Polyenes stabilization

C. W. Spangler, P.-K. Liu, and K. O. Havelka, Comparison of bipolaron-like charge-state generation in the oxidative doping of a,o>-dithienyl and a,w-di-phenyl polyenes stabilized by methoxy and meth-ylthio substituents, J. Chem. Soc., Perkin Trans. 2 1207 (1992). 

Among planar monocyclic fully conjugated polyenes only those possessing (4n + 2) TT electrons where n is a whole number will have special stability that be aromatic... 

Other, removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a sdyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. The key step, treatment of (257) with methyl aluminum chloride in methylene chloride at —78° C, followed by acylation and chromatographic separation, affords (258) in 55% yield (two steps). When this cyclization was attempted on similar compounds that did not contain the C7P-silicon substituent, no tetracycHc products were observed. Steroid (258) is converted to lanosterol (77) in three additional chemical steps (225). 

A modified definition of resonance energy has been introduced by Dewar (66T(S8)75, 69JA6321) in which the reference point is the corresponding open-chain polyene. In principle this overcomes the difficulties inherent in comparing observed stability with that of an idealized molecule with pure single and double bonds, as thermochemical data for the reference acyclic polyenes are capable of direct experimental determination. In practice, as the required data were not available, recourse was made to theoretical calculations using a semiempirical SCF-MO method. The pertinent Dewar Resonance Energies are listed in Table 30. 

Monocyclic conjugated polyenes are referred to as annulenes, and there exists ample experimental evidence to support the conclusions based on application of HMO theory to neutral and charged annulenes. The relationship between stability and structure in cyclic conjugated systems will be explored more fully in Chapter 9. 

Both thermochemical and MO approaches agree that benzene is an especially stable molecule and are reasonably consistent with one another in the stabilization energy which is assigned. It is very significant that MO calculations also show a destabilization of certain conjugated cyclic polyenes, cyclobutadiene in particular. The instability of cyclobutadiene has precluded any thermochemical evaluation of the extent of destabilization. Compounds that are destabilized relative to conjugated noncydic polyene models are called antiaro-maticf ... 

The Z,Z,Z,Z,Z-isomer is required by geometry to have bond angles of 144° to maintain planarity and would therefore be enormously destabilized by distortion of the normal trigonal bond angle. The most stable structure is a twisted form of the , Z,Z,Z,Z-isomer. MO (MP2/DZd) calculations suggest an aromatic stabilization of almost ISkcal for a conformation of the , Z,Z,Z,Z-isomer in which the irmer hydrogens are twisted out of the plane by about 20°, but other calculations point to a polyene structure. ... 

There are a number of other systems for eomparing the stability of eonjugated cyelie compounds with reference polyenes. For example, see L. J. Schaad and B. A. Hess, Jr., Pure Appl. Chem. 54 1097 (1982) J. Aihara, Pure Appl. Chem. 54 1115 (1982) K. Jug, J. Org. Chem. 48 1344 (1983) W. Griindler, Monatsh Chem. 114 155 (1983). 

The predictions of relative stability obtained by the various approaches diverge more widely when nonbenzenoid systems are considered. The simple Hiickel method using total n delocalization energies relative to an isolated double-bond reference energy (a + fi) fails. This approach predicts stabilization of the same order of magnitude for such unstable systems as pentalene and fulvalene as it does for much more stable aromatics. The HMO, RE, and SCF-MO methods, which use polyene reference energies, do much better. All show drastically reduced stabilization for such systems and, in fact, indicate destabilization of systems such as butalene and pentalene (Scheme 9.2). 

In contrast to the significant resonance stabilization of azulene, pentalene and heptalene are indicated to be destabilized relative to a reference polyene ... 

The fulvalene systems are not predieted to be aromatie by any of the theoretieal estimates of stability. Even simple resonanee eonsiderations would suggest polyene behavior, sinee only dipolar resonance stmctures can be drawn in addition to the single nonpolar stmeture. 

In general conclusion, the HMO and SCF methods both appear able to make reasonably accurate predictions about the stabilization in conjugated moleeules. The stabilization is general for benzenoid compounds but quite restricted in nonbenzenoid systems. Because the HMO method of estimating stabiUty is based on the ideas of HMO theory, its general success vindicates the ability of this very simplified MO approach to provide insight into the structural nature of the aimulenes and other conjugated polyenes. More sophisticated MO methods, of course, are now accessible and should be applied for more detailed analysis of the structures of these molecules. 

Braun [22] showed from ozonolysis that for fractions of bulk PVC the number of internal double bonds and the rate of thermal degradation, although dependent on each other, were independent of the molecular weight. This clearly demonstrated the role of internal unsaturation on the stability of the polymer. After careful chlorination of the double bonds, an increase in thermal stability was observed and the number of double bonds as determined by oxidation with potassium permanganate were reduced. It was also shown that one polyene sequence was formed from each isolated double bond. 

In the stabilization of PVC, the principal mode of action of the various stabilizer systems has been explained in terms of the Frye and Horst mechanism, i.e., substitution of labile chlorines by more stable groups. Evidence for other actions, such as HCl neutralization, addition to polyene sequences, and bimetallic complex formation have also been given. Despite the wide acceptance of the Frye and Horst mechanism, researchers have frequently contended that this could not be the dominant mechanism in the stabilization of PVC. 

In 1982 the present author discovered cyclic orbital interactions in acyclic conjugation, and showed that the orbital phase continuity controls acyclic systems as well as the cyclic systems [23]. The orbital phase theory has thus far expanded and is still expanding the scope of its applications. Among some typical examples are included relative stabilities of cross vs linear polyenes and conjugated diradicals in the singlet and triplet states, spin preference of diradicals, regioselectivities, conformational stabilities, acute coordination angle in metal complexes, and so on. 

Hydrogen abstraction — The abstraction of a hydrogen atom H from a saturated carbon atom in a position allylic to the polyene chain can generate a resonance-stabilized neutral radical by homolytic cleavage of a C-H bond CAR = X - H. Then X - H -H R- X + RH. 

In the carotenoid radicals, the unpaired electron is highly delocalized over the conjugated polyene chromophore. This has a stabilizing effect and also allows subsequent reactions. The cation and anion radicals can be detected by their characteristic spectral properties, with intense absorption in the near-infrared region. 

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