Partial bond fixation

Oxadiazole (1 R = R = H) is calculated to be 8.9 kcal moU less stable than its open chain tautomer (2) (85AG(E)713>. On the other hand, 1,2,3-benzoxadiazole is about 1 kcal mol more stable than 2-diazocyclohexadienone (91JST(247)135>. The position of the equilibrium is markedly affected by substituents. The benzoxadiazole (4) is more stable than its tautomer (5) by about 1.5 kcal mol but tetrafluorobenzoxadiazole is substantially less stable than the corresponding diazocarbonyl isomer. The open-chain forms are also favoured by polar solvents and by hydrogen bonding. In the naphthalene series, the stabilization of the fused oxadiazole (6) relative to its open chain tautomer can be ascribed to partial bond fixation in the naphthalene system, which disfavours... 

Some pK values are collected in Table 2. Thiones are ca. 2 pK units more acidic than the corresponding azinones. Fused benzene rings have little effect except in the 3-substituted isoquinoline series where partial bond fixation lowers the acidity. Additional aza substitution increases the acidity significantly. 

The effect of one substituent on the reactivity of another is generally similar to that observed in the corresponding polysubstituted benzenes. However, the partial bond fixation in an azole can lead to differential effects in the mutual interactions of substituents, similar to those found in naphthalene where the benzene ring fusion induces bond fixation. 

It would be of interest to find a fused system where CC bonds have the same hybridization and the partial bond fixation is produced by 7r-localization only. There is, fortunately, such a molecule which is shown in Fig. 9. Although triphenylene itself does not represent a MN-system in a sense of the definition given in section 2.1 belonging actually to... 

The biphenylene system has interested theoretical chemists for over 20 years. LCAO-MO calculations93 c 18>62,63,69 predicted that the four-membered ring is closer in structure to tetramethylenecyclobutane than to cyclobutadiene. Structural analyses70 have supported this prediction and have shown 56 to be completely planar with the bond-lengths and angles shown in Fig. 2. Evaluation of the proton NMR spectrum of biphenylene indicated that the partial bond fixation shown by... 

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]. 

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