Biomimetical catalytic activity of iron(III) porphyrins encapsulated in the zeolite X

The approach adopted for the obtention of zeolite-encapsulated FeP led to clean syntheses of biomimetical catalyst. The catalysts were obtained through the zeolite synthesis method, where NaX zeolite was synthesised around one of the cationic FePs: iron(III) 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (FeP1) or iron(III) 5-mono(2, 6-dichloro-phenyl)10,15,20-tris(4-N-methylpyridyl)porphyrin (FeP2). The syntheses yielded pure FeP1NaX and FeP2NaX catalysts without any by-products blocking the zeolite nanopores. FeP1NaX and FeP2NaX efficiently catalysed the epoxidation of (Z)-cyclooctene by iodosylbenzene (PhIO) in DCE, giving rise to cis-epoxycyclooctane yields of 85% and 95%, respectively. Hydroxylation of adamantane shows a preferable alkane oxidation at the tertiary C-H bond, indicating a hydrogen abstraction through the Fe-IV(O)P (.+) species in the initial step. The total adamantanol yields were 52% and 45% for FeP1NaX and FeP2NaX, respectively. Concerning selectivity, FeP1NaX and FeP2NaX gave an 1-adamantanol (Ad-1-ol)/2-adamantanol (Ad-2-ol) ratio of 20:1 and 11:1, respectively (after statistical correction). Therefore, these results indicate a free radical activation of the C-H bonds of adamantane as expected for P-450 models. In the cyclohexane oxidation catalysed by FeP1NaX in DCE, a cyclohexanol (C-6-ol) yield of 50% and an alcohol/ketone ratio of 10 was obtained. The hydroxylation occurs according to the so-called oxygen rebound mechanism, as expected for a P-450 model system. FeP2NaX is less selective (C-6-ol yield = 25% and alcohol/ketone = 1.2). One possible explanation is that a Russell-type mechanism involving O-2 imprisoned within the zeolite cages may be operating parallelly, generating both C-6-ol and cyclohexanone. (C) 2000 Elsevier Science B.V. All rights reserved.