Of 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... 

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. 

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. 

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. 

A convenient synthesis of a cubane system, in which the cyclobutadiene transfer reaction plays a key role, was reported by Pettit et al. (Scheme 11). By the oxidative decomposition of cyclobutadiene-iron tricarbonyl 56 with Ce + ion in the presence of a dienophile, a molecule of cyclobutadiene can be transferred from the iron to the dienophile. Decomposition of 56 in the presence of 2,5-dibromobenzoquinone 57 yielded the Diels-Alder adduct 58 with e do-configuration. Irradiation of 58 in benzene with a high-pressure Hg lamp afforded the bishomocubane derivative 59, which gave cubane-1,3-dicarboxylic acid 60 (80%) by treatment of 59 with aqueous KOH at 100°C. 

The oxidation of cyclobutadiene iron tricarbonyl with ceric ion releases the cyclobutadiene which may be characterized by its in situ reactions ... 

Examination of the proton magnetic resonance spectrum of cyclobutadiene iron tricarbonyl in the liquid crystal phase (i.e. in 4,4 -di-n-hexyloxyazobenzene) shows that, at the most, there is only a very small departure of the C4H4 ring from square symmetry [50Z>]. 

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. 

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

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. 

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. 

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. 

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). 

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. ... 

---