| Abstract
| - Upon reaction with osmium tetraoxide a series ofporphyrins and chlorins were converted into thecorresponding vic-dihydroxychlorins and bacteriochlorins.The presence of an electron-withdrawingsubstituent at a peripheral position on the porphyrins or chlorinsdeactivated that particular pyrroleunit toward oxidation, and also directed the oxidation regioselectivelyto the pyrrole ring oppositeto the one bearing the electronegative group. Thevic-dihydroxychlorins and bacteriochlorins wereconverted into the corresponding oxochlorins and dioxobacteriochlorinsunder pinacol−pinacolonereaction conditions. The migratory behavior of the varioussubstituents were found to be quitecomplex, since distant conjugated peripheral substituents were able toaffect the stability of thecarbocation intermediates during the process; the ability to rearrangewas affected not only by theintrinsic nature of the migratory group but also by steric andelectronic factors operative elsewhereon the porphyrin and chlorin macrocycles. Preferential migrationof the propionic ester over themethyl substituent in dioxobacteriochlorins obtained from2,3,12,13-tetrahydroxycoproporphyrinII tetramethyl ester (IUPAC nomenclature) under pinacol−pinacoloneconditions was confirmedby a single crystal X-ray study. The dioxobacteriochlorinsobtained from mesoporphyrin III dimethylester and coproporphyrin II tetramethyl ester were converted into thecorresponding dithio-analogues using Lawesson's reagent; this caused a red shift of 62 nm(compared with the dioxocompounds) affording long wavelength absorption at λmax746 nm.
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