Stability of polyene and polyenyls

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

It is this mechanism which forms the primary focus of the present study, and we shall be particularly concerned with the effects of unsaturation on the electronic structures, equilibrium geometries, and energetics of the polyenyl cations and the polyenes derived therefrom. Some of the more practical aspects of the degradation and stabilization of PVC will then be discussed in terms of our results. The question of initiation by defect structures will be considered elsewhere, and we shall be mainly concerned in this work with the propagative or "unzipping phase of the mechanism. 

When the metal fragment is a poor ir base, the L model (5.4) applies and the vinylic carbons bound to the metal behave as masked, metal-stabilized carbonium ions. In such a case we often see nucleophilic attack (e.g., Eq. 5.10)." This is an example of a more general reaction type—nucleophilic attack on polyenes or polyenyls, and will be discussed in more detail in S tion 8.3. 

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