Iron hydrides reduction

Keywords Catalysis Electrochemical reduction Hydroboration Hydrogenation Hydrosilylation Iron hydride complex Photochemical reduction... 

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

Knowledge of the active site allows for speculation on the mechanism of H2-D20 exchange which these Fe4 systems catalyze 473,483). Ruthe-nium(III) systems catalyze such an exchange via a ruthenium(III) hydride intermediate (7, p. 73 Section II,A), as exemplified in reactions (82) and (83), and iron hydrides must be involved in the hydrogenase systems. Ruthenium(III) also catalyzes the H2 reduction of ruthenium(IV) via reaction (82), followed by reaction (84) (3), and using these ruthenium systems as models, a very tentative scheme has been proposed 473) for... 

It is unlikely that the hydride addition to the cation proceeds via an isolable iron hydride, 77-CpFe(CO)2H, plus CH3CH=CH2, followed by readdition of the hydride to the olefin, because it was not possible to transfer the iron to either 1-hexene or butadiene during the reduction. 

The mechanism of the C—H and C—C bond activation of bare Fe+ with n-heptyltrimethylsilane has been elucidated with the help of extensive labeling studies71. The system was found to display a rather rich chemistry. Loss of neutral tetramethylsilane from the ion-molecule complex (equation 11) was explained by an initial insertion of the metal ion into the Cl— C2 bond to form 21, and a subsequent fi-H shift giving rise to the iron-hydride complex 22. This ion can then lose a tetramethylsilane molecule via reductive elimination. 

The first experiments which were carried out in the author s laboratory on organometallic phase-transfer catalysis were concerned with the reduction of nitrobenzenes (4) to anilines (5) by triiron dodecacarbonyl. Such a conversion was reported to occur in benzene containing methanol at reflux for 10-17 h, with the hydridoundecacarbonyltriferrate anion as the likely key intermediate (16). It was our expectation that the trinuclear iron hydride should be generated by phase-transfer catalysis and if so, effect reduction of nitro compounds (4) under exceedingly mild conditions. Indeed this was the case, as illustrated by the results shown in Table I (17). Not only is the reaction complete in 2 h or less using sodium hydroxide as the aqueous phase, benzene as the organic phase, and benzyltrieth-ylammonium chloride as the phase-transfer catalyst, but it occurs at room temperature and requires less metal carbonyl than when the reaction was... 

The highly reactive nature of this cluster together with the proposed formation of an iron-hydride intermediate during proton reduction (6) may make the search for an oxygen stable hydrogenase a rather elusive goal. 

Thus they treated the cisltrans mixture obtained by isomerization with iron pentacarbonyl and after decomposition of the complex with ferric chloride obtained the desired all-trans-tetraene ester (4) in 51% yield. The remaining steps in the synthesis involved lithium aluminum hydride reduction (80% yield) and Mn02 oxidation (52% yield). 

Another example is the conversion of alkyl bromides into aldehydes using Fe(CO)s 97). Fe(CO)5 is reduced by sodium amalgam and the result is a co-ordinatively unsaturated iron dianion. Oxidative addition of alkyl bromide affords the saturated alkyl iron anion. Alkyl-acyl rearrangement takes place by the addition of triphenylphosphine to give acyl iron anion. Protonation of the anion results in the intermediate acyl iron hydride which undergoes reductive elimination to yield aldehydes. 

Subsequent studies by Hayes and Weitz [64] reported that addition of ethylene to (CO)3FeH2 or H2 to (CO)3Fe(Ti -CH2=CH2) yielded singlet (CO)3Fe(Ti -CH2=CH2) H2 based on observation of a strong infrared band near 2051 cm . Olefin insertion into an iron hydride followed by C-H reductive elimination yields ethane. The... 

After protonation of the pendant amine, catalysis involves reduction of the diiron unit, which then sustains a second protonation to give an iron hydride, which couples, presumably intramolecularly, with the ammonium center to liberate H2. 

Unequivocal syntheses of cis- and rans-( decahydroquinoxalincs have been achieved by lithium aluminum hydride reduction of the corresponding cis- and trans-decahydroquinoxalin-2-ones. The latter compounds were prepared by condensation of chloroacetic acid and ds- and irons-1 -diaminocyclohexane, respectively. The resolution of irans-dUdecahydroquinoxaline was effected by use of first di-benzoyl-d-tartaric acid and then of dibenzoyl-i-tartaric acid. (C/. p- 215.)... 

Ketones are reduced to alcohols in the phase-transfer catalysed H-transfer reduction with isopropanol, or better PhCH2CH20H, in the presence of Fe3(C0)i2 Fe(C0)5 is far less active. Mono-, di- and trinuclear iron hydride carbonyl anions are generated in situ. a-Trimethylsilylketones can be prepared via Rh catalysed oxidations with butenones (eqn.9). Azobenzene is isomerised and reduced to o-phenylenediamines by a RuCl3/PPh3/C0/Li0Ac system in secondary alcohols. By contrast, n-butanol leads to formation of benzimidazoles (Scheme 3). 

Square-redox scheme for the CO-induced reductive elimination of hydride from an iron-hydride complex... 

The slow step in the iron carbonyl-catalyzed water gas shift reaction (Equation (3)) is the proton transfer from water to the iron hydride anion (Equation (7)). Ab initio calculations have been made on this process." It was found that in solution, the proton transfer was still endothermic (as in the gas phase) however, there was a reduction in the reaction energy of about SOkcalmoF. ... 

Anionic (77 -allyl)iron tricarbonyl complexes are easily prepared by the hydride reduction of (butadiene)iron tricarbonyl or (l-phenylbutadiene)iron tricarbonyl complexes with Li[BHEt3] in THF. Tricarbonyl(r7" -l,3-diene)-iron(O) complexes undergo addition reactions with reactive carbanions, such as LiCHPh2, to form anionic tricarbonyl(77 7 -but-3-en-l-yl)iron(0) complexes. 

A well-defined iron hydride complex FeH(CO)(NO)(Ph3P)2 is highly active as a catalyst for selective hydrosilylation of internal alkynes to vinylsilanes. Depending on the silane employed, either E- or Z-selective hydrosilylation products are formed in excellent yields and good to excellent stereoselectivities. The stereochemical course of this transformation is dependent on the steric demand of the substituents on the silane. " A new 0 family of Lewis-basic 2-pyridyloxazolines catalyses the enantioselective reduction of prochiral aromatic ketones and ketimines using trichlorosilane. 1-Isoquinolyloxazoline (20) derivative was identified as the most efficient catalyst of the series capable of delivering high enantioselectivities in the reduction of both ketones (up to 94% ee) and ketimines (up to 89% (p)... 

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