Cycloheptatriene electrocyclic closure

The two pathways, electrocyclic closure to the bicyclo[3.2.0]heptadiene and the [l,7]-shift of hydrogen, are in competition. This makes the experimental work more difficult, because the four possible isomers of the cycloheptatriene interconvert during the photolysis. Nevertheless the pattern of reactivity is clear, and the observation is that the better the donor substituent, the more it favours the formation of the bicyclo[3.2.0]heptadiene at the expense of the [l,7]-shift. This too is reasonable, since the hydride migration towards the donor-substituted atom 8.80 leads to a less stable cation, and ought to be slow. 

Norcaradiene formation from a cycloheptatriene corresponds to a 1,3-ring-closure. On the basis of this reaction, aminocyclopropane 317 was obtained as a solid in 94% yield from piperidine and the tropylium ion 315 (equation 74). A rapid equilibrium between 316 and 317 was postulated in solution. Electrocyclic 1,3-bond connections also were involved in the fluctual behaviour of 9-azabarbaralanes " , in the formation of homoazepines (from nitrenes and cycloheptatriene" " ) and in a 6-azabenz[10]annulene system . ... 

Problem 4.5. Cycloheptatriene undergoes rapid electrocyclic ring closure under thermal conditions. Determine the structure and stereochemistry of the product. 

A more detailed investigation of the photochemistry of 5-substituted 1,2-benzo-cycloheptatrienes has shown that [2,3]benzonorcaradienes can be obtained by [1,7]H shift. However, electrocyclic ring-closure across C-3—C-6 competes favourably, and (44) is unusual in that only the norcaradiene product is obtained... 

---