1,3-Dipolar cycloadditions with meso-tetraarylchlorins - site selectivity and mixed bisadducts

The 1,3-dipolar cycloaddition reaction of meso-tetraarylporphyrins with nitrones gives isoxazolidine-fused chlorins. Depending on the substitution pattern on the meso-aryl groups and the nitrone, the chlorins can be obtained in high yields (up to 91%). Bacteriochlorin-type bisadducts are also obtained, although in low yield, from the reaction of meso-tetrakis(pentafluorophenyl) porphyrin with N-methyl, N-cyclohexyl or N-benzyl nitrone. The structure of a bis(N-benzyl isoxazolidine-fused) bacteriochlorin was determined by single-crystal X-ray diffraction and rationalized by DFT calculations. To further explore the nature of site selectivity in the formation of bisadducts, isomeric mixed bacteriochlorins and iso-bacteriochlorins were synthesized by two complementary routes that involved the sequential addition of two types of 1,3-dipoles to the porphyrin macrocycle: a nitrone and an azomethine ylide. The photo-physical properties of the mixed bacteriochlorins were evaluated and the results were compared with the reported data for other meso-tetraarylbacteriochlorins.