Cyclic polyenes Cycloheptatrienes

By analogy with azepines and 1,2-diazepines <8iH(i5)i569>, the fully unsaturated 1,4-diazepines with their 8ti electron system should not be aromatic but should behave as cyclic polyenes and, hke cycloheptatriene should have a chair or boat conformation. If the ring system were able to become planar, one would expect the fully unsaturated 1,4-diazepines to be anti-aromatic. Additional evidence for the nonaromatic nature of the 1,4-diazepines is provided by the NMR spectra, where the ring protons are found in the typical range for alkenic protons, i.e. 5 5.0-7.0. 

But not all ring compounds containing double bonds behave in this way. Some cycloalkenes react as expected cycloheptatriene and cyclooctatetraene are examples of this kind of cyclic molecule. Others are exceptionally unstable cyclobutadiene is the archetype of this kind of cyclic polyene (Fig. 13.3). 

Cyclic polyenes are expected to show similar behavior thus, cyclopentadiene has El I of 9.00 e.v. rather similar to the methyl-butadienes and lower than that of butadiene itself. Cycloheptatriene, however, has El / = 8.55 e.v., somewhat higher than that of hex-atriene even allowing for the difference between UV and El determinations. The value is intermediate between that which would normally be expected for a cycloheptatriene structure ( 7.89 e.v. (47)] and that of toluene, El / = 9.23 e.v. (46), and suggests that tropylidene or tropylidene cation actually has r-bonding between the 2- and 7-positions, VI. 

Mass spectrometry of certain cyclic dienes and polyenes desowes special discussion owing to their prototypical isomerization and fragmentation behaviour. Among them, CsHe" " ions from 1,3-cyclopentadiene, CeHg" " ions from the cyclohexadienes, C7H8" " and C7H9+ ions from 1,3,5-cycloheptatriene and its isomras, as well as ions derived from... 

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