Propellanes thermal stability

Considering their high expected strain energy, [1.1.1 [propellane and its derivatives possess substantial thermal stability. The neat parent hydrocarbon la is moderately stable at room temperature, particularly in the absence of air, but after a few hours, a considerable amount of polymer builds up36 37. A 3-5% solution can be stored in a refrigerator for weeks without noticeable losses. 

The unusual high thermal stability is observed in highly fluorinated [2.2.2] propellane (38) in contrast to the unstable hydrocarbon analogue (39). The fluorinated propellane 38 is extremely thermally stable andso nochangeis observed evenat 100°Cfor lOh [12]. In contrast, nonfluorinated propellane 39 undergoes retrocycloaddition to yield dimethylenecyclohex-anes (40) and (41) with a half-life of 25 min at 25°C (Scheme 1.40) [13]. 

In this paperthe relative stabilities of various small-ring propellanes are discussed in terms of enthalpies of hydrogenolysis of the conjoining bond and dissociation energies of this bond in the various substrates. This is perhaps the place to state that the mechanism of addition of bromine, in the dark, to the conjoining bond of several [m.n.l]propellanes, has been discussed in generaF. It is concluded that thermally initiated low temperature radical chain addition to the cyclopropane rings is involved. 

Other interesting examples of compounds with this structural feature are the propellanes, such as [l.l.ljpropellane (23). A theoretical analysis indicated that the C1-C3 bond has a bond order of about 0.7 and that the orbitals that comprise the bond have much p character. ° The compound has surprising stability, with a half-life for thermal rearrangement of 5 minutes at 114°. This example illustrates that we should not necessarily equate high strain with high chemical reactivity, because there may be a high activation barrier for chemical reaction, even for a very strained molecule. 

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