Cyclobutadienes thermolysis

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. 

Fritch, J. R., Vollhardt, K. P. C. Cyclobutadiene-metal complexes as potential intermediates of alkyne metathesis flash thermolysis of substituted 4-cyclobutadienyl- 5-cyclopentadienylcobalt complexes. Angew. Chem. 1979,91,439-440. 

The photoisomer of a-pyrone was synthesized in order to prepare a cyclobutadiene.64 It was pyrophoric in air at room temperature and its mass spectrum was similar to that of furan. Flash thermolysis was reported to give many compounds, as shown in Scheme 26.65 No evidence for the formation of cyclobutadiene was obtained, but carbon dioxide and acetylene products seem to support its intervention. 

Allenes can also be synthesized from diazocyclopropanes by chemical or photochemical [l,2,l]-elimination of nitrogen. In the thermolysis of 28 to give 30 the carbene-intermediate 29 could be trapped 18), and in the low-temperature photolysis of 31 the triplet carbene 32 could be detected by EPR-spectroscopy 17). 32 is longlived in a polycrystalline matrix and rearranges to 33 (28 %) at a temperature of —154 °C 19>. Numerous applications are included in Ref. 20). Especially noteworthy are the syntheses of stable cyclobutadienes by Masamune (90%)21) and Regitz (67%)22). 

The adduct generated from the benzvalene and butadiene was brominated and dehydrobrominated to a cyclohexadiene which seems to be a suitable precursor of tetrahedrane. However, its photolysis yields (2+2)intramolecular cycloadducts and its thermolysis gives only a cyclobutadiene dimer (99)99). 

Thermolysis or photolysis of the diazo cyclopropenes 51 4-167 yhich bear very bulky substituents leads to a fragmentation to two alkynes, although in related cases cyclobutadienes were formed, e.g. photolysis of 6. Photolysis of cyclopropenyl diazo ketone 7 also gave acetylene." ... 

Azete is iso- r-electronic with cyclobutadiene and is therefore the simplest antiaromatic het-eroannulene. It would be expected to be thermally unstable and extremely reactive, and as yet the parent compound has not been synthesized. In 1973, tris(dimethylamino)azete was described. In 1986, Regitz succeeded in synthesizing tri- er -butylazete by thermolysis of 3-azido-l,2,3-tri- er -butylcyclopropene [6] ... 

The approach to cubane by Pettit and coworkers also made use of two quasi-Favorskii rearrangements fScheme 7.4ST The reaction between cyclobutadiene and 2,5-dibromoquinone afforded exclusively the endo cycloadduct 205 in 80% yield. Subsequent photolysis afforded the [2+2]-photocycloadduct 206. When this compound was treated with aqueous KOH at 100 °C, an 80% yield of the diacid 38 was obtained. This diacid could be converted to cubane by conversion to the di-t-butyl perester followed by thermolysis, as per the method developed by Eaton and Cole. 

Thermolysis of the Dewar-benzene-cyclobutadiene adduct (601 R = H) proceeds via central bond cleavage, giving (602), whereas (610 R = Me) thermolyses by cyclobutene ring-opening. In the case of (603), cleavage of the central bond is impeded for steric reasons, and (604) is obtained. ... 

Figure 10.4. Correspondence diagram for fragmentation of Cyclobutadiene (D2/1). Solid lines thermolysis broken lines photolysis. (From Fig. 6 of reference [18])...

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