Mechanism of Photochemical O‑Atom Exchange in Nitrosamines with Molecular Oxygen

• Published in: Journal of organic chemistry 2015-06, Vol.80 (12), p.6119-6127
• Main Authors: Oliveira, Marilene Silva, Ghogare, Ashwini A, Abramova, Inna, Greer, Edyta M, Prado, Fernanda Manso, Di Mascio, Paolo, Greer, Alexander
• Format: Article
• Language: English
• Subjects: Amines - chemistry; Molecular Structure; Nitrosamines - chemistry; Photochemical Processes; Singlet Oxygen - chemistry; Tandem Mass Spectrometry; Ultraviolet Rays
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/acs.joc.5b00633

The detection of an oxygen-atom photoexchange process of N-nitrosamines is reported. The photolysis of four nitrosamines (N-nitrosodiphenylamine 1, N-nitroso-N-methylaniline 2, N-butyl-N-(4-hydroxy­butyl)­nitrosamine 3, and N-nitroso­diethylamine 4) with ultraviolet light was examined in an 18O2-enriched atmosphere in solution. HPLC/MS and HPLC-MS/MS data show that 18O-labeled nitrosamines were generated for 1 and 2. In contrast, nitrosamines 3 and 4 do not exchange the 18O label and instead decomposed to amines and/or imines under the conditions. For 1 and 2, the 18O atom was found not to be introduced by moisture or by singlet oxygen [18(1O2 1Δg)] produced thermally by 18O–18O labeled endoperoxide of N,N′-di­(2,3-hydroxy­propyl)-1,4-naphthalene dipropanamide (DHPN18O2) or by visible-light sensitization. A density functional theory study of the structures and energetics of peroxy intermediates arising from reaction of nitrosamines with O2 is also presented. A reversible head-to-tail dimerization of the O-nitrooxide to the 1,2,3,5,6,7-hexaoxa­diazocane (30 kcal/mol barrier) with extrusion of O18O accounts for exchange of the oxygen atom label. The unimolecular cyclization of O-nitrooxide to 1,2,3,4-trioxazetidine (46 kcal/mol barrier) followed by a retro [2 + 2] reaction is an alternative, but higher energy process. Both pathways would require the photoexcitation of the nitrooxide.