Tricarbonyliron transfer

Because of the high lability of the reagents described above, (T 4-l-azabuta-l,3-dienejtricarbonyliron complexes have been developed as alternative tricarbonyliron transfer reagents. They are best prepared by an ultrasound-promoted reaction of 1-azabuta-l,3-dienes with nonacarbonyldiiron in tetrahydrofuran at room temperature. Using (ri4-l-azabuta-l,3-diene)tricarbonyliron complexes the transfer of the tricarbonyliron unit proceeds in refluxing tetrahydrofuran in high yields [55a,b]. 

Ghiral 1-aza-l,3-butadienes react with nonacarbonyldiiron in THF at RT using ultrasound, to yield a 5 1 mixture of the (S,R)-32 and R,R)-33 diastereoisomers, respectively. The crystal structure of 32 has been reported (Scheme 27). The free ligands of 1-aza-l,3-dienes are reported to be efficient catalysts for the complexation of 1,3-dienes with either pentacarbonyliron or nonacarbonyldiiron.In this respect, the heterodiene complexes serve as useful tricarbonyliron transfer reagents, to yield free 1-azabuta-1,3-dienes (Scheme 28). 

Scheme 4-103. Tricarbonyliron transfer using Grevels reagent.

Scheme 4-104. Tricarbonyl(T -1 -aza-1,3-butadiene)iron complexes as tricarbonyliron transfer reagents.

Tricarbonyl(T -l-aza-l,3-butadiene)iron complexes constitute convenient tricarbonyliron transfer reagents, which are easier to handle than the corresponding T -benzylideneacetone(tricarbonyl)iron complexes. They are prepared from 1-aza-1,3-butadienes with nonacarbonyldiiron in tetrahydrofuran at room temperature. The... 

Scheme 4-105. Mechanism of the tricarbonyliron transfer by tricarbonyl(T -l-aza-l, 3-butadiene)iron complexes.

Scheme 4—106. Mechanism of the azabutadiene-catalyzed tricarbonyliron transfer.

Due to the extensively represented oxidative behaviour of the carbenium ions as hydride abstractor or one-electron oxidant [157], attempts were made to employ the carbocations as reagents. Recently the enantioselective outcome in a hydride transfer reaction was reported [158, 159]. The abstraction of the exo hydrogen atoms from the tricarbonyliron complex 57 resulted in a yield up to 70% and enantiose-lectivity of 53% (Scheme 62) [158]. 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

Tricarbonyliron diene complexes have found many uses in synthetic chemistry but their synthesis is often not easy. Knolker has developed a range of tricarbonyl(7] -l-aza-l,3-butadiene) iron complexes that are excellent transfer agents for the Fe(CO)3 complexation of 1,3-dienes, and showed their versatility. As an extension to this work, Knolker and Gonser have prepared a polymer-supported l-aza-l,3-butadiene 321 by reaction of Merrifield s resin with phenolic l-aza-l,3-butadiene 320, formed from cinnamaldehyde and /> ra-hydroxyaniline (Scheme 105). The corresponding tricarbonyl iron complex 322 was formed by treatment of 321 with an excess of Fe2(CO)9 in THF using ultrasound. The iron complex was subsequently used efficiently as a transfer agent for the tricarbonyliron complexation of 1,3-dienes. 

Other Reactions of Unsaturated Steroids.—Asymmetric synthesis of optically active tricarbonyliron complexes of 1,3-dienes was achieved using the tricar-bonyliron complex (25) as a transfer agent for Fe(CO)3. Further investigation of the stereochemistry of formation of a-(4-6i7)-PdCl complexes from A -3-oxo-... 

The relative and absolute configuration of 35,125-dihydroxypalmitic acid, a constituent of the Ipomea operculata M. resin, was confirmed by synthesis starting with dimethyl L-malate (35). An efficient synfliesis of (55)-hydroxy-20 4(6 ,8Z,l 1Z,14Z) was accomplished by the coupling of two readily accessible synthons, methyl (55)-hydroxy-7-iodo-heptanoate and 4Z,7Z- tridecadien-l-yne (36). A highly stereoselective synthesis of P-dimorphecolic acid, (95)-hydroxy-18 2(10.E,12 ), has been reported the synthesis features a diastereoselective reduction of a keto intermediate [4] in which the tricarbonyliron lactone tether induces a 1,5-transfer of chirality followed by a stereoselective decarboxylation to create all of the stereochemical ele-... 

Isomerization of conjugated dienes is possible by protonation of the corresponding tricarbonyliron complexes [Eqs. (147) (Birch and Williamson, 1973) and (148) (Birch et ai, 1975)]. These isomerizations presumably also involve transfer of allylic hydrogen to the transition metal (cf. Scheme 11). 

In contrast to the trans addition observed for irreversible nucleophilic attack on tricarbonyliron dienylium cations and olefins complexed to Fp (Section III,A), irreversible nucleophilic attack (e.g., by carbon nucleophiles) on olefins coordinated to Pt(II) and Pd(II) appears to result in cis addition, presumably via initial attack at the coordinatively unsaturated Pt(II) or Pd(II), followed by intramolecular transfer of the coordinated nucleophile to the double bond (Section III,A). 

The tricarbonyliron fragment can be attached to a diene system by a variety of transfer reagents. Homoleptic carbonyliron complexes are the most simple ones and have been known for the longest time. The replacement of two carbonyl ligands in pentacarbonyliron by a diene ligand requires quite harsh conditions. A temperature of approximately 140 °C or irradiation is necessary to induce this dissociative process. (Scheme 4-101). ° ... 

The mechanism involves formation of a reactive 16-electron tricarbonyliron species by sequential loss of two carbon monoxide ligands and coordination of allyl alcohol to pentacarbonyliron (Scheme 4-291). Oxidative addition under activation of the sp C-H bond adjacent to the hydroxy group leads to an Ti -allyl(hydrido)iron complex. Subsequent reductive elimination transfers the hydrido ligand to the 3-position of the allyl ligand to afford an ri -alkene(tricarbonyl)iron complex. Dissoziation of the enol ligand releases a 14-electron tricarbonyliron complex that will start the catalytic cycle de novo by complexation of allyl alcohol. The enol will finally tautomerize to the ketone. In total, a formal [l,3]-hydride shift is achieved at the allyl alcohol. ... 

Enone Complexation. Various a,p-unsaturated ketones form moderately stable tricarbonyliron complexes (18), and offer access to 1,4-diketones (19) by reaction with Grignard reagents, organolithium reagents (eq 14), or organocuprates. Trimethylsilyloxybutadienes require phenyl substituents to form stable complexes by reaction with Fc2(CO)9. Enone complexes are also useful as transfer reagents to place Fe(CO)3 on diene ligands. A related procedure employs complexes of a,3-unsaturated imines in the same way. ... 

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