Selective photoreduction of nitrophenyl ethers by amines in ternary β-cyclodextrin complexes. A spectroscopic and mechanistic studyDedicated to Professor Frank Wilkinson on the occasion of his retirement.

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.