Pinacol-Pinacolone Rearrangements in vic-Dihydroxychlorins and Bacteriochlorins: Effect of Substituents at the Peripheral Positions

Upon reaction with osmium tetraoxide a series of porphyrins and chlorins were converted into the corresponding vic-dihydroxychlorins and bacteriochlorins. The presence of an electron-withdrawing substituent at a peripheral position on the porphyrins or chlorins deactivated that particular pyrrole unit toward oxidation, and also directed the oxidation regioselectively to the pyrrole ring opposite to the one bearing the electronegative group. The vic-dihydroxychlorins and bacteriochlorins were converted into the corresponding oxochlorins and dioxobacteriochlorins under pinacol-pinacolone reaction conditions. The migratory behavior of the various substituents were found to be quite complex, since distant conjugated peripheral substituents were able to affect the stability of the carbocation intermediates during the process; the ability to rearrange was affected not only by the intrinsic nature of the migratory group but also by steric and electronic factors operative elsewhere on the porphyrin and chlorin macrocycles. Preferential migration of the propionic ester over the methyl substituent in dioxobacteriochlorins obtained from 2,3,12,13-tetrahydroxycoproporphyrin II tetramethyl ester (IUPAC nomenclature) under pinacol-pinacolone conditions was confirmed by a single crystal X-ray study. The dioxobacteriochlorins obtained from mesoporphyrin III dimethyl ester and coproporphyrin II tetramethyl ester were converted into the corresponding dithio-analogues using Lawesson's reagent; this caused a red shift of 62 nm (compared with the dioxo compounds) affording long wavelength absorption at λ_max 746 nm.