1,3-cyclobutadiene reactions

THE cvcLOBUTADENE-TETRAHEDRANE SYSTEM. A related reaction is the photoisomerization of cyclobutadiene (CBD). It was found that unsubstituted CBD does not react in an argon matrix upon irradiation, while the tri-butyl substituted derivative forms the corresponding tetrahedrane [86,87]. These results may be understood on the basis of a conical intersection enclosed by the loop shown in Figure 37. The analogy with the butadiene loop (Fig. 13) is obvious. The two CBDs and the biradical shown in the figure are the three anchors in this system. With small substituents, the two lobes containing the lone electrons can be far... 

Cyclobutadiene itself is not stable at room temperature. Several derivatives with stabilizing groups have been prepared by the acid-catalyzed dimerization of alkjmes (R. Gompper, 1975). Less substituted cyclobutadienes could be obtained by photolytic reactions in solid matrix at low temperatures (G. Maier, 1973, 1974). 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

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). 

Reactions of acetylene and iron carbonyls can yield benzene derivatives, quinones, cyclopentadienes, and a variety of heterocycHc compounds. The cyclization reaction is useful for preparing substituted benzenes. The reaction of / fZ-butylacetylene in the presence of Co2(CO)g as the catalyst yields l,2,4-tri-/ f2 butylbenzene (142). The reaction of Fe(CO) and diphenylacetylene yields no less than seven different species. A cyclobutadiene derivative [31811 -56-0] is the most important (143—145). 

Extrapolation from the known reactivity of cyclobutadiene would suggest that azetes should be highly reactive towards dimerization and as dienes and dienophiles in cycloaddition reactions and the presence of a polar C=N should impart additional reactivity towards attack by nucleophiles. Isolation of formal dimers of azetes has been claimed as evidence for the intermediacy of such species, but no clear reports of their interception in inter-molecular cycloaddition reactions or by nucleophiles have yet appeared. 

NMR, 3, 542 oxidation, 3, 546 phosphorescence, 3, 543 photoelectron spectra, 3, 542 photolysis, 3, 549 reactions, 3, 543-555 with alkenes, 3, 50 with alkynes, 3, 50 with IH-azepines, 3, 552 with azirines, 3, 554 with cyclobutadiene, 3, 551 with cyclopropenes, 3, 550 with dimethylbicyclopropenyl, 3, 551 with heterocyclic transition metal complexes, 7, 28 29... 

This dimerization is an extremely fast reaction and limits the lifetime of cyclobutadiene, except at very low temperatures. 

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. 

The cyclobutadiene analogue diazadiboretidine reacts with hexafluoroacetone to give a nng expansion product [776] (equation 90) 6-(3-Fluoroaryl)decaborane is formed by alkylation of decaboranyl anion and separation of the two isomers (5- and 6-benzyl) formed by reaction with dimethyl sulfide [777] (equation 91). 

Retro Diels-Alder reaction of nitrogen bridgehead compound 415 at 100 °C afforded 6,7,8,9-tetrahydro-4//-pyrido[l,2-u]pyrimidin-4-one and cyclobutadiene (97SC195). 

Cyclo butadiene is highly reactive and shows none of the properties associated with aromaticity. In fact, it was not even prepared until 1965, when Rowland Pettit of the University of Texas was able to make it at low temperature. Even at —78 °C, however, cyclobutadiene is so reactive that it dimerizes by a Diels-Alder reaction. One molecule behaves as a diene and the other as a dienophile. 

The reaction of diazirines 2 with the kinetically stabilized cyclobutadiene 1 gives 577-1,3-di-azepines 4 via the initial cycloadducts 3.159... 

For the cyclotrimerization of alkynes, several mechanisms have been proposed. The most plausible ones are a concerted fusion of three ir-bonded alkyne molecules, and stepwise processes involving a cyclobutadiene complex or a five-membered metallocyclic intermediate (98). In the case of the cyclotrimerization of a-alkynes it is possible to discriminate between a reaction pathway via a cyclobutadiene complex and the other reaction pathways, by analysis of the products. If cyclotrimerization proceeds via a cyclobutadiene complex and if steric factors do not affect the reaction,... 

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... 

It would be useful if triple bonds could be similarly epoxidized to give oxirenes. However, oxirenes are not stable compounds.Two of them have been trapped in solid argon matrices at very low temperatures, but they decayed on warming to 35 Oxirenes probably form in the reaction, but react further before they can be isolated. Note that oxirenes bear the same relationship to cyclobutadiene that furan does to benzene and may therefore be expected to be antiaromatic (see p. 58). 

For other reactions between cyclobutadienes and triple bonds to give Dewar benzenes, see Wingert, H. Regitz, M. Chem. Bet, 1986, 119, 244. 

Attempts to use in the alkynylation reaction not tin-substituted alkynes, but to couple 26 directly under the conditions developed by Heck, Cassar, Sono-gashira, and Hagihara, surprisingly enough gave rise to the formation of the corresponding amino-substituted cyclobutadiene complex 30 in good yields. 

The palladium catalyst is essential in this reaction, as was shown in control experiments to make sure that this was not a direct nucleophilic addition of the amine to the electron-poor (regarding the low lying LUMO ) cyclobutadiene ligand. A series of amino-substituted cyclobutadiene complexes have been synthesized by this methodology [29]. 

With application of reasonable values for trapping parameters and AS2, it was possible to bracket the enthalpy and entropy of activation for isomerization of cyclobutadiene. Hence, A/Zj was estimated to fall between 1.6 and lOkcal/mol, where the upper limit was consistent with theoretical predictions for square-planar cyclobutadiene. Most surprising, though, was the conclusion that AS for automeriza-tion must lie between -17 and -32cal/(molK), based on the AS values normally observed for Diels-Alder reactions as a model for AS2. ... 

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