Cyclobutadiene, iron tricarbonyl complex

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... 

Cyclobutadiene iron tricarbonyl complexes also stabilized carbocations on an adjacent carbon. The cation reacts with silyl enol ethers to afford alkylated complexes such as (127) (Scheme 187). A samarimn diiodide -mediated intermolecular radical cychzation of iron tricarbonyl complex (128) is depicted in Scheme 188. An excellent stereocontrol at three contiguous centers is observed. 

Cyclobutadiene-iron tricarbonyl is prepared through reaction of S,4-dichlorocydolmtene and diiron enneacarbonyl. In an analogous manner, one can prepare 1,2-diphenyl- 1,2,3,4-tetramethyl- and benzocyclobutadiene-iron tri-carbonyl complexes. Cyclobutadiene-iron tricarbonyl is aromatic" in the sense that it undergoes facile attack by electrophilic reagents to produce monosubstituted cydo-butadiene-iron tricarbonyl complexes. Functional groups in the substituents display many of their normal chemical reactions which can be used to prepare further types of substituted cyclobutadiene-iron tricarbonyl complexes. 

One of the most interesting properties of the complex concerns its reactions with electrophilic reagents. It is found that these reactions lead to substituted cyclobutadiene-iron tricarbonyl complexes and, in this sense, the complex is classified as aromatic just as ferrocene may be so classified. The substitution reactions which have been performed so far are summarized below. 

CYCLOBUTADIENEIRON TRICARBONYL (1,3-Cyclobutadiene, iron tricarbonyl complex)... 

In M-generated metal carbonyl-complexed cyclopropenylphosphinidenes undergo a sequence of structural changes leading to phosphorus analogues of Pettit s seminal ( i(4)-cyclobutadiene)iron tricarbonyl complex via multiple valence isomers and the elimination of one molecule of carbon monoxide. ... 

Stabilization of Unstable Intermediates. Transition metals can stabilize normally unstable or transient organic intermediates. Cyclobutadiene has never been isolated as a free molecule, but it has been isolated and fully characterized as an iron tricarbonyl complex (138) ... 

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. 

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 4.5. Count the skeletal electron pairs (or cluster valence electrons) in the organometallic complex shown below at the left. Is there a relationship to cyclobutadiene iron tricarbonyl shown next to it ... 

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... 

In 1964 we prepared cyclobutadiene-iron tricarbonyl (III), a complex possessing an unsubstituted cyclobutadiene ligand (5). The reaction employed in this preparation involves the interaction of c2s-3,4-dichlorocyclobutene with Fe2(CO)9. It has subsequently been found that... 

Cyclobutadiene-iron tricarbonyl is a pale, yellow solid, m.p. 26°C., which exhibits a single, sharp NMR absorption at r 6.09. In common with other diene-iron tricarbonyl complexes, the material displays appreciable thermal stability, as well as a pronounced resistance to further replacement of the CO ligands several hours treatment with triphenylphosphine in refluxing toluene leaves the complex unaffected. 

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... 

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

The C NMR spectrum of cyclobutadiene-iron tricarbonyl consists of a singlet at 209.0 ppm for the carbonyl carbons and a doublet centered at 61.0 ppm, Jc-H = 191 Hz (193). The large C-H coupling constant suggests hybridization at carbon between sp and sp which is consistent with the strained cyclobutadiene ring. In a plot of C versus chemical shifts the cyclobutadiene resonances fall on the same line as resonances for aromatic molecules, their ions and their metal n complexes. 

Some monosubsdtuted cyclobutadiene-iron tricarbonyls and related complexes are also mentioned in the Appendix. 

Pettit et al. (39a) have found that cyclobutadieneiron tricarbonyl (XVIII) undergoes electrophilic substitution under very mild conditions to give a variety of monosubstituted cyclobutadiene-iron tricarbonyls (CII R = D, COCH3, CHO, CHjCl, HgCl, CH2NMC2). The authors have suggested that the reactions proceed via cyclobutenyliron tricarbonyl cationic complexes (Cl). 

Phenylbenzocyclobutadiene is an intermediate in the photolysis of diphenyl-acetylene, but is too unstable to be isolated and undergoes dimerization. A cyclobutadiene has also been proposed as an intermediate in the complex reaction of perfluoro-l,l-dimethylallene with chloroform and caesium fluoride. Cyclobutadiene is generally trapped and stored as its iron tricarbonyl complex. A normal co-ordinate analysis of the latter has been reported, as has a convenient synthesis of the carboxylic acid complex (823), ... 

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. ... 

Once freed from its iron tricarbonyl complex, cyclobutadiene is unstable and dimerizes readily. The structure of the dima- is ... 

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