Polyenyl stability

Evidence has accumulated from various sources which supports the idea that polyenylic cations are implicated in other aspects of PVC degradation. Molecular orbital calculations carried out by Starnes (45) show that the charge is better stabilized at the center of the formal delocalized length of the ion than at the end and that since the process represented by equation 16 becomes more favorable with increasing sequence length, this may provide an explanation for the relatively short sequence lengths. 

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. 

A minor part was assigned to radicals—CH2—CH=CH—CH—CH2— formed by hydrogen abstraction from one of the a methylene groups. These radicals were not resonance-stabilized at -196° C due to steric hindrance. On heating, a singlet spectrum similar to the one observed for 1,4-polyisoprene was observed and interpreted as due to polyenyl radicals -fCH=Cf% H—. 

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. 

At the present time, it is not known whether any of the common PVC stabilizers do, indeed, react preferentially at the centers of polyenyl cations. However, it would seem that this possibility should be amenable to experimental tests. 

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. 

The stability of the polyene complexes L toward dissociation is in general Jess iJian iJiat of polyenyl complexes h X. because the free polyene is usually... 

Polyenyl complexes the most common is cyclopentadienyl (Cp), usually pentahapto (sometimes mono- or trihapto) CsMcs (Cp ) stabilizes the complexes indenyl facilitates the trihapto mode. 

Many other species are stabilized in 18-electron organometallic complexes car-benes and carbynes, enyls and polyenyls (XL ligands), o-xylylene (o-quinodime-thane), trimethylenemethane, benzyne, norbornadiene-7-one, cyclohexyne, 1,2-di-hydropyridines (intermediates in biological processes), thermodynamically unfavorable organic tautomers such as vinyl alcohols [less stable by 14 kcafrmol (58.5 kJ mol ) than their aldehyde tautomers], aromatic anions resulting from deprotonation in juxta-cyclic position such as tautomers of phenolates and benzylic carbanions. All these species have a specific reactivity that can lead to synthetic applications in the same way as cyclobutadiene above. 

Figure 6 Dependence of non-bonded resonance stabilization energies, Fnobo, of linear a.ll-trans polyenyl cations CH2-(CH=CH) -H on the chain length n...

---