Cyclobutadieneiron tricarbonyl

The complex exhibits remarkable stabiUty, and the cyclobutadiene undergoes reactions without destmction of the ring (139). Cyclobutadieneiron tricarbonyl [12078-17-0] can be oxidized to generate cyclobutadiene in situ (140). 

Other oxidizing agents may he used to degrade cyclobutadieneiron tricarbonyl in those cases in which the reactants or products are sensitive to the acidic ceric ammonium nitrate solutions, lead tetraacetate in pyridine can be used. 

Carboxylic acids, a-bromination of 55, 31 CARBOXYLIC ACID CHLORIDES, ketones from, 55, 122 CARBYLAMINE REACTION, 55, 96 Ceric ammonium nitrate [Ammonium hexa mtrocerate(IV)[, 55, 43 Chlorine, 55, 33, 35, 63 CHROMIUM TRIOXIDE-PYRIDINE COMPLEX, preparation in situ, 55, 84 Cinnamomtnle, a-phenyl- [2-Propeneni-tnle 2,3-diphenyl-], 55, 92 Copper(l) iodide, 55, 105, 123, 124 Copper thiophenoxide [Benzenethiol, copper(I) salt], 55, 123 CYCLIZATION, free radical, 55, 57 CYCLOBUTADIENE, 55, 43 Cyclobutadieneiron tricarbonyl [Iron, tn-carbonyl(r)4-l,3-cyclo-butadiene)-], 55,43... 

CYCLOBUTADIENEIRON TRICARBONYL, 50, 27 Cyclobutane, 52,32 CYCLOBUTANE, l-BROMO-3-CHLORO-,51, 706... 

Cyclobutadiene, [4]annulene, is so far too elusive for the kinds of investigations one would like to be able to carry out. But the great difficulty that chemists have experienced in its preparation alone justifies the conclusion that it lacks any aromatic stabilization. The compound can now be prepared by oxidation of cyclobutadieneiron tricarbonyl (36) 13 it dimerizes instantaneously but is stable... 

When cyclobutadiene is generated by oxidation of cyclobutadieneiron tricarbonyl, most of the product is a mixture of the dimers 1 and 2. Is this dimerization thermally allowed or forbidden, and which isomer is expected to predominate ... 

The conversion of the dichlorocyclobutene to cyclobutadieneiron tricarbonyl can be conveniently monitored by vapor phase chromatography. On a 5 ft. x in. column of 20% Carbowax on Chromosorb W, under conditions where the retention time of dichlorocyclobutene is 2.6 minutes, the retention time of cyclobutadieneiron tricarbonyl is 2.4 minutes. 

Cyclobutadieneiron tricarbonyl may also be produced through reaction of 3,4-dichlorocyclobutene with Na2Fe(CO)4,5 and by irradiation of a-pyrone followed by treatment with Fe2(CO)9 . The method outlined here is the most convenient procedure especially when considerable quantities (10 g. or more) of cyclobutadieneiron tricarbonyl are required. The analogous reaction of derivatives of 3,4-dihalocyclobutenes with Fe2(CO)9 affords the corresponding cyclobutadieneiron tricarbonyl complexes. Cyclobutadieneiron tricarbonyl can be oxidized to generate cyclobutadiene in situ.7... 

Cyclobutadiene. Cyclobutadiene itself is unknown but the complex cyclobuta-dieneiron tricarbonyl (1, 2,140 3, 101) is readily available. The diene can be liberated from the complex as a transient intermediate for use in synthesis by oxidative degradation with ceric ammonium nitrate. If the reactants or products are sensitive to the acidic solutions of CAN, lead tetraacetate in pyridine can be used as oxidant. Thus the reaction of cyclobutadieneiron tricarbonyl with p-benzoquinone in the presence of CAN leads to formation of endo-tricyclo(4.4.0.02 5]deca-3,8-diene-7,10-dione (2) in about 40% yield.3... 

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

In a well-ventilated hood a 1-1. three-necked flask is immersed in an oil bath and fitted with a mechanical stirrer and condenser with aT-piece at the top with one lead connected to a nitrogen supply and the other to a gas bubbler. The flask is charged with 60 g. (0.49 mole) of c/i-3,4-dichlorocyclobutene (this volume) in 250 ml. of benzene, and the system is flushed with nitrogen. A first 50-g. batch of diiron nonacarbonyl (2) is added and the mixture is heated at 50-55° with stirring. After about 15 min. the initial rapid evolution of carbon monoxide becomes greatly diminished and a further 25 g. of Fe2(CO)9 is added. Further 25-g. portions are added until no more carbon monoxide is liberated a total of approximately 275 g. of Fe2(CO)9 is required and the total reaction time is about 5 hrs. The mixture is then filtered with suction through Celite and the Buchner funnel is washed thoroughly with pentane until the filtrate is colorless. Fractional distillation at reduced pressure removes benzene, then iron pentacarbonyl (b.p. 20730 mm.) when the Fe(CO)5 has been removed, cyclobutadieneiron tricarbonyl is collected as a pale yellow oil, b.p. 4773 mm. 

When cyclobutadiene is liberated as a transient species by oxidative decomposition of cyclobutadieneiron tricarbonyl (lead tetraacetate in pyridine) in the presence of cyclopentadicnone diethyl ketal the adduct I is obtained. Irradiation of 1 in acetone gives 9,9-diethoxyhomocubane, which on hydrolysis gives homocubanone. II. The corresponding alcohol, homocubanol. is of interest because the derived cation is a fluctional molecule.2... 

CYCLOBUTADIENEIRON TRICARBONYL, 50, 21 Cyclobutane, 52, 32 CYCLOBUTANE, l-BROMO-3-CHLORO-, 51, 106 Cyclobutanecarbonyl chloride, reaction with eo fhro-2,3-butane-diol monomesylate, 51,12 CYCLOBUTANECARBOXALDE-HYDE,51, 11... 

Diels-Alder reactions (2, 95 4, 72). Preparation of derivatives of Dewar benzene from cyclobutadiene and various acetylenes was first reported by Pettit et al. Addition of ceric ammonium nitrate (CAN) to an ice-cold acetone solution of equimolar amounts of cyclobutadieneiron tricarbonyl (1) and... 

This procedure is illustrative of the synthetic use of cyclobutadieneiron tricarbonyl as a source of highly reactive cyclobutadiene. Cyclobutadiene has been employed, for example, in the synthesis of cubane, Dewar benzenes, and a variety of other systems. ... 

Bis(p-chlorophenyl)acetylene undergoes a similar reaction to give tetrakis(/ -chlorophenyl)cyclobutadieneiron tricarbonyl in 8% yield (14). Cyclo-... 

Little meaningful physical data on these complexes has appeared yet. The study of their reactivity is complicated by the effects of the substituents on the cyclobutadiene ring and the other ligands present. Until more information is available on the properties of the recently prepared unsubstituted cyclobutadieneiron tricarbonyl (XVIII) 38) and similar molecules, it is hard to be certain which properties are due to the presence of a cyclobutadiene group. Thus while cyclobutadieneiron tricarbonyl (XVIII) is easily oxidized by ferric chloride in ethanol (as are other diene-iron tri-carbonyl complexes, albeit to different types of product), tetraphenyl-cyclobutadieneiron tricarbonyl (XIII) is very resistant to this reagent, and indeed to most others, presumably mainly due to the steric hindrance of the phenyls. 

A very recent paper 39a) has now shown that cyclobutadieneiron tricarbonyl (XVIII) is an aromatic system and that it easily undergoes electrophilic substitution reactions (see Appendix). 

The proton NMR spectra of some of these complexes have been determined. The spectrum of cyclobutadieneiron tricarbonyl (XVIII) shows a singlet at 6.09r and that of benzocyclobutadieneiron tricarbonyl (XIX) a singlet at 5.98r due to the cyclobutadiene protons, as well as a multiplet due to the aromatic protons at 3.05r (3S). The NMR spectra of monosubstituted cyclobutadieneiron tricarbonyls (see Appendix) show the equivalence of the two cyclobutadiene ring protons adjacent to the substituent. This implies that the four-membered ring must be square (39a). Tetramethylcyclo-butadienenickel chloride in water shows only a single resonance due to the 12 equivalent methyl protons (32). The spectra of the tetraphenylcyclo-butadiene-metal complexes are those due to phenyl protons and are usually complex. In the (cyclopentadienyl)(tetraphenylcyclobutadiene)nickel and -palladium bromides (XLIV), however, sharp single phenyl proton resonances are obtained at 2.39r (65). The reason for the apparent equivalence of all the phenyl protons in (XLIV) is not clear. 

Cyclobutadieneiron tricarbonyl (XVIII) on oxidation with ethanolic ferric chloride (or ceric ammonium nitrate-lithium chloride) gave trans-dichlorocyclobutene (LXVII) (38), and other products (92a) (see also Appendix). 

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