CATALYTIC CYCLOPHANES .8. CYTOCHROME-P-450 ACTIVITY OF A PORPHYRIN-BRIDGED CYCLOPHANE

Following a known synthetic procedure, the porphyrin-cyclophane 1 having a porphyrin attached by two straps to an apolar cyclophane binding site was prepared. Upon metallation, the Zn(II) and Fe(III) derivatives 2 and 3, respectively, were obtained in good yields. Treatment of 3 with base yielded the mu-oxo dimer 4 in which the two oxo-bridged porphyrins moieties are both capped by cyclophane binding sites. All compounds 1-4 are freely soluble in protic solvents such as MeOH and CF3CH2OH, and the Fe(III) derivatives 3 and 4 are active cytochrome P-450 mimics in these protic environments. Strong inclusion complexation of polycyclic aromatic hydrocarbons by 1 and 3 in alcoholic solvents was observed and quantified by H-1-NMR and UV/VIS titrations. Acenaphthylene binds in an 'equatorial' orientation which locates its reactive 1,2-double bond near the porphyrin center, whereas phenanthrene binds 'axially' with the reactive 9, 1 0-double bond oriented away from the porphyrin. The reduction potential of 3 was not significantly altered by substrate binding. In the unbound form, the Fe(III) center in porphyrin 3 was found by ESR and H-1-NMR to prefer a high-spin state (S = 5/2). In CF3CH2OH, using iodosylbenzene as 0-transfer agent, the Fe(III) derivative 3 catalyzed the oxidation of acenaphthylene to acenaphthen-1-one (14). Phenanthrene inhibited the reaction, possibly as a result of strong but nonproductive binding. Under similar conditions, isotetralin (18) was aromatized with high turnover to 1,4-dihydronaphthalene. The mu-oxo dimer 4 also showed high catalytic activity in the oxidation of acenaphthylene in MeOH, a result which provides strong evidence for efficient supramolecular catalysis. Due to as yet unknown reaction channels leading to polymeric products, poor mass balances were generally obtained in the oxidations effected in MeOH and CF3CH2OH in the presence of PhIO.