Methane monooxygenase catalyst

Itoh et al. used Cu yd-diketiminato complexes with general formula 4, and their reactivity has been described as a functional model for pMMO (particulate methane monooxygenase). Initially, the Hgands were reacted with both Cu and Cu precursors, with a variety of species formed, depending on the specific conditions employed [111, 112]. It was then shown that both Cu and Cu complexes ultimately led to bis(/z-oxo)(Cu )2 species upon reaction with O2 and H2O2, respectively. Use of these Cu complexes as the pre-catalysts for the oxidation of alkanes (cyclohexane and adamantane) in the presence of H2O2 resulted in low yields ( 20%). 

Since H202 is easier to handle than 02, we will focus on the use of the former. Many metals can be used for this transformation [50]. Among them, iron compounds are of interest as mimics of naturally occurring non-heme catalysts such as methane monooxygenase (MMO) [51a] or the non-heme anti-tumor drug bleomycin [51b]. Epoxidation catalysts should meet several requirements in order to be suitable for this transformation [50]. Most importantly they must activate the oxidant without formation of radicals as this would lead to Fenton-type chemistry and catalyst decomposition. Instead, heterolytic cleavage of the 0—0 bond is desired. In some cases, alkene oxidation furnishes not only epoxides but also diols. The latter transformation will be the topic of the following section. 

Some preparations of iron exchanged into zeolite H-MFI by vapor-phase FeCL are known to be active and selective catalysts for the reduction of NO, with hydrocarbons or ammonia in the presence of excess oxygen and water vapor (45,46). The active centers in Fe/MFI are assumed to be binuclear, oxygen-bridged iron complexes, as follows from H2-TPR, CO-TPR, and ESR data (45,47) and EXAFS and XANES results (48,49). These complexes are structurally similar to the binuclear iron centers in methane monooxygenase enzymes that are employed by methanotrophic bacteria in utilization of methane as their primary energy source (50). It is believed that molecular oxygen reacts with these centers to form peroxide as the initial step in this chemistry (50). 

Various systems oxidizing hydrocarbons and containing iron ions have been described which can be considered as models of non-heme mono- and dioxygenases [87] (see also Chapter X). Mononuclear iron derivatives have been used as catalysts in oxidations modeling the action of methane monooxygenase [88]. For example, a mononuclear iron carboxylate complex immobilized on a modified silica surface catalyzes oxidation of hexane in the presence of mercap-... 

Steric effect is very crudal in the selectivity of ds-diol [79]. While the (6-Me2-BPMEN)Fe(OTf)2 produces ds-l,2-cylcooctanediol as the major product (Table 1.5, entry 4), the 5-methyl analog performs as an epoxidation catalyst (Table 1.5, entry 5). In the presence of acetic add, the nonsubstituted (B PM EN) Fe(SbF6)2, also referred to as (MEP)Fe(SbF6)2, self-assembled to a dimer as a stmdural mimic of methane monooxygenase (MMO) [80]. It catalyzes the epoxidation of a range of aliphatic alkenes. Even the relatively nonreactive substrate, 1-decene, can be oxidized to the corresponding epoxide in 85% yield in 5 min. 

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