Cyclobutadiene-iron tricarbonyl

In a series of classic studies, Pettit et al. reported the synthesis of (cyclobutadiene)iron tricarbonyl together with a variety of electrophilic substitution reactions of this aromatic system (Scheme S).25-27... 

D. Seyferth, (Cyclobutadiene)iron Tricarbonyl - A Case of Theory before Experiment,... 

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 ... 

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. 

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... 

Cyclobutadiene-iron tricarbonyl itself is unique in that it undergoes many aromatic-type ring substitution reactions, such as Friedel-Crafts acylation. 

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. 

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. 

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. 

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. 

Polymercuration frequently occurs. The Hg atom is not large [the non-bonded radius is 1.50-1.73 A (150-173 pm), compared, e.g., with 1.95 A (195 pm) for Br], and its electronic effect is weak. Permercuration of aromatics is possible, and not all compounds require drastic conditions, such as fusion with reactive Hg salts . Furan , pyrrole , thiophene and cyclobutadiene iron tricarbonyl are mercurated under mild circumstances. For the monomercuration of the more reactive aromatics, the extent of reaction must be limited, e.g., by using the mild HgCl2 in the presence of Na02CCH3 to buffer the solution. 

Cyclobutadiene iron tricarbonyl is permercurated easily , but benzenechromium tricarbonyl is less reactive than free benzene ... 

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

Cyclobutadiene has been observed directly by kinetic mass spectrometry in the flash photolysis of cyclobutadiene iron tricarbonyl 513). 

Rosenblum et al. 386> reported a very elegant synthesis of cyclobutadiene iron tricarbonyl starting from oc-pyrone. The corresponding reaction with cyclopentadienylcobalt dicarbonyl produces cyclobutadiene-(cyclopentadienyl)cobalt 88S>. 

Diels-Alder reactions. The diazene (1) undergoes a Diels-Alder reaction with cyclopentadiene to give the expected adduct in >90% yield. The reaction with cyclobutadiene (liberated by oxidation of cyclobutadiene iron tricarbonyl) has been studied in detail. The adduct (2) is formed in 45% yield. Deblocking of (2) by treatment with base and then acid leads to 2,3-diazabicyclo[2.2.0]-hexene-5 (4). Oxidation of a hydrazo group is known to lead to an azo group. 

DIELS-ALDER REACTIONS Benzocyclopropene. 1,2-BisC3-tosyIethoxycarbonyl)-diazene. Cyclobutadiene iron tricarbonyl. 1,2-Dicyanocyclobutene. Diethyl ketomalonate. 1,3,4,6-Heptatetraene. 2-Hydroxy-5-oxo-5,6-dihydro-2/f-pyrone. 3-Hydroxy-2-pyrone. Isopropylidene isopropylidenemalonate. Lithium tetra-methylpiperidide. 4-Methoxy-5-acetoxymethyl-o-benzoquinone. 4-Methoxy-5-methyl-o-benzoquinone. frans-l-Methoxy-3-trimethyl-silyloxy-l,3-butadiene. Per-fluorotetramethylcyclopentadiene. 4-Phenyl-l, 2,4-triazoline-3,5-dione. Potassium f-butoxide. 

Cycloalkene-l,4-diols, 608-609 Cycloalkenes, 162,417-418 Cyclobutadiene, 55, 149 Cyclobutadiene iron tricarbonyl, 55, 149-... 

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). 

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