Reactions of cyclobutadiene

The area of synthesis and structure determination of molecules constructed 

Tsutsui, M. N. Levy, A. Nakamura, M. Ichikawa, and K. Mori, Introduction to Metal n-Complex Chemistry, Plenum Press, New York, NY, 1970, pp. 44-45. 

Numerous chemical reactions have been carried out on ferrocene and its derivatives. The molecule behaves as an electron-rich aromatic system, and many of the typical aromatic substitutions occur readily. Reagents such as the halogens that are relatively strong oxidizing agents, however, react by oxidation-reduction, with a change in the oxidation state and coordination sphere of the iron atom. 

The most useful tt complexes of aromatic compounds from the synthetic point of view are the complexes obtained by heating benzene and substituted derivatives with Cr(CO)6. The aromatic rings in these complexes are subject to nucleophilic 

Organometallic chemistry remains a very active field of research. New types of compounds, new reactions, and useful catalytic reactivity are being discovered at a rapid rate. The examples given in this section are some of the best known and most useful reactions but a wide variety of others are known which have not been included for lack of space. 

Coates and K. Wade, Organometallic Compounds, Vol. I, Methuen, London (1967). 

Wakefield, The Chemistry of Organolithium Compounds, Pergamon Press, Oxford (1974). 

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Hogeveen and co-workers <82JOC1909 83JOC4275> also reported the synthesis of tricyclic sulfinamides by the reaction of cyclobutadiene aluminum halide o complexes (65 or 66) with 53 at low temperature. Treatment of 65 or 66 with 53 at -60 °C resulted in the formation of 67 or 68 in 52% and 55% yields, respectively (Scheme 18). 

Neutral (cyclobutadiene)Fe(CO)3 complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate and with the nitrosonium cation to generate the corresponding (cyclobutadiene)Fe(CO)2L complexes15. These types of complexes are presumably intermediates in the reaction of (cyclobutadiene)Fe(CO)3 complexes with perfluorinated alkenes and alkynes to generate the insertion products 266 or 267 respectively (Scheme 70)15,238. 

The reaction of (cyclobutadiene)metal complexes with X2 results in the oxidative decomplexation to generate either dihalocyclobutenes or tetrahalocyclobutanes. In comparison, substitution of (cyclobutadiene)MLn complexes 223 [MLn = Fe(CO)3, CoCp, and RhCp] with a variety of carbon electrophiles has been observed (equation 34)15. Electrophilic acylation of 1-substituted (cyclobutadiene)Fe(CO)3 complexes gives a mixture of regioisomers predominating in the 1,3-disubstituted product and this has been utilized for the preparation of a cyclobutadiene cyclophane complex 272 (equation 35)246. For (cyclobutadiene)CoCp complexes, in which all of the ring carbons are substituted, electrophilic acylation occurs at the cyclopentadienyl ligand. 

A substituted Dewar form of benzene has also been prepared by the reaction of cyclobutadiene with an acetylenic compound.57... 

Hoffmann and Woodward (1965b) consider possible 2 + 2, 2 + 4, and 4 + 4 reactions of cyclobutadiene with itself. The fact is that the 2 + 2 and the 2 + 4 processes are not distinguishable, but the result is an endo favored cyclization (64). We have already discussed the forbidden 4 + 4... 

Very similar results were found in Jorgensen s AMl-OPLS MC simulation of the reaction of cyclobutadiene with butenone (Reaction 7.1), acrylonitrile, and l,4-naphthoquinone. ° The computed decreases in the activation free energy... 

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. 

The cycloaddition reaction of cyclobutadienes with azodicarboxylates gives 3,6-bonded Dewar pyridazines. An example of this type of reaction has already been reported before94. A further example is described by the reaction sequence (147)158,159). The formed cycloadduct a hydrolyzed, decarboxylated, and oxidized to 3,6-bonded Dewar pyridazine which loses nitrogen to form cyclobutadiene (147). 

Cyclobutadiene (4. 72). The reaction of cyclobutadiene in a cycloaddition rtaction with p-benzoquinone has now been published. The diene can also be liberated from the iron Iricarbonyl complex with lead tetraacetate in pyridine. ... 

Reactions with Dienophiles. Inevitably, because of its transient nature, reactions of cyclobutadiene itself have involved trapping reactions using dienophiles. The reactions of dideuteriocyclobuta-diene with some dienophiles have been discussed in detail in the previous paragraph. Cyclobutadiene itself has been trapped with maleic and fumaric esters [42,43], maleimides [45], acetylenic esters [10,46], benzoyl ethylene [47], and o- and p-qui nones [11,48]. 

Most of the reactions of cyclobutadiene-metal complexes so far reported were carried out with the aim of confirming the structure of the complex. As mentioned in Section V, A, however, evidence for the structure from chemical degradation is not always very convincing. 

Mono- and polyanionic species were obtained through deprotonation reactions of cyclobutadienes by excesses of methyllithium. The dianionic complex readily rearranged to a cobaltacyclopentadienyl unit (Scheme 44). ... 

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