Iron complexes, with cyclobutadiene

A rare example of a ferracycloheptane 108 was obtained as the product of the photochemical reaction of a Petitt s cyclobutadiene iron complex with dimethyl-maleate [Eq. (43)].118 The ferracycloheptane arises from the insertion of a maleate into each of two Fe-C bonds and might therefore be considered a special case of alkene trimerisation (vide infra). The cyclobutene fragment in the final metallacycle remains coordinated to iron, as established crystallographically (Fig. 34). 

Liberation of cyclobutadiene from its tetrahapto-iron tricarbonyl complex is believed to proceed via a dihapto iron complex, and the first such complex to be prepared by an independent method is [824 Fp — T -CpFe(CO)2]. On oxidation with iodide ion in the presence of diphenylisobenzofuran the adduct (825) of benzo-cyclobutadiene is obtained. A dinuclear complex with cyclobutadiene acting as a bridging ligand between two iron atoms has also been prepared. ... 

The smallest member, cyclobutadiene, was the objective of attempted synthesis for many years. The first success was aehieved when cyclobutadiene released from a stable iron complex was trapped with various reagents. ... 

The reaction of o-diphenylcyclobutadiene (generated in situ by oxidation of its iron tricarbonyl complex) with p-benzoquinone yields A as the exclusive product. With tetracyanoethylene, however, B and C are formed in a 1 7 ratio. Discuss these results, and explain how they relate to the question of the square versus rectangular shape of cyclobutadiene. 

Cycloarsa(V)azanes, 28 216-223 Cyclobutadienes, 31 144, 166 Cyclobutene complexes with iron, 12 254,... 

Several transition-metal complexes of cyclobutadiene have been prepared, and this is all the more remarkable because of the instability of the parent hydrocarbon. Reactions that logically should lead to cyclobutadiene give dimeric products instead. Thus, 3,4-dichlorocyclobutene has been de-chlorinated with lithium amalgam in ether, and the hydrocarbon product is a dimer of cyclobutadiene, 5. However, 3,4-dichlorocyclobutene reacts with diiron nonacarbonyl, Fe2(CO)9, to give a stable iron tricarbonyl complex of cyclobutadiene, 6, whose structure has been established by x-ray analysis. The 7r-electron system of cyclobutadiene is considerably stabilized by complex formation with iron, which again attains the electronic configuration of krypton. 

Exercise 31-2 The cyclobutadiene iron complex, 10, has been prepared optically active, and when oxidized with Ce(IV) in the presence of tetracyanoethene gives a mixture of cyclobutadiene cycloadducts, all of which are optically inactive. 

Comparison of the tropone ring geometry in iron complex 282 with that of the parent tropone shows that the double bond character of the carbonyl group and the bond between C-2 and C-3 in 282 is even higher, probably as a result of dominant contributions from two cyclobutadiene resonance... 

Neutral (cyclobutadiene)iron complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate, and with the nitrosonium cation. Cationic nitrosyl complexes (e.g. 210) undergo ligand substitution by treatment with phosphines. Photolysis of (tetraphenylcyclobutadiene)Fe(CO)3 in THF at -40 °C is reported to give the novel bimetallic complex (214), which reacts with carbon monoxide (140 atm, 80 °C) to regenerate the starting material.An X-ray diffraction analysis of (214 R = Ph, R = t-Bu) reveals a very short Fe-Fe distance of 2.117 A. 

Cyclobutadiene iron tricarbonyl complexes can be isolated and have been utilized in organic synthesis. Both intra- and intermolecular [2 + 2] cycloadditions of alkenes with cyclobutadiene complexes are observed upon decomplexation using CAN or TMANO (Schemes 164-165). The stereochemistry of the aUcene is retained in the product. Iron tricarbonyl diene complexes are compatible with metathesis reactions... 

The area of cyclobutadiene-transition metal chemistry has expanded rapidly since these initial findings, largely through the work of Maitlis 163), Nakamura 183), Freedman 104), and others, but details will not be presented here. Several recent important discoveries by Pettit and co-workers 22, 79,102, 24I), however, relate to the formation and chemistry of cyclobutadiene-iron tricarbonyl (XVII). This product is formed from the reaction of cis-3,4-dichlorocyclobutene and diiron nonacarbonyl and can be isolated in the form of yellow crystals of excellent stability. Cyclobutadiene can be liberated by treating the complex with oxidizing agents such as ferric or ceric ion. The free ligand has been trapped and demonstrated to possess a finite lifetime. It has also been shown to... 

Reaction of cyclobutadiene-iron tricarbonyl with methylchlorothio-formate followed by hydrolysis gives rise to cyclobutadienecarboxylic acid-iron tricarbonyl (XII). A Curtius rearrangement of the acid azide derived from Complex XII affords aminocyclobutadiene-iron tricarbonyl (XIII). The dimethylaminomethyl derivative (XIV) is readily available through the Mannich reaction with formaldehyde and dimethylamine. The chloromercury cyclobutadiene complex (XV) is produced upon reaction of Complex III with Hg(OAc)2, followed by treatment with hydrochloric acid. In the simplest substitution reaction, treatment of cyclobutadiene-iron tricarbonyl with CF3COOD produces a mixture of deuterated derivatives of Complex III. 

One final comment relating to the aromaticity of the cyclobutadiene ligand concerns the orientation effect of substituents towards introduction of a second substituent. To date, the only reaction bearing on this question which has been performed is the acetylation of methylcyclobutadiene -Fe(CO)3 (XVIII), which was prepared by reducing the chloromethyl complex (IX). Acetylation of Complex XVIII produces a mixture of 2-and 3-acetyl-1-methylcyclobutadiene-iron tricarbonyl complexes with the latter isomer (XIX) predominating ( 2 1). This is not the orientation... 

The cyclobutadiene system can be stabilized as a ri" -complex with metals,as with the iron complex 84 (see Chapter 3), but in these cases electron density is withdrawn from the ring by the metal and there is no aromatic quartet. In fact, these cyclobutadiene-metal complexes can be looked upon as systems containing an aromatic duet. The ring is square planar, the compounds undergo aromatic substitution, and nmr spectra of monosubstituted derivatives show that the C-2 and C-4 protons are equivalent. 

Cyclopropene provides isomeric tricycles upon reaction with cyclobutadiene generated from its iron tricarbonyl complex (equation 54) but this thermal reaction is more likely to... 

Although several derivatives of cyclobutadiene are known and are discussed shortly, cyclobutadiene itself has been observed only as a matrix-isolated species, that is trapped at very low temperature in a frozen inert gas. The first successful synthesis of cyclobutadiene was achieved by release from a stable iron complex." " Various trapping agents react with cyclobutadiene to give Diels-Alder adducts, indicating that it is reactive as both a diene and a dienophile." Dehalogenation of trans-3, 4-dibromocyclobutene gave the same reaction products." ... 

Cyclobutadiene (4. 72). The reaction of cyclobutadiene in a cycloaddition rtaction with p-benzoquinone has now been published. The diene can also be liberated from the iron Iricarbonyl complex with lead tetraacetate in pyridine. ... 

Bunz. U.H.L. Wiegelmann-Kreiter, J.E.C. Alkynyl-sub-stituted tricarbonyl(cyclobutadiene)iron complexes Stille coupling of iodocyclobutadiene complexes with stannyl-alkynes. Chem. Ber. 1996,129. 785-797. 

Optically active 1,2-disubstituted cyclobutadiene iron complexes have been oxidised in this way and the resultant cyclobutadienes have been trapped, and in every case racemic products have been obtained, showing that the free cyclobutadiene, which is achiral, has been formed [61], Had the trapping molecules reacted with the complexes prior to loss of the metal moiety the chirality would have been retained. 

The iron complex has a square-planar cyclobutadiene ring with C—C distances averaging 1.46 A and internal bond angles 90°+ 1°. The substituent phenyl groups are bent back out of the plane of the cyclobutadiene ring away from the iron by an average of 11 ° and are all twisted about the... 

The smallest member, cyclobutadiene, has been the objective of synthetic efforts for many years, but success was achieved only relatively recently. It was first detected by trapping with various reagents after release from an iron complex ... 

Liberation of excess cyclobutadiene from its iron complex in the presence of acetylenes leads to a double addition, affording the previously unknown tetracyclic system (831 R = Ph or OMe). Trapping with phenyl vinyl ketone affords (832X nhich has been converted by reduction, alkylation, and photolysis into the new tri-cyclo[3,1,1,0 ]heptanols (833). ... 

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