Biomimetic iron complexes

Insights have also been provided by the first examples of biomimetic iron complexes that catalyze olefin cis-... 

One-step hydroxylation of aromatic nucleus with nitrous oxide (N2O) is among recently discovered organic reactions. A high eflSciency of FeZSM-5 zeolites in this reaction relates to a pronounced biomimetic-type activity of iron complexes stabilized in ZSM-5 matrix. N2O decomposition on these complexes produces particular atomic oj gen form (a-oxygen), whose chemistry is similar to that performed by the active oxygen of enzyme monooxygenases. Room temperature oxidation reactions of a-oxygen as well as the data on the kinetic isotope effect and Moessbauer spectroscopy show FeZSM-5 zeolite to be a successfiil biomimetic model. 

Results discussed above show in several lines a distinct biomimetic-type activity of iron complexes stabilized in the ZSM-S matrix. The most important feature is their unique ability to coordinate a very reactive a-oxygen form which is similar to the active oxygen species of MMO. At room temperature a-oxygen provides various oxidation reactions including selective hydroxylation of methane to methanol. Like in biological oxidation, the rate determining step of this reaction involves the cleavage of C-H bond. 

Clearly, while porphyrin complexes are obvious candidates for modelling these kinds of biomimetic oxidations, a range of non-heme iron complexes based on macrocyclic and podand ligand have also proved to be successful structural and functional mimics.19 To take one example, Figure 12.13 shows the X-ray structure of the iron (IV) tetramethylcyclam (tmc) oxo complex [Felv(tmc)(0)(MeCN)]2+... 

In the early 1970s it was discovered that P-450 cytochromes are irreversibly inhibited during the metabolism of xenobiotics (1). The formation of a modified heme prosthetic group is associated with enzyme inhibition and subsequent studies have identified these modified complexes as N-alkylated protoporphyrin-IX (2). The chemistry of N-sub-stituted porphyrins was comprehensively reviewed by Lavallee in 1987 (3). Since that time, there have been many significant contributions to this field by several groups. The goal of this chapter is to summarize some of this work as it relates to the mechanism of formation and reactivity of iron N-alkyl porphyrins. Biomimetic model complexes have played an important role in elucidating the chemistry of N-alkyl hemes in much the same way that synthetic iron tetraarylporphyrins have aided... 

P. Comba, G. Rajaraman, H. Rohwer, A density functional theory study of the reaction of the biomimetic iron(II) complex of a tetradentate bispidine ligand with H2O2, Inorg. Chem. 46 (2007) 3826. 

Biomimetic oxidation of 3,4-dimethoxybenzyl alcohol (veratryl alcohol), with HjOj in the presence of catalytic amounts of iron(III) porphyrin complex (ClgTAPS FeCl) and/or horseradish peroxidase (HRP) in [bmim][PEg], was studied by Kumar et al. (Scheme 14.6) [6]. Comparison of the obtained results showed better catalytic activity for iron complex. 

With regard to the biomimetic non-heme iron complexes, the work devoted to develop catalysts that perform catalytic alkane hydroxylation has resulted in a large number of iron complexes, which generate Fe =0 iron-oxo species characterized by different spectroscopic techniques. There is now direct evidence that the involvement of high-valent iron-oxo species leads to stereospecific alkane hydroxylation, while hydroxyl radicals contribute to non-selective oxidations. The impressive work performed by Que and co-workers has demonstrated that olefin epoxidation and cis-dihydroxylation are different facets of the reactivity of a common Fe -OOH intermediate, whose spin state can be modulated by the electronic and steric properties of... 

Lu TT, Chiou SJ et al (2006) Mononitrosyl tris(thiolate) iron complex [Fe(NO) (SPh)3] and dinitrosyl irrai ctnnplex [(EtS)2Fe(NO)2] formation pathway of dinitrosyl iron complexes (DNICs) frmn nibosylatirai of biomimetic rubredoxin [Fe(SR)4] (R = Ph, Et). Inorg Chem 45 8799-8806... 

The development of more sustainable methodologies is of particular interest to afford carbonyl compounds with industrial and biological interest. In this context, iron-based alcohol oxidations may be of great use. Since cytochrome P450 was presented as an active catalyst in the synthesis of carbonyl compounds,biomimetic synthetic complexes have also been used in this context, e.g. TPA-Fe =0. In the proposed mechanism, the oxidation of the alcohol takes place via a-CH hydrogen-atom abstraction, followed by an electron transfer to yield the corresponding carbonyl compound and an iron(ii) complex that could be reoxidised toward the active catalytic species, L-0=Fe(iv) (Scheme 13.27). 

As mentioned in the Introduction, the group 3 biomimetic approach (see Scheme 1) has been the most popular route to the trichothecene skeleton. Two different moieties have served as the electrophilic site for biomimetic cyclization. When the cyclization proceeds via an allylic carbonium ion (127) (Path A, Scheme 9), the desired trisubstituted olefin (126) is obtained directly. On the other hand, the Michael acceptor (128) (PathB) yields, upon cyclization, a ketone (129) which must then be transformed into the olefin (126), a process which shows good but not complete regioselectivity. Hence, Path A, which can also be entered from the enone (128), is the superior route. Further analysis of Scheme 9 reveals that the primary stereochemical challenge of the biomimetic approach is to control the relative stereochemistry at the two quaternary centers C-5 and C-6. Within the context of trichothecene synthesis, a number of useful protocols have been devised for this purpose and include photocyclization (99), selective ring contraction (134), Diels-Alder cycloaddition (117,125) conjugate addition (27,120), and interconversion of dienyl iron complexes (114). 

This methodology has been applied to a highly stereoselective biomimetic polyene cyclization using chiral tricarbonyl(ii -l-pentadienol)iron complexes to give rise to franj-decaline (Ti -diene)iron complexes. The transformation represents a potential route to bicyclic sesquiterpenes and tricyclic diterpenes (Scheme 4-191). ... 

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

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