H-Naphthalene and Their Derivatives

In the case of the [4+ 2]-cycloadditions, the diradical analogous to 172 should contain an allyl radical subunit in the side-chain having the Z-configuration. There the closure of the six-membered ring occurs also employing the central carbon atom of the pentadienyl radical system. A quantum-chemical study reproduced the preference of the step 172 —y 163 over that from 172 to 173 [47]. This may have its origin in the higher spin density at C3 of the cyclohexadienyl radical as compared with Cl and C5 [108]. 

In spite of the fact that the yields of the trapping products of 162 are far from quantitative, a dimer of 162 or an oligomer has never been observed. Most probably, 162 oligomerizes unspecifically or polymerizes. 

The discrepancy between calculated and experimental AHf° value of 162 was resolved as it turned out that the latter has to be considered as the upper limit resulting from the assumption of the interception of 162 by 302 under encounter control. However, the numerical simulation of the experimental rate data was also perfectly successful by imposing an activation barrier of 7 kcal mol-1 on the trapping step [13], This brought the AH/ of 162 down to 78 kcal mol-1 above that of benzene [13], in good agreement with calculated values [13,116,117]. 

If both ethylene subunits of a l,3-hexadien-5-yne derivative are members of benzene entities, the cycloaromatization, caused by flash vacuum thermolysis, may give rise to bowl-shaped molecules such as corannulene or semibuckminsterfullerene. However, in those cases, the initial step is not a Hopf cydization but an isomerization of the ethynyl to a vinylidenecarbene group [125]. 

Diels-Alder reaction of a pair of phenylacetylene moieties such as 204. 

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