Iron complexes cyclobutadiene

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. ... 

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. 

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). 

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... 

In fact, cubane has been made, cyclobutadiene has a fleeting existence but can be isolated as an iron complex, and a few substituted versions of tetrahedrane have been made. The most convincing evidence that you have made any of these three compounds would be the extreme simplicity of the spectra. Each has only one kind of hydrogen and only one kind of carbon. They all belong to the family (CH) . 

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 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. 

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. 

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." ... 

An elegant use of [4]annulene chemistry utilizes the stable tricarbonylcyclobutadiene iron complex as a protected cyclobutadiene.One advantage over the aluminum cyclobutadiene complex is the ability of the tricarbonylcyclobutadiene iron complex to be amenable to synthetic modifications of the four-membered ring. Typically, ceric ammonium nitrate (CAN or other oxidants) may be used to oxidize the iron and liberate free cyclobutadiene. An example of this methodology is shown in Scheme 2, as applied to the synthesis of (-i-)-asteriscanolide. Further examples of reactions utilizing this chemistry are shown in Table 1. 

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. 

Further evidence that cyclobutadiene itself is formed as a compound having a finite independent existence comes from an ingenious experiment in which it was generated by the oxidation of an iron complex which was bound to a resin. It was trapped in near quantitative yield by a maleimide derivative which was itself fixed onto another resin. Cyclobutadiene molecules must therefore travel through the solvent to pass from one resin to the other [62]. 

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... 

As iron forms cyclobutadiene complexes preferentially in the (0) oxidation state, the best starting materials for these complexes are Fe(0) compounds such as the carbonyls. Cobalt, however, tends to form Co(I) (also cyclobutadiene complexes. The best known one (cyclopentadienyl)(tetra-phenylcyclobutadiene)cobalt (XXVIII), has been prepared from cyclo-pentadienylcobalt(I) derivatives and also from cobaltocene, a cobalt(II)... 

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