Biphenylene, rearrangement

By a similar reaction phenanthraquinone yields biphenylene-glycollic acid. (The equation should be written.) The benzilic acid rearrangement also plays a part in many other reactions (croconic acid, purpurogallin). 

The formation of reactive carbenes from alkylidene Meldrum s acids has also been observed. Thus pyrolysis of 1-indanylidene Meldrum s acid at 640 °C gave the corresponding carbene which further rearranged to benzofulvene and naphthalene (Scheme 12) <1998JA8315>. Similarly, EVP of 9-fluorenylidene Meldrum s acid at 1100°C provided a mixture of phenanthrene and biphenylene <1996TL6819>. 

It has been observed that sulfones do not decompose photochemically in the absence of sensitizer on irradiation above 3200 A so that the excited state is likely to be a triplet. It is possible that intersystem crossing to the vibrationally excited ground state occurs prior to bond breakage, but this is by no means certain . Occasionally rearrangement is observed in the decomposition of sulfones e.g., pyrolysis of dibenzothiophene-5,5-dioxide gives dibenzofuran instead of the de-sulfonylation product, biphenylene, viz. 

Benzyne (1) and cyclopentylidenecarbene (121) are isoelectronic. The formation of benzyne from 121 in the gas phase was first proposed in 1979, and the ensuing studies have recently been reviewed. The rearrangement of 121 -> 1 was suggested from pyrolysis results with the alkylidene Meldrum s acid 122. Besides high yields of the expected carbon dioxide, acetone and cyclopentadiene, substantial amounts of aromatic products such as biphenylene (27%) and triphenylene (12%) were obtained that were most easily accounted for as a consequence of rearrangement of carbene 121 to benzyne and subsequent di- or trimerization. 

Whereas diazotization of labelled anthranilic acid 130 at 84 °C (in the presence of excess unlabelled acid) gave biphenylene 131 without label rearrangement, pyrolysis of labelled phthalic anhydride 132 (again, with excess unlabelled anhydride, so that the presence of tetra-labelled biphenylene in the product is virtually nil) at 830 °C gave approximately a 1 1 mixture of 131 and 133. Similar labelling results were obtained in the pyrolysis of 134 cfll) at 650-830 °C. 

Addition of Carbenes. Addition of dichlorocarbene to either 1--methoxy- or 1,2-dimethoxy-biphenylene takes place in the ring bearing the substituents, occurring only at the bonds which the accepted formula of biphenylene implies as having the highest double bond character [168,172,173], These results have been taken to provide evidence of partial bond fixation in the biphenylene molecule [172,173] Earlier work had shown that the product of addition of ethoxy-carbonylcarbene to biphenylene was ethyl fluorene-2-carboxylate and its formation was rationalised in terms of an initial addition of the carbene to the 2,3-bond of biphenylene, followed by a series of rearrangements [125,174]. 

Now the subsequent rearrangement of (222) or (223) to (224) involves transition states isoconjugate with naphthalocyclobutadiene (225) or biphenylene (226). Each of these has two aromatic benzene rings and one antiaromatic cyclobutadiene ring. The latter will cancel the effect of one of the benzenes, so (225) and (226) should have resonance energies similar to that of benzene. The same should be true of the corresponding transition states for (222) and (223). 

A rhodium complex similar to the platinum complex described in (4) also showed an ability to catalyze formation of phenanthrenes from biphenylene and alkynes [22]. Cyclotrimerization was observed as a side reaction, and with silyl substituted alkynes some [1,2]-silyl rearrangements were seen, leading to [1,1] addition of the alkyne across the C-C bond (13). 

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