Dimerization of Cyclobutadiene

After CBD has been generated photo chemically in an argon matrix at low temperature and the matrix is thawed at 35 K, it dimerizes to 5T/n-tricyclo-[4.2.0.0 ]octa-3,7-diene (TCOD), [18] which calculations indicate to be less stable than its trans isomer [19, Table 1]  

Woodward and Hoffmann [4, p. 147] characterize the reaction as an Homed [ 4a H- 2a]-cycloaddition, in which 5 /n-TC0D is formed rather than its anti isomer as a result of secondary orbital interactions. The reaction was analysed by OCAMS [5, p. 598] for a nearly-coplanar axial approach of the two CBD molecules, leading to an impossibly strained dimer with axial symmetry and a puckered central ring. The axial dimer , however, is not intended to depict a real molecule, but is a purely formal model produced by anasymmetrization to D2 of either the syn (Cs) or the anti (C2) isomer, and can thus represent them both. The correspondence diagram [5, Fig. 7] then shows that relaxation to the syn isomer involves a lower investment in distortional energy, in agreement with experiment. 

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T. Bally, S. Bernhard, S. Matzinger, J.-L. Roulin, G. N. Sastry, L. Truttmann, Z. Zhu, A. Marcinek, J. Adamus, R. Kaminski, J. Gebicki, F. Williams, G.-F. Chen, and M. P. Fulscher, The Radical Cation of i-yn-Tricyclooctadiene and Its Rearrangement Products, Chem. Eur. J. 2000, 6, 858. A combination of CASPT2 calculations of UV spectra and B3LYP and CCSD(T) calculations of PESs is used to identify the intermediates formed sequentially in the rearrangement of the radical cation of the syn dimer of cyclobutadiene and to understand why they differ from those formed from the anti dimer. 

Similarly, cycloaddition of 4-toluenenitrile oxide to cyclobutadiene gave a 2 2 adduct,26 whose configuration was not determined, but it is believed that this adduct was formed from the cycloaddition of nitrile oxide to the C —C double bonds of the major dimer of cyclobutadiene, namely A > -tricyclo[4.2.0.02 5]octa-3,7-diene.26... 

Several transition-metal complexes of cyclobutadiene have been prepared, and this is all the more remarkable because of the instability of the parent hydrocarbon. Reactions that logically should lead to cyclobutadiene give dimeric products instead. Thus, 3,4-dichlorocyclobutene has been de-chlorinated with lithium amalgam in ether, and the hydrocarbon product is a dimer of cyclobutadiene, 5. However, 3,4-dichlorocyclobutene reacts with diiron nonacarbonyl, Fe2(CO)9, to give a stable iron tricarbonyl complex of cyclobutadiene, 6, whose structure has been established by x-ray analysis. The 7r-electron system of cyclobutadiene is considerably stabilized by complex formation with iron, which again attains the electronic configuration of krypton. 

Explain why the dimerization of cyclobutadiene is the fastest known bimolecular pericyclic reaction ... 

Figure 8. VB diagram (ref 37) describing the barrier for dimerization of cyclobutadiene. The barrier is small since the excitation gap, G, is small.

The dimerization of cyclobutadiene is a Diels-Alder reaction in which one molecule of cyclobutadiene acts as a diene and the other as a dienophile. 

Fig. 17. Orbital correlation for the dimerization of cyclobutadiene to cubane, a forbidden process.

Show the product of the Diels-Alder dimerization of cyclobutadiene. (This reaction is similar to the dimerization of cyclopentadiene, discussed in Section 15-11.)... 

Dehalogenation (1, 1278-1279). Treatment of either dibromocyclobutene or diiodocyclobutene (1) with zinc dust in DMF results in a vigorous exothermic reaction and formation of the two dimers of cyclobutadiene (3) and (4) in 60% yield, the former predominating. The simplest explanation of the reaction is that dehalogenation occurs to yield cyclobutadiene (2), which then undergoes a Diels-Alder type of dimerization.8... 

The dimer of cyclobutadiene is formed by a thermally allowed [4+2] cycloaddition reaction. One molecule of cyclobutadiene reacts as the diene and the other as the dienophile in a Diels-Alder reaction. 

Dehalogenation of dihalogenocyclobutenes was historically the first of these methods and has provided dimers of cyclobutadiene itself [45] and of tetramethylcyclobutadiene[5,6]. 

Oxidation of (XVIII) with ceric nitrate gives a dimer of cyclobutadiene and other products (Section VI, C). In the presence of acetylenes the bicyclohexadienes (CIII) can be isolated they isomerize to the benzenes (CIV) on heating. 

Figure 7.8 Prescribed symmetry coordinates for dimerization of Cyclobutadiene to Cubane. (a) D2/1 orientation (b) D2d orientation.

Phosphenated silica Ni(COD)j Dimerization of cyclobutadiene British Petroleum Co. Ltd., 1973... 

In dimerization of cyclobutadiene examination of secondary orbital interaction reveals that endo-approach leads to syn-dimer in contrast to exo-approach which provides ond-dimer. 

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