Catalytic Cyclophanes. Part VIII. Cytochrome P‐450 activity of a porphyrin‐bridged cyclophane

Abstract

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 ZnII and FeIII derivatives 2 and 3, respectively, were obtained in good yields. Treatment of 3 with base yielded the μ‐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 FeIII 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 1H‐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 9,10‐double bond oriented away from the porphyrin. The reduction potential of 3 was not significantly altered by substrate binding. In the unbound form, the FeIII center in porphyrin 3 was found by ESR and 1H‐NMR to prefer a high‐spin state (S = 5.2). In CF3CH2OH, using iodosylbenzene as O‐transfer agent, the FeIII 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 μ‐oxo dimer 4 also showed high activity in the oxidation of acenaphthylen in MeOH, a result which provides strong evidence for efficent 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.

UV/Vis Spectra of Cyclophanes

“…As acceptors different groups such as quinones [109][110][111][112][113][114][115][116], 7,7,8,8-tetracyanoquinodimethane (TCNQ) [117], pyromellitic diimide [118], and others [119,120] were utilized. As acceptors different groups such as quinones [109][110][111][112][113][114][115][116], 7,7,8,8-tetracyanoquinodimethane (TCNQ) [117], pyromellitic diimide [118], and others [119,120] were utilized.…”

Supramolekulare Elektrochemie

“…[96] Etwas später wurden Eisen-Schwefel-Cluster in einem Cyclophankäfig [97] und ein diamagnetischer Di-Eisen-[2.2]Paracyclophan-Komplex [98] untersucht. Metallkomplexe von Cyclophanen auf Porphyrinbasis sind gegenwärtig ein aktuelles Thema, das durch Arbeiten von Staab et al [99] sowie Diederich et al [100] sprechenden Radikalkation, während das zweite Reduktionspotential bei À 0.703 V der anschlieûenden Reduktion (zwei Elektronen) zu den neutralen Viologenen zuzuordnen ist. Das Voltammogramm läût deutlich erkennen, daû zwischen den Viologeneinheiten keine elektronische Wechselwirkung besteht.…”

Electrochemistry of Supramolecular Systems

“…[96] More recently, iron ± sulfur clusters in a cyclophane cage [97] and a diamagnetic diiron ± [2.2]paracyclophane complex [98] have been studied. Metal complexes of porhyrin-based cyclophanes are currently a topic of interest, studied by Staab, and co-workers [99] and Diederich et al [100] In the area of metal complexes of cyclophanes, an elegant example studied by electrochemistry was recently provided by Sauvage and co-workers: linkage isomerism in a catenane system 18 that contains an interlocking diphenylphenanthroline (dpp) and terpyridine (terpy) cyclophane and a dpp crown ether as metal binding ligands (Scheme 12). [101] As in Scheme 12.…”