1,3-cyclobutadiene generation

Cyclization, solvolytic, 54, 84 Cycloalkene oxides, 1-methyl, conversion to exocyclic methylene alcohols, 53, 20 Cyclobutadiene, generation in situ, 50, 23... 

Cyclopropene provides isomeric tricycles upon reaction with cyclobutadiene generated from its iron tricarbonyl complex (equation 54) but this thermal reaction is more likely to... 

The detailed structure of cyclobutadiene continues to be the subject of interest. Both Masmune and Maier have commented on differences in the i.r. spectrum of cyclobutadiene produced from different precursors. The assignment of a square ( 4fc) structure to cyclobutadiene rests on an analysis of the spectrum of cyclobutadiene generated by photolysis of the lactone (5) in an argon matrix at 4—10 K. The material thus prepared shows four bands at 3400, 1240 (or 1236), 650 (or 643), and 570 (or 573)... 

Cyclobutadiene is expected to have strong donor characteristics, and the cyclobutadiene generated by the photolysis of (8) does seem to be complexed with the phthallic anhydride which is also formed. The 1240 cm" band, in this case, has a shoulder at 1238 cm" and the 570 cm" band is broadened, possibly because the four out-of-plane C—H vibrations are no longer degenerate in the complexed species. Complex formation is also reflected in changes in the properties of the phthallic anhydride, as the anhydride vibrations are distinctly different from those of free phthallic anhydride observed under the same conditions. The photolysis product also shows a u.v. absorption consistent with complex formation. 

Cyclobutadiene generated by cerium (iv) oxidation of the cyclobutadieneiron tricarbonyl complex undergoes a [4 + 2] addition to tropone to give the endo-adduct (179). Tropone reacts with l,3-diphenylindan-2-one to give the decarbonyl-ated [4 + 2]adduct (180). " The activation volume of the [4 + 6] cycloaddition of tropone to cyclopentadiene is —7.5+1 cm mol in p-dioxan at 60 °C and the overall reaction volume is —4.3 1 cm mol . The small partial molar volume of tropone is responsible for these small negative values. ... 

Cyclobutadiene escaped chemical charactenzation for more than 100 years Despite numerous attempts all synthetic efforts met with failure It became apparent not only that cyclobutadiene was not aromatic but that it was exceedingly unstable Beginning m the 1950s a variety of novel techniques succeeded m generating cyclobutadiene as a transient reactive intermediate... 

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). 

It is of interest to consider at this point some of the specific molecules in Scheme 9.2 and compare their chemical properties with the calculated stabilization energies. Benzo-cyclobutadiene has been generated in a number of ways, including dehalogenation of... 

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 compound was generated (as an intermediate that was not isolated) and two isomers were indeed found. The cyclobutadiene molecule is not static, even in the matrices. There are two forms (52a and 52b) that rapidly interconvert. ... 

Secondly, the carbon framework holding the exocyclic double bonds could be extended. This is demonstrated by naphtharadialene 5, a highly reactive intermediate which has been generated by thermal dehydrochlorination from either the tetrachloride 178 or its isomer 179106. Radialene 5 has not been detected as such in these eliminations rather, its temporary formation was inferred from the isolation of the thermolysis product 180 which was isolated in 15% yield (equation 25). Formally, 5 may also be regarded as an [8]radialene into whose center an ethylene unit has been inserted. In principle, other center units—cyclobutadiene, suitable aromatic systems—may be introduced in this manner, thus generating a plethora of novel radialene structures. 

The reaction of alkynes with AIX3 at —78 °C has been shown, by NMR spectroscopy, to generate a zwitterionic cr-cyclobutadiene aluminum species 225 (Scheme 58)211a. Transfer of the cyclobutadiene ligand from 225 to a variety of transition metals has been reported211. 

Flash vapor pyrolysis of the rf -thiophene l,l-dioxide)cobalt complexes results in extrusion of SO2 to generate (cyclobutadiene)cobalt complexes (Scheme 63)229. The absence of ligand crossover products indicates that this reaction occurs in a unimolecular fashion. Pyrolysis of the diastereomerically pure complex 240 gave the cyclobutadiene complex as an equimolar mixture of diastereomers 241a and 241b. In addition, the recovered starting material (37%) was shown to have ca 40% scramble of the diastereomeric... 

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