Cycloheptatrienes bicyclo -2,5-heptadiene

Bicyclo[2.2.1]heptadiene rearranges at elevated temperatures to cycloheptatriene and toluene. The reaction is facilitated by substituents at C-7 such as phenyl and alkoxy, in which case cycloheptatrienes are the dominant products. 

Exercise 21-22 Unlike the conversion of bicyclo[2.1,0]-2-pentene to 1,3-cyclo-pentadiene, bicyclo[4.1.0]-2,4-heptadiene is transformed to 1,3,5-cycloheptatriene very rapidly at low temperatures by what appears to be a wholly concerted mechanism. Account for this difference. 

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. 

It is evident that substituents play an important role in the photo-chemically induced cydization and migration reactions of conjugated trienes. Cycloheptatrienes isomerize to other trienes by migration and the direction of migration is frequently controlled by substituents present in the ring. The direction of cydization to the bicyclo[3.2.0]-heptadiene system is also dependent upon substituents. The relative competition of these two types of reactions has been shown to depend on the type of substituent present in the ring system. Most evidence indicates that these reactions are most likely excited singlet state reactions. 

BICYCLO(2.2.1)HEPTADIENE (121-46-0) Forms explosive mixture with air (flash point -5°F/-21°C). Elevated temperatures form cycloheptatriene and other isomers. Violent reaction with strong oxidizers. May accumulate static electrical charges, and may cause ignition of its vapors. 

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). 

Cycloheptatrienes are in many cases in rapid equilibrium with an isomeric bicyclo[4.1.0]heptadiene. The thermodynamics of the valence isomerism has been studied in a number of instances and some of the data are given below. Calculate the equilibrium constant for each case at 25 C. Calculate the temperature at which = 1 for each system. Are the signs of the enthalpy and entropy as you would expect them to be Can you discern any pattern of substituent effects from the data ... 

Method 5. Other Methods of Initiation Catalysts were prepared by the interaction of molybdenum or tungsten hexacarbonyl with bicyclo(2.2.1]heptadiene or cycloheptatriene (Si). They were described as being effective for the cyclopolymerization of [9] and [143]. 

---