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

The detection of an oxygen-atom photoexchange process of <i>N-</i>nitrosamines is reported. The photolysis of four nitrosamines (<i>N</i>-nitrosodiphenylamine <b>1</b>, <i>N</i>-nitroso-<i>N</i>-methylaniline <b>2</b>, <i>N</i>-butyl-<i>N</i>-(4-hydroxy­butyl)­nitrosamine <b>3</b>, and <i>N</i>-nitroso­diethylamine <b>4</b>) 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 <b>1</b> and <b>2</b>. In contrast, nitrosamines <b>3</b> and <b>4</b> do not exchange the ¹⁸O label and instead decomposed to amines and/or imines under the conditions. For <b>1</b> and <b>2</b>, the ¹⁸O atom was found not to be introduced by moisture or by singlet oxygen [¹⁸(¹O₂ ¹Δ_g)] produced thermally by ¹⁸O–¹⁸O labeled endoperoxide of <i>N,N</i>′-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 <i>O-</i>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 <i>O-</i>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

Experimental and DFT Computational Insight into Nitrosamine PhotochemistryOxygen Matters

A nitrosamine photooxidation reaction is shown to generate a peroxy intermediate by experimental physical-organic methods. The irradiation of phenyl and methyl-substituted nitrosamines in the presence of isotopically labeled 18-oxygen revealed that an O atom was trapped from a peroxy intermediate to trimethylphosphite or triphenylphosphine, or by nitrosamine itself, forming two moles of nitramine. The unstable peroxy intermediate can be trapped at low temperature in postphotolyzed solution in the dark. Chemiluminescence was also observed upon thermal decomposition of the peroxy intermediate, that is, when a postphotolysis low-temperature solution is brought up to room temperature. A DFT study provides tentative information for cyclic nitrogen peroxide species on the reaction surface