Iron, diene complexes

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. 

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). 

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-... 

Acyclic tricarbonyl(diene)iron complexes may be prepared by reaction of a 1,3-diene with (1), (2), or (3) either thermally, photochemically, or under the influence of ultrasonic stirring (Scheme 62). Ligand transfer species, such as (benzylideneacetone)Fe(CO)3, (l-aza-1,3-diene)Fe(CO)3, or bis( -cyclooctene)Fe(CO)3 are also useful for the complexation of dienes nnder mUd reaction conditions. The thermal reaction of nnconjugated dienes with (1) generally results in double bond migration... 

Cationic complexes formed from acyclic diene iron complexes react in a fashion very similar to dicobalt hexacarbonyl stabilized propargylic cations. The cations can... 

Previous syntheses of tricarbonyl( /-diene)iron complexes have relied mainly on the reaction of Fe(CO)s, Fc3(CO)i2, or Fc2(CO)9 with the free diene. The use of the first two carbonyls suffers from the prolonged reflux times and/or ultraviolet irradiation necessary to obtain reaction and the consequent low yields and mixtures of complexes obtained with heat- and ultraviolet-sensitive dienes. The latter reagent, although utilized at lower temperatures, may react with polyenes (n > 3) to give mixtures containing, in addition to the expected product, binuclear derivatives containing a metal—metal bond.2... 

While a great number of tricarbonyl( -diene)iron complexes have been reported and their reactivity investigated, much less is known of the corresponding heterodiene complexes. In recent years, synthesis of several tricar-bonyl(heterodiene)iron systems involving r] coordination of the heterodiene unit has been achieved. Among the tetracarbonyl(/ -olefin)iron complexes prepared by Weiss was tetracarbonyl(cinnamaldehyde)iron, which converts on heating to the //-bonded tricarbonyl(cinnamaldehyde)iron. The preparation and synthetic utility of (benzylideneacetone)tricarbonyl iron, an analogous complex of an ar,/9-unsaturated ketone, are reported here. 

The complex functions as a convenient source of the tricarbonyliron moiety by displacement of the unsaturated ketone. For example, reaction with 1,3-cycloheptadiene results in a 78% yield of (//-l,3-cycloheptadiene)tricar-bonyliron. More importantly, it may be used in syntheses of tricarbonyl-(diene)iron complexes where the iron carbonyls are not satisfactory. Several complexes of sensitive heptafulvenes have been prepared in this way, and the reagent has been used in the synthesis of tricarbonyliron complexes of several steroids. 

In contrast to the very large number of tricarbonyl( -diene)iron complexes described in the literature,thg corresponding ruthenium compounds have received very little attention. This may reflect the well-documented tendency of ruthenium to form metal—metal bonds as opposed to iron. particular, while the metal—metal bonds in Fe3(CO)i2 are easily broken, Ru3(CO)i2 undergoes a variety of reactions in which the Rus cluster is retained. 

A useful synthesis of racemic methyl lipoate from the key intermediate 348, prepared via a six-step reaction sequence, starts from tricarbonyl(diene)iron complex 350 (Scheme 67) <1998EJ01949>. The main goal of this practical method, based on the use of the optically active iron complex 350, was a possible stereoselective synthesis of LA and other structural analogues. 

Isolated double bonds can be oxidatively cleaved in systems containing a conjugated diene moiety if it is protected as a tricarbonyl(diene)iron complex . Dienal 39 was acquired in 49% yield by a two-step osmylation-periodate cleavage sequence (equation 27). In contrast, ozonolysis of the polyene complexes is reported to lead to destruction of the complex. 

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