title: Kinetic analysis of demetalation of synthetic zinc cyclic tetrapyrroles possessing an acetyl group at the 3-position: effects of tetrapyrrole structures and peripheral substitution.
abstract: Demetalation of three synthetic zinc cyclic tetrapyrroles that possess identical peripheral substituents, zinc methyl bacteriopyropheophorbide a (zinc bacteriochlorin 1), zinc methyl 3-devinyl-3-acetyl-pyropheophorbide a (zinc chlorin 2), and zinc methyl 3-devinyl-3-acetyl-protopyropheophorbide a (zinc porphyrin 3), was kinetically analyzed under acidic conditions to examine the effects of macrocyclic structures on demetalation without peripheral substitution effects. Zinc bacteriochlorin 1 exhibited much slower demetalation kinetics than zinc chlorin 2 and zinc porphyrin 3. These results indicate that the bacteriochlorin skeleton provides significant resistance to the removal of the central metal from the tetrapyrrole ligand. Comparison of demetalation kinetics of 3-acetyl zinc complexes 2 and 3 with that of 3-vinyl zinc complexes under the same reaction condition demonstrated that the relative ratio (5.0 × 10(-2)) of the demetalation rate constant of the 3-acetyl zinc chlorin 2 to that of the corresponding 3-vinyl zinc chlorin 4 resembled the case of the 3-acetyl zinc porphyrin 3 to the 3-vinyl zinc porphyrin 5 (the relative ratio was 6.8 × 10(-2)). These suggest that the electron-withdrawing 3-acetyl group slows down the demetalation from the tetrapyrrole ligands more than the 3-vinyl group and that the 3-acetyl effect is analogous in both chlorin and porphyrin π-systems.
authors.affiliation: Department of Chemistry, Faculty of Science and Engineering, Kinki University, Higashi-Osaka, Osaka 577-8502, Japan. saga@chem.kindai.ac.jp
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Saga
authors.forename: Ryosuke
authors.initials: R
authors.lastname: Miura
authors.forename: Kana
authors.initials: K
authors.lastname: Sadaoka
authors.forename: Yuki
authors.initials: Y
authors.lastname: Hirai
mesh_terms: D002734:Chlorophyll / Q000031:analogs & derivatives / Q000737:chemistry; D007700:Kinetics; D011166:Porphyrins / Q000737:chemistry; D045725:Tetrapyrroles / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry*
chemical_list: D011166:Porphyrins; D045725:Tetrapyrroles; C452433:bacteriochlorin; D002734:Chlorophyll; C040298:pyropheophorbide a; C006969:chlorin; D015032:Zinc