Cyclobutadiene tricarbonyliron

As we saw in Section 11.17, cyclobutadiene is antiaromatic and exceedingly difficult to prepare and study. Its successful preparation by Rowland Pettit (University of Texas) in 1965 demonstrated how transition-metal organometallic cbemistry can provide access to novel reactions and structures. His approach was to prepare cyclobutadiene as a transition-metal complex, then destabilize the complex to trigger its dissociation. The sequence for cyclobutadiene begins with the reaction of cis-3,4-dichlorocyclobutene with diiron nonacarbonyl [Fe2(CO)9]. The resulting iron-cyclobutadiene complex satisfies the 18-electron rale, is stable, and undo-goes a variety of reactions. Most importantly, oxidation with ceric ammonium nitrate (a source of Ce ) lowers the electron count Irom 18 to 16, causing the complex to dissociate and Ubo-ate free cyclobutadiene. 

38 The tricarbonyliron complex of cyclobutadiene is sufficiently stable to undergo reactions typical of aromatic hydrocarbons. What is the product of the reaction shown  

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

40 Oxidation of (cyclobutadiene)tricarbonyliron with Ce in the presence of ethyl propynoate yielded a product corresponding to a Diels-Alder adduct of cyclobutadiene and ethyl propynoate. 

41 The product of this reaction is formed by an intramolecular Diels-Alder cycloaddition. What is its structure  

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The irradiation of a-pyrone in the presence of Fe(CO)5 results in formation of (cyclobutadiene)tricarbonyliron and the yellow (a-pyrone)-tricarbonyliron, m.p. 140°-141°C (141). This latter complex can also be... 

Thus, every chapter concludes with a self-contained D scnp-tive Passage and Interpretive Problems unit that complements the chapter s content while emulating the MCAT style. These 27 passages—listed on page xix—are accompanied by more than 100 total multiple-choice problems. Two of these More on Spin-Spin Splitting and Coupling Constants in Chapter 13 and Cyclobutadiene and (Cyclobutadiene)tricarbonyliron in Chapter 14 are new to this edition. 

Although cyclobutadiene itself is unknown, it can be liberated as an intermediate by oxidative degradation of the organometallic compound cyclobutadiene tricarbonyliron by ammonium hexanitratocerate(IV) (Watts et al., 1965 Paquette and Kelly, 1969). The transient intermediate can be trapped by a dienophile (scheme 57). When the oxidation is carried out in presence of a CAN solution saturated with lithium chloride, tran5 -3,4,-dichloro-cyclobutene is formed (Emerson et al., 1965). Because tricarbonyliron complexes are often used as reagents or intermediates in organic synthesis, the possibility to remove the Fe(CO)3 group by decomplexation... 

Takanashi K, Lee VY, Ichinohe M, Sekiguchi A (2006) A (tetrasilacyclobutadiene) tricarbonyliron complex [ ri -(tBu2MeSi)4Si4 Fe(CO)3] the silicon cousin of Pettit s (cyclobutadiene)tricarbonyliron complex [(ti -H4C4)Fe(CO)3]. Angew Chem Int Ed 45 3269... 

The mass spectrum of tricarbonyliron cyclobutadiene derivative 282 shows fragments corresponding to the stepwise loss of four CO molecules and finally of C2H2 (78AJC1607). 

Similarly, in the tricarbonyliron cyclobutadiene complex 285 (78AJC1607), the metal carbonyl groups stabilize the positive charge in the a-positions to the complexed diene system (H-2 and H-6 8 8.29 ppm). 

The Ce(IV)-oxidation of a tricarbonyliron(0)-cyclobuta-diene complex releases the cyclobutadiene (la-c) that subsequently oligomerizes to yield a mixture of [n]-ladderanes (Figure 14). ... 

Tricarbonyliron complexes of cyclobutadienes. Roberts et al. have reported a useful synthesis of (2) by reaction of (1) with sodium tetracarbonyl-ferrate in THF. Use of diiron nonacarbonyl leads to (2) in lower yields. The... 

The reactive species, trimethylenemethane, has been observed only by photolysis of an adduct of allene and diazomethane in a matrix of hexafluorobenzene at — 185°C. The e.s.r. spectrum shows that it has two unpaired electrons (a triplet ground state) which is consistent with a planar structure with 3-fold symmetry (Djh). This species can, like cyclobutadiene, be stabilized by coordination to tricarbonyliron. The parent compound, a diamagnetic pale yellow solid, m.p. 28°C can be obtained by the two routes shown. 

Tricarbonyl(cyclobutadiene)iron is also readily acylated as shown in Eq. (68) (Fitzpatrick et ai, 1965), thus providing easy access to substituted cyclobutadienes by removal of the tricarbonyliron group with ceric ion (Section VI,A). Other cyclobutadiene derivatives have been prepared (Roberts et al., 1969). 

Variously substituted tricarbonyliron-cyclobutadiene complexes are readily prepared (Fitzpatrick et ah, 1965 Roberts et al., 1969 Agar et al., 1974) and cyclobutadiene complexes with other transition metals are known (Maitlis, 1966), but few of these have been used in organic synthesis. 

The [ 2s + 2sJ electrocyclic photochemical cyclization is important from the preparative viewpoint. The interesting but labile compounds Dewar benzene (19) and cyclobutadiene have been prepared by such means. Equations (6.19) and (6.20). The anti-aromatic cyclobutadiene is too reactive to be isolated, but is conveniently trapped as the tricarbonyliron(0) complex from which it can be later regenerated under oxidative conditions, for example by using cerium (IV) salts. 

A pentafulvalene has been trapped as a diiron complex using Fe(CO)5. Thiophene dioxide has also been stabilized as its tricarbonyliron complex. The irradiation of a-pyrone (45) in the presence of Fe(CO)5 gives cyclobutadiene stabilized by complexation with the Fe(CO)3 moiety (46), together with the (ct-pyrone)Fe(CO)3 complex (47) (eq 29). ... 

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