Photochemical reactions of 1,10-phenanthroline N-oxide in solutions and mechanistic studies by DFT calculations

Abstract

From experiments and DFT calculations about photolysis of 1,10-phenanthroline N-oxide, it is indicated that reaction mechanism of the product in photochemical secondary process in aprotic solvent is different from that in protic solvent. • Isolation and identification of the reaction products in protic and aprotic solvents. • Measurement of quantum yields for the formation of products. • Deduced reactive state: singlet exited nπ* state. • Primary photochemical process: the formation of oxaziridine. • Study on the mechanism of secondary photochemical process using DFT calculation. Photolysis of 1,10-phenanthroline N-oxide in aprotic solvents, benzene and acetonitrile, results in the formation of 2-hydroxy-2,3-dihydro-1 H -pyrrolo[3,2- h ]quinoline-1-carboxaldehyde ( A ) and 1 H -pyrrolo[3,2- h ]quinoline-1-carboxaldehyde ( B ), while that in protic solvents, ethanol and water, gives 1,10-phenanthroline-2(1 H )-one ( C ). Three major intermediates are responsible for product formation: oxazepine, oxirane, and oxaziridine. DFT calculations at the B3LYP/6-31+G(d) level suggest that: (1) in aprotic solvents, A and B are produced from the seven-membered oxazepine, [1,3]oxazepino[5,4- h ]quinoline, formed via a three-membered oxirane, 7a,8a-dihydrooxireno[2,3- b ][1,10]phenanthroline, and (2) in protic solvents, C is produced from a three-membered oxaziridine, 1a H -[1,2]oxazireno[2,3- a ][1,10]phenanthroline. The three-membered ring is opened by the addition of a protic solvent molecule, followed by the removal of the solvent molecule to give C .