Heptatrienes, isomerization

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

The formation of the same cyclopropylamine from 2-methyl-l,3-pentadiene as from 4-methyl-l,3-pentadiene (entries 2 and 3 in Table 11.11) can most probably be attributed to initial isomerization of the former to the latter under the conditions employed. The fact that the conjugated 6-methyl-l,3,5-heptatriene yields only the 2,3-dialkenylcyclopro-... 

Much experimental and theoretical work has been performed with the two allenes 1,2,6-heptatriene (32) and 1,2,6,7-octatetraene (34). Thermal isomerization of 32 leads to 3-methylene-l,5-hexadiene (346), a process that at first sight looks like a typical Cope rearrangement. However, trapping experiments with either oxygen or sulfur dioxide have shown that at least half of the rearrangement passes through the diradical 345 (Scheme 5.52) [144],... 

The cycloaddition of 4-methyl-3//-l,2,4-triazole-3,5(4/f)-dione to spiro[fluorenyl-l,3,5-cyclo-heptatriene] at — 10°C gives the norcaradiene-type cycloadduct, monitored by NMR spectroscopy, which by warming at room temperature is converted to an isomeric triazolidinedione, whose structure is confirmed by X-ray analysis22. 

An experimental distinction among these three alternatives proved to be difficult, particularly as norcaradienes and cycloheptatrienes usually interconvert easily via valence bond isomerization (norcaradiene - cycloheptatriene EA = 6.5 kcal/mol log A = 11.8) (64) and cycloheptatriene ring inversion (1,3,5-cyclo-heptatriene EA - 6.3 kcal/mol) (Figure 4 ref. 65). Therefore it was necessary to resort to such devices as introducing chirality into the molecules by way of an additional stereochemical label. 

Substituted norbornadienes isomerize at 170°C to substituted cyclo-heptatrienes , viz. 

The energy of activation for isomerization of norbornadiene itself is considerably largeThe reaction is mechanistically complex, and yields toluene and decomposition products as well as cycloheptatriene. Under the conditions used, part of the toluene is formed by isomerization of the cyclo-heptatriene , possibly via norcaradiene. In the gas phase the energy of activation for formation of cycloheptatriene from norbornadiene is about 51 kcal.mole and log is about 14.8 (refs. 24, 190). Activation parameters for the formation of toluene directly from norbornadiene in the gas phase are Ecf = 53 kcal.mole and log A — 14.2. These reactions probably involve initial cleavage of the C-1, C-7 bond in norbornadiene to yield an allylic diradical which can cyclize to norcaradiene (a precursor for both cycloheptatriene and toluene) as well as undergo other reactions. 

Treatment of the tricyclic dienones (149) with l,5-diazabicyclo[5,4,0]undec-5-ene (DBU) gives the 3,4-benzocycloheptadienone (150). The mechanism suggested involves the formation and selective aromatization of isomeric fused cyclohexadiene-cyclo-heptatriene systems via the norcaradiene tautomer (151), The polycyclic nitriles (152) fragmented on treatment with pyridinium acetate to give the substituted cyclo-... 

C-Labelling showed that the thermal conversion of the phenyl trimethylsilyl diazo-compound (168) into (169) involved isomerization of a phenyl carbene into a cycloheptatrienylidene, silacyclopropanes were not involved. It was shown that the P-naphthyl carbene obtained by rearrangement of 4,5-benzocycloheptatrienylidene enters the singlet-triplet manifold as a singlet. Evidence has been reported that aryl carbenes can show both electrophilic and nucleophilic properties in their intramolecular ring expansion. Low-temperature photolysis of the sodium salt of the toluene-p-sulphonyl hydrazone (170) gave, by intramolecular carbene addition, the dibenzobicyclo[4,l,0]heptatriene (171) which was trapped by buta-l,3-diene. ... 

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