Iron complexes, cationic

The molybdenum complex 1, a typical high-valent Schrock-type carbene, efficiently catalyzes the self-metathesis of styrene. On the other hand, the cationic iron complex 3 does not induce metathesis but stoichiometrically cyclopropanates styrene. The tungsten complex 2, again a Fischer-type carbene complex, mediates... 

Alkene synthesis. A regio- and stereoselective alkene synthesis is formulated in equations (I)—(III). The first step involves formation of the alkyliron complex 1. Treatment of 1 with trityl tetrafluoroborate abstracts a -hydrogen to give a cationic iron complex 2. Liberation of the free alkene is effected by Nal in acetone. This sequence is useful because 1- and 2-alkyliron complexes (1) are converted into 1-alkcncs exclusively 3-alkyliron complexes are converted exclusively into the less stable (Z)-2-alkenes. The paper includes some 20 examples of alkenes prepared in this way. 

Although alkynes are highly reactive toward a wide range of transition metals, few instances of metal-catalyzed reactions of carbanions with alkynes are known. The most extensively developed system involves cationic iron complexes of internal alkynes. These complexes underwent alkylation by a range of carbanions to produce stable [Pg.582]

Based on the observation that 1,4-dimethoxycyclohexadiene cationic iron complex 310 undergoes regiospeciflc nucleophilic addition at C(l) and not at C(S) Stephenson et al [81] developed a new formal synthesis of ( )-lycoramine (300) (Scheme 47). Thus, the arylcarbanion 311, derived from the bromocompound 312 by halogen-metal exchange, reacted with 310 to provide the ipso addition product 313. Oxidative elimination of the C(l) methoxyl group generated the salt 314. 

To prepare the enantiomerically pure iron acyl complex (R)-(39), a precursor diastereomeric menthoxyaUcyl complex was resolved and then manipulated (Scheme 14). More recently resolution of the chiral-at-metal acyl complexes themselves was achieved, and this has become the basis for a commercial preparation of the iron acyl developed for use as a chiral auxiliary (see below). Cationic iron complex (43) was treated with potassium L-mentholate to produce diastereomeric esters (44) that were not isolated but were reacted with LiBr/MeLi (Scheme 15). After chromatography and recrystallization the enantiomerically pure ironacyl complex (5 )-(39a) was obtained. It was suggested that only one diastereomeric ester can react (with inversion of configuration at iron, as shown) with the methyl nucleophile the unreactive diastereomer suffers from severe steric congestion about the electrophilic CO ligand. 

Le Caer S, Heninger M, Lemaire J, Boissel P, Maitre P, Mestdagh H. (2004) Structural characterization of selectively prepared cationic iron complexes bearing monodentate and bidentate ether ligands using infrared photodissociation spectroscopy. Chem Phys Lett 273-279. 

The complexes are isolated, characterized and used as chiral Lewis acids. Dissociation of the labile ligand liberates a single coordination site at the metal center. These Lewis acids catalyze enantioselective Diels-Alder reactions. For instance, reaction of methacrolein with cyclopentadiene in the presence of the cationic iron complex (L = acrolein) occurs with exo selectivity and an enantiomeric excess of the same order of magnitude as those obtained with the successful boron and copper catalysts (eq 3). ... 

Reaction of the lithium diphospholide 37 with the cationic iron complex 38 resulted in the formation of the diphosphaferrocene 39 (Scheme 15) <1995AG623>. 

The first procedure involves ionization of a leaving group attached to Ccarbene (perhaps more accurately described as an electrophilic abstraction, Section 8-4-2). The second procedure occurs when an electrophile (usually H+) undergoes electrophilic addition (Section 8-4-2) to a nq vinyl complex. The cationic iron complexes produced are usually thermally unstable and may either react with a nucleophile or rearrange at low temperature to an alkene complex via a 1,2-H-shift (Scheme 10.5). 

The cationic iron complex 23 was prepared by several routes in 8-60% yield. ... 

Fig. 25. X-ray structure of 39b and proposed transition state assembly for cationic iron complex 39a...

Electrophilic carbene complexes of tungsten or iron smoothly transfer their phenylcarbene ligand to electron rich olefins. Pentacarbonyl(phcny]carbcnc)tungsten(0) (4) is generated in situ by acid treatment of the precursor salt 36, while the cationic iron complex 5 is a stable, isolable compound7-10. 

Li an alternative valence bond language, two resonance forms can be considered to describe metal-olefin complexes, as shown in Figure 1.22. An olefin bound to a mildly electron-rich or electron-poor metal center in which the a-donation dominates file bonding will correspond predominantly to resonance form A in Figure 1.22. One example of such a complex is the cationic iron complex in Figure 1.22. In contrast, olefin complexes of strongly backbonding metals, such as those with or d electron counts, will correspond... 

Hydrolysis of a cationic iron complex proceeds similarly [see equation (7.89)]. 

A similar cationic iron complex [76] with a bridging methyl was investigated by Casey et al. (1982). The averaged structure of the p-CHj group shows apparent symmetry in the H and C nmr spectra, but shift differences between p-CHj-[76] ( — 1.85 ppm) and p-CH2D-[76] ( — 2.62 to... 

These complexes can be made using many different transition metals and are often stable. Cationic iron complexes can be made from allylic alcohols or dienes via their t - or if -complexes by treatment with acid (Schemes 9.1 and 9.2). The non-coordinating Bp4 counter ion is often used. 

The regiochemistry of addition to conjugated dienes normally occurs at the ends of the extended -it system. One of the most well studied examples involves the T -dienyl cationic iron complexes shown in Eqs. 12.70 and 12.71. Addition to either end of the extended it system gives regioisomeric products, which are difficult compounds to obtain using other synthetic routes. 

Figure 24 shows the H and NMR spectra of complex 49. The methylene protons resonate at 5.35 ppm, the Cp resonance of the cationic iron complexes appear at 5.33 ppm, and the ferrocenyl resonances appear at 4.50 and 4.82ppm. The com-plexed aromatic protons appear as two sets of doublets at 6.49 and 6.79 ppm, while the uncomplexed aromatic protons appear as two doublets at 7.41 and 7.73 ppm. In the NMR spectrum, the methylene resonance appears at 65.56 ppm. The CH carbons of the ferrocenyl Cp rings appear at 72.19 and 73.65ppm, wMe the carbons ipso to the carbonyl appear at 73.53 ppm. The Cp carbons coordinated to the cationic iron center resonate at 80.25 ppm. The complexed aromatic (CH) carbons resonate at 76.97 and 87.56 ppm, while the quaternary complexed aromatic carbons appear at 104.74 and 133.37ppm. The uncomplexed aromatic carbons appear at 121.51 and 131.54ppm, and the quaternary aromatic carbons resonated at 136.09 and 153.52 ppm. 

The protected diene group can also be formed from a cationic iron complex by nucleophilic attack (see below for examples of applications of cationic iron complexes in synthesis). This principle has been extended to other metal complexes and functional groups and applications in prostaglandin, steriod, pyrethroid, peptide chemistry [27, 60, 64] and various pheromone syntheses. 

Different types of cationic iron complexes because of their high electro-philicity are now currently reported as highly efficient and selective reagents in organic synthesis. 

Iron carbonyl complexes containing 77 -alkyl-77 -allyl coordinated hydrocarbon ligands are obtained in several ways. Nucleophilic addition to cationic iron complexes containing 77 -pentadienyl ligands yields (pentenediyl)iron complexes. Oxidatively-induced reductive elimination of these complexes can be utilized as a means to generate 1,2,3-trisubstituted cyclopropanes.The reaction of cationic cycloheptadienyl complexes (Scheme 22) with appropriate nucleophiles also yields the alkyl-allyliron carbonyl complexes. Fe(CO)s also reacts with a- or /3-pincnc in refluxing dioxane (Scheme 22) to produce an alkyl-allyliron complex. Recently, 1,2- and 1,4-disubstituted [(pentadienyl)Fe(CO)3] cations were shown to react with carbon nucleophiles, such as sodium dimethylmalonate, to yield 77 77 -allyl complexes as products. 

Thus, DFT calculations complemented by statistical thermodynamic analysis showed that a realistic Fe/ZSM-5 catalyst should contain a small fraction of isolated Fe + species at specific positions inside the zeolite chaimels, while the predominant part of iron is present in the form of oxygenated cationic iron complexes. The questions about which of these different extraframework complexes is actually responsible for the specific catalytic properties of Fe/ZSM-5 and what the role of other species were still open. They were addressed to a large extent only when the mechanisms of different catalytic reactions over different potential intrazeolite iron sites were thoroughly investigated by DFT calculations [43,46]. The influence of the nature and structural properties of Fe sites on two important catalytic processes promoted by Fe/ ZSM-5, namely, the selective oxidation of benzene to phenol and the direct catalj4ic N O decomposition, was investigated [43,46]. 

It is possible to perform the paired-ion determination without using a water-immiscible solvent. The ion pair formed by an anionic surfactant with many cationic dyes, including methylene blue and rhodamine 6G, is water-insoluble and, with shaking, will adsorb to the walls of the vessel. After discarding the aqueous solution, the ion pair can be washed from the walls with ethanol or methyl cellosolve and measured spectrophotometri-cally. Best results are obtained with a poly(tetrafluoroethylene) vessel (52). In a similar vein, the ion pair can be filtered out, then dissolved and measured spectrophotometrically. This was demonstrated using a cationic iron complex with a nitrocellulose membrane filter which was soluble in the 2-methoxyethanol used for the spectrophotometric measurement (53). 

---