Norbornadiene, 1,3,5-cycloheptatriene from

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. 

Such investigations gave a hint as to the structure of the protonated toluene, since it was observed (Field, 1967) that the ionic species from the reactions of toluene, cycloheptatriene and norbornadiene with CHf differ significantly, in that the amount of fragment ions increases progressively from toluene to norbornadiene. 

Bicyclo[2.2.1]heptadiene (norbornadiene) gives cycloheptatriene upon heating with log k = 14.68 — 50 610/2.3/ r. Also formed in the reaction is cyclopentadiene and acetylene, the retro Diels-Alder products with log k = 14.68 — 51900/2.3/ r and toluene with log k = 14.23 — 53 A0/23RT. Most likely, the initial reaction proceeds via cleavage of the C1-C7 bond to give a biradical which can form norcaradiene and then cycloheptatriene or undergo a hydrogen shift to toluene, but the retro 4 -h 2 reaction must result from C1-C2 (and C3-C4) bond fission (Scheme 8.18). 

Norcaradienes are intermediates in several types of reaction, including intramolecular and intermolecular additions of carbenes to aromatic rings, the chromic acid oxidation of cycloheptatriene to benzaldehyde, the thermal isomerization of 7-substituted norbornadienes to cycloheptatrienes, °° and the reaction of pyridine iV-oxides with benzynes. An interesting feature of the latter reaction is the transfer of oxygen from the pyridine to the benzene nucleus (Scheme 61). 

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