Cyclobutadiene, attack

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. 

Alkvnes do rot dimerize photochemically to give cyclobutadienes, but dimers are formed from arylalkynes under conditions of electron-transfer sensitization (2.105). These dimers arise from a reaction of the alkyne radical cation with ground-slate alkyne, followed by intramolecular electrophilic attack on the benzene ring. 

These have previously been obtained by electrophilic attack on ene-yl complexes [equation (a) Y = CH(C02Me)2, OMe ch = 1J-C5H5 diene = 1,5-cyclooctadiene]1 or by reaction of the compounds (diene)MBr2 with 57-C5H6Fe(CO)2Br (diene = 1,5-cyclooctadiene or 1,2,3,4-tetraphenyl-l,3-cyclobutadiene).2 An example of the former method is given in which the methoxy-cyclooctenyl derivative is used as the substrate and tetrafluoro-boric acid as the electrophile. The substrate is conveniently prepared and used without isolation, and in this way the reaction takes only a few hours, starting with dichloro(l,5-cycloocta-diene)palladium, prepared as described above. 

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. 

One exception to this statement is cyclobutadiene. This molecule has two degenerate (equal energy) HOMOs, each of which is singly occupied with an electron. Nucleophilic attack on other even polyenes is preferred according to Rule 1. Cyclobutadiene is, however, attacked in preference to odd polyenyl ligands. 

The Diels-Alder reaction of hexakis(trifluoromethyl)benzvalene with furan had been reported 63) but not its stereochemistry. We have examined this reaction and found that it proceeds smoothly with many 1.3-dienes if the steric requirement of the olefin is not large in the transition state where exo-attack occurs as confirmed by X-ray analysis (97)96). Therefore, 2,3-dimethylbutadiene reacts smoothly with hexakis(trifluoromethyl)benzvalene whereas 1,3-pentadiene reacts very slowly and 2,4-hexadiene does not at all. The cycloaddition to cyclobutadiene gives a mixture of endo- and exo-adducts (98)97) ... 

All these push-pull cyclobutadienes react readily with both electrophiles and nucleophiles [72,78,79]. With electrophiles reaction takes place at a ring atom carrying an ester group with nucleophiles ring-opening follows the initial nucleophilic attack, e.g. 

Many of the previously described reactions may well also go via an initial nucleophilic attack on the cyclobutadiene ring however, from the products the first step is usually not obvious. In some cases, however, notably attack by alkoxide or cyclopentadienide ions, metal complexes which have been fully characterized are isolated. 

While sodium cyclopentadienide attacks tetramethylcyclobutadiene-nickel chloride both at carbon and nickel (Section VI, F), the discovery of a novel cyclopentadienylation reaction which is in effect a ligand-transfer reaction involving attack at the metal only has allowed other types of cyclobutadiene complexes to be prepared. Thus on reaction of tetra-phenylcyclobutadienenickel and -palladium bromides (LXXXVI) with cyclopentadienyliron dicarbonyl bromide, the paramagnetic (cyclopentadi-enyl)(tetraphenylcyclobutadiene)nickel and palladium tetrabromoferrates (LXXXVII M=Ni, Pd) are obtained 64, <55). 

Substituted cyclobutenes (225) are formed by the reaction of tricarbonylcyclo-butadiene iron complexes (226) with electrophiles. With a chiral complex (226a) the cyclobutenes formed are also chiral, and the inference is that the reaction must involve attack of electrophile on complexed cyclobutadiene rather than on free cyclobutadiene. In contrast, the cyclobutadiene Diels-Alder adducts (227) produced by oxidation of the optically active complexes in the presence of dieneophiles are racemic, indicating the intermediacy of the free cyclobutadienes. ... 

In the first quantitative study of nucleophilic attack upon coordinated cyclobutadiene, Choi and Sweigart have reported kinetics for reactions (4) in nitromethane, using a wide range of spectator ligands (L) and various phosphorus nucleophiles (PR3). All the reactions obeyed equation (5), as... 

Then we set up the six simultaneous equations for the coefficients and the corresponding 6x6 secular determinant and apply the Hiickel approximations. Its form resembles that for cyclobutadiene in Exercise 10.33, but with six rows and six columns. Full-frontal attack on it to determine the six values of E is rather tedious, especially as there are procedures that make use of symmetry that greatly simplifies the solution. As should be verified, the energies and the corresponding (unnormalized) molecular orbitals obtained are as follows (Fig. 10.39) ... 

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