Selective photoreduction of nitrophenyl ethers by amines in ternary β-cyclodextrin complexes. A spectroscopic and mechanistic study

Abstract

The photoreduction of nitrophenyl ethers by amines has been studied in aqueous solution in the presence of β-cyclodextrin (β-CD), and in ternary β-cyclodextrin complexes in the solid state. NMR (NOESY) and induced circular dichroism (ICD) studies in solution suggest the operation of a cooperative effect by the amine in the complexation of the nitrophenyl ether by the β-CD, evidencing the existence of interactions between complementary groups that govern the structure of the ternary complexes. Irradiation of the solutions shows an inhibitory effect of the β-CD, together with a significant increase in the chimioselectivity of the photoreaction. This tendency is confirmed by irradiation of the solid ternary complexes that show a complete selectivity, only producing the nitroso derivative. A time-resolved diffuse reflectance laser flash photolysis (DRLFP) mechanistic study on the solid complexes has demonstrated that in the ternary β-CD complexes, the evolution of the first transient (assigned to the triplet excited state of the nitrophenyl ether) is completely different than in solution or in mechanical cellulose mixtures, this approach being a very valuable tool to investigate bimolecular photoinduced reactions difficult to study in solution due to the existence of other competitive processes.