triplet energy transfer to form triplet alkyl nitrene. The major photoproduct 3 arises from interception of the triplet alkyl nitrene with benzoyl radicals. The triplet alkyl nitrene intermediates are also trapped with molecular oxygen to yield the corresponding 2-nitrophenylethanone. Laser flash photolysis of 1 reveals that the triplet alkyl nitrenes have absorption around 300 nm. The triplet alkyl nitrenes were further characterized by obtaining their UV and IR spectra in argon matrices. 13C and 15N isotope labeling studies allowed us to characterize the C−N stretch of the nitrene intermediate at 1201 cm-1.