| Abstract
| - To investigate the effects of electron-donating and electron-withdrawing substituents upon thereaction of porphyrins with osmium tetraoxide, and the pinacol−pinacolone rearrangement of theresulting diols, a series of meso-substituted porphyrins were prepared by total synthesis. Porphyrinswith electron-donating substitutents at the meso-positions gave vic-dihydroxychlorins in which theadjacent pyrrole subunit was predominantly oxidized. No such selectivity was observed in aporphyrin containing a methoxycarbonyl as the electron-withdrawing group, whereas a formylsubstituent again resulted in oxidation at the pyrrole unit adjacent to the meso-substituent. Underpinacol−pinacolone conditions, vic-dihydroxy chlorins containing 4-methoxyphenyl or 3,5-dimethoxyphenyl groups at the meso-position showed preferential migration of the ethyl group over themethyl group to give 8-ketochlorins, whereas the diol with an n-heptyl substituent under similarreaction conditions gave both 7- and 8-ketochlorins. In contrast, the diol containing a meso-formylsubstituent produced the corresponding 7-ketochlorin exclusively. These results indicate that it isnot possible to predict the reactivity of meso-substituted porphyrins in the osmium tetraoxidereaction nor the general substituent migratory aptitudes in the pinacol−pinacolone rearrangementbased on simple electronic arguments, most likely because many parameters (e.g., meso-β-pyrrolicsteric crowding and long-range electronic effects) ultimately determine the reactivity. The structuralassignments of the porphyrin diols and the keto-analogues were confirmed by extensive 1H NMRstudies; some of the dihydroxychlorins and ketochlorins were found to display unusual features intheir 1H NMR spectra.
|
