Benzene, aromatic-Cope rearrangement

Compared to the example above, the participation of benzene in the aromatic-Cope rearrangement has been more difficult to achieve. However, it has recently been utilized for the preparation of helicenes and other polycyclic compounds.One of the first and most impressive demonstrations of this reaction was utilized in the synthesis of tricyclic compounds such as 252. As above, the anionic oxy-aromatic-Cope rearrangement of 251 proceeded in excellent yield to give 252, resulting from rearomatization of the benzene ring. 

The Cope rearrangement mechanism can be also strongly affected by other substituents. Thus, the normal electrocyclic process in the thermal isomerization of divinyl aromatics has been suppressed relative to the thermolysis of l,2-bis(trifluorovinyl)naphthalene 438 (in benzene, at 193 °C, 24 h)231. Three major products 440-442 were isolated from the reaction mixture, but none of them was the expected product 439. Also formed in low... 

NMR and kinetic studies have been carried out on the antibody-catalysed oxy-Cope rearrangement of hexadiene (100) to aldehyde (101). An aromatic oxy-Cope rearrangement involving a benzene ring [see (102) (103)] has been observed to... 

In a later report by Hill and Newkome105, iV-allylenamines 134, prepared from 133, are transformed to 2-butenylquinolines 135 in refluxing benzene (equation 29). The lower temperature required for the 3-aza-Cope rearrangement is due to the regain of aromaticity of the quinoline ring in the conversion of 134 to 135. 

Figure 3.5. Reaction lattices of benzene like aromaticity and of the sigmatropic Cope rearrangement.

Addition of carbenes to aromatic systems leads to ring-expanded products. Methylene itself, formed by photolysis of diazomethane, adds to benzene to form cycloheptatriene in 32% yield a small amount of toluene is also formed by an insertion reaction. The cycloheptatriene is formed by a Cope rearrangement of the intermediate cyclopropane (a norcaradiene). More satisfactory is the reaction of benzene with diazomethane in the presence of copper salts, such as copper(I) chloride, which gives cycloheptatriene in 85% yield (4.87). The reaction is general for aromatic systems, substituted benzenes giving mixtures of the corresponding substituted cycloheptatrienes. 

---