Norcaradiene heptadiene

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). 

Pyrolyses of 7-alkoxy- and 7-phenyl-bicyclo[2,2,l]heptadienes at 140—175 °C provide almost quantitative yields of mixtures of the corresponding cyclohepta-trienes. Because of the absence of aromatic products, it is suggested the first step of these rearrangements involves a non-concerted, radical isomerization to the corresponding norcaradiene isomer. Photolysis of 7-methylenenorbomadienes has been... 

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