Dendritic iron porphyrins with a tethered axial ligand as new model compounds for heme monooxygenases

The novel iron(III) porphyrin dendrimers of generation zero ([1 . Fe-III]Cl), one ([2 . Fe-III]Cl), and two ([3 . Fe-III]Cl) (Fig. 1) were prepared (Schemes 1 and 3) as models of heme monooxygenases. They feature Controlled axial ligation at the Fe center by one imidazole tethered to the porphyrin core and possess a vacant coordination site available for ligand binding and catalysis. The high purity of the dendrimers and the absence of structural defects was demonstrated by matrix-assisted laser-desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry (Fig. 3). The electronic properties of the Fe-III porphyrin dendrimers and comparison compounds [4 . Fe-III]Cl and [12 . Fe-III(1,2-Me(2)Im)]Cl (1,2-Me(2)Im = 1,2-dimethylimidazole) were investigated by UV/VlS and EPR (electronic paramagnetic resonance) spectroscopy, as well as by measurements of the magnetic moments by the Evans-Scheffold method. Epoxidation of olefins and oxidation of sulfides to sulfoxides, catalyzed by the new dendritic metalloporphyrins, were investigated in CH2Cl2 with iodosylbenzene as the oxidant (Tables 1 and 2). The total turnover numbers were found to increase with the size of the dendrimer, due to improved catalyst stability at higher dendritic generations (Figs. 4 and 5). The second-generation complex [3 . Fe-III]Cl was, therefore, the most efficient catalyst in the series, despite the fact that its active site is considerably hindered by the encapsulation inside the sterically demanding, fluctuating dendritic wedges, Very high product selectivities were observed in all oxidation reactions, regardless of dendrimer generation.