Dienes iron complexes

Diene)tricarbonyliron complexes are able to react with a variety of nucleophiles, mediated by a Lewis acid. The Darzens-Nenitzescu reaction was utilized to prepare i7" -dienetricarbonyliron complexes containing carbonyl groups. Lewis acid-catalyzed nucleophilic additions have been reported to occur at carbonyl functionalities adjacent to an iron-complexed diene. In a separate study, facile racemization or epimerization was observed... 

Iron-complexed diene systems have a reduced electron density due to 7t-donation to the iron center. This makes them less reactive towards electrophilic attack that stands for the majority of reactions at olefmic systems. But also oxidation, reduction, and cycloaddition reactions proceed more slowly or can be completely suppressed when the diene is ligated to an iron center. Moreover, the iron complex blocks one face of the diene system. Incoming reagents, whether at the diene unit or at the periphery, are directed anti to the iron complex fragment. This allows stereospecific reactions that are otherwise difficult to achieve. The stereodirecting effect can be exploited for reactions... 

HWE reaction of aldehydes adjacent to tricarbonyliron complexed dienes gives rise to trienes having a free olefin and an iron-complexed diene. 1,3-Migrations of the tricarbonyliron fragments in such acyclic (polyene)iron complexes bearing an electron-withdrawing group at the uncomplexed olefin unit can be induced by treatment with base. The thermodynamically stable product is formed with the tricarbonyliron moiety next to... 

Chiral diene—iron tricarbonyl complexes were acylated using aluminum chloride to give acylated diene—iron complexes with high enantiomeric purity (>96% ee). For example, /ra/ j -piperjdene—iron tricarbonyl reacted with acyl haUdes under Friedel-Crafts conditions to give l-acyl-l,3-pentadiene—iron tricarbonyl complex without any racemization. These complexes can be converted to a variety of enantiomericaHy pure tertiary alcohols (180). 

ALKYLATION OF DIMEDONE WITH A TRICARBONYL(DIENE)IRON COMPLEX TRlCARBONYL[2-[(2,3,4,5-t))-4-METHOXY 2,4 CYCLOHEXADUEN-l-YLJ-5,5-DIMETHYL-l,3-CYCLOHEXANEDIONE]IRON... 

Syntheses of heterocycles, among them carbazole alkaloids, with participation of tricarbonyl(Ti -diene)iron complexes 99CSR151. 

Scheme 26 1,4-Hydroboration of 1,3-diene derivatives with pinacolborane catalyzed by an iron complex...

Diastereoselective intermolecular nitrile oxide—olefin cycloaddition has been used in an enantioselective synthesis of the C(7)-C(24) segment 433 of the 24-membered natural lactone, macrolactin A 434 (471, 472). Two (carbonyl)iron moieties are instrumental for the stereoselective preparation of the C(8)-C(ii) E,Z-diene and the C(i5) and C(24) sp3 stereocenters. Also it is important to note that the (carbonyl)iron complexation serves to protect the C(8)-C(ii) and C(i6)-C(i9) diene groups during the reductive hydrolysis of an isoxazoline ring. 

However, upon thermolysis of 178.d or 178.h, isomeric mixtures of the butadiene complexes 183.a and 183.b were formed. Since intramolecular hydrogen transfer within (3,3-dime thyl-773 r -allylacy iron complexes is well precedented101 (see Section VI,B), it seems likely that this process is responsible for diene complex formation. Note that only the Z-diene complex was isolated from the reaction mixture, a surprisingly stereoselective result. 

An alternative method for the oxidative cyclization of the arylamine-substituted tricarbonyl(r -cyclohexa-l,3-diene)iron complex (725) is the iron-mediated arylamine cyclization. Using ferricenium hexafluorophosphate in the presence of sodium carbonate provided hyellazole (245) directly, along with the complex 727, which was also converted to the natural product (599,600) (Scheme 5.71). 

A recently described method for insertion of a carbon monoxide molecule into the monoepoxide of a conjugated diene gives /3-lactones in high yield. This is achieved by reaction of iron pentacarbonyl with the starting vinyloxirane to give the 7r-allyl iron complex (66), which on oxidation with cerium(IV) ammonium nitrate gives the /3-lactone. In some cases, y-Iactone products can also be obtained from this reaction (8lJCS(Pi)270). 

Tricarbonyl)iron complexes of 1,2-diazepines do not show the rapid isomerization found in their azepine counterparts (Scheme 41) the iron forms a diene complex with the C = C double bonds in the 4- and 6-positions. The chemistry of the 1,2-diazepine complexes is similar to that of the azepine complexes (CHEC 5.18.2.1) (81ACR348). 

Dicarbonyltris(phosphane)iron complexes have been known for more than 30 years. In 1960, Manuel and Stone prepared the complex Fe(CO)2[P(C6H5)3]3 in low yields (4-13%) by the reaction of a (diene)- or (triene)tricarbonyliron complex with an excess of triphenylphosphine in refluxing ethylcyclohexane.1... 

A related >/4-norcaradiene tricarbonyliron complex is obtained upon reaction of tricy-clo[4.3.1.0l6]deca-2,4-diene with Fe3(CO)12 in boiling benzene (equation 143). However, the [4.3.1]propellane ring system is not retained in the analogous tricarbonylchromium complex. Instead, as suggested from solution NMR and solid state X-ray analyses, the complex assumes a homoaromatic structure, which is intermediate between a norcaradi-ene and a cycloheptatriene system (equation 144)193,194. It is noteworthy that the Cr(CO)3 group prefers the same conformation as the Fe(CO)3 group in the analogous norcaradiene iron complex. 

The iron complexes 35 (78) and 36(79) have been reported. Crystallographic studies on 35 indicate that the (> 4-diene)metal interaction is best represented by the valence bond description shown, in which the perfluorodiene acts as a... 

IRON COMPLEXES OF OFCOT AND PERFLUOROTRICYCLO[4.2.0.02s]OCTA-3,7-DIENE... 

Dienes form very stable complexes with a variety of metal caibonyls, particularly Fe(CO)s, and the neutral V-diene metal carbonyl complexes are quite resistant to normal reactions of dienes (e.g. hydrogenation, Diels-Alder). However, they are subject to nucleophilic attack by a variety of nonstabilized carbanions. Treatment of -cyclohexadiene iron tricarbonyl with nonstabilized carbanions, followed by protonolysis of the resulting complex, produced isomeric mixtures of alkylated cyclohexenes (Scheme 15).24 With acyclic dienes, this alkylation was shown to be reversible, with kinetic alkylation occurring at an internal position of the complexed dienes but rearranging to the terminal position under thermodynamic conditions (Scheme 16).2S By trapping the kinetic product with an electrophile, overall carbo-... 

Cleavage of acylated diene-iron complexes with LiAlH4 is illustrated by the following reports of Nes-meyanov and Anisimov (Scheme 12).28,29... 

A stable tricarbonyl-(r 4-l,3-diene)-iron complex is formed22 This causes the 1,4-diene 14-Z to isomcrizc to 1,3-dicnc 16, which in turn leads to the hexahydro l//-pyrindine present in streptazolin (1). 

Iron carbonyls have been used in stoichiometric and catalytic amounts for a variety of transformations in organic synthesis. For example, the isomerization of 1,4-dienes to 1,3-dienes by formation of tricarbonyl(ri4-l,3-diene)iron complexes and subsequent oxidative demetallation has been applied to the synthesis of 12-prostaglandin PGC2 [10], The photochemically induced double bond isomerization of allyl alcohols to aldehydes [11] and allylamines to enamines [12,13] can be carried out with catalytic amounts of iron carbonyls (see Section 1.4.3). 

Removal of the 0-substituted Fp group can be achieved by conversion into the cationic alkene-Fp complex using Ph3CPF6 and subsequent treatment with iodide, bromide or acetonitrile. Oxidative cleavage with ceric ammonium nitrate in methanol provides the methyl esters via carbon monoxide insertion followed by demetallation. The [3 + 2]-cydoaddition has been successfully applied to the synthesis of hydroazulenes (Scheme 1.11) [34]. This remarkable reaction takes advantage of the specific nucleophilic and electrophilic properties of V-allyl-, cationic t 5-dienyl-, cationic ri2-alkene- and ti4-diene-iron complexes, respectively. 

The first aspect is illustrated by the synthesis of tricarbonyl(ri4-l,3-diene)iron complexes from pentacarbonyliron in the presence of catalytic amounts of a 1-azabuta-... 

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