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

Abstract

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