Conjugated polyenes acyclic

TABLE 5. Structural parameters determined for acyclic conjugated polyenes (distances in A, angles in degrees)... 

By connecting double and single bonds, formally five classes of hydrocarbons can be constructed which differ considerably from one another not only chemically and physically but also in terms of their practical significance [1] the linear polyenes 1, the annulenes 2, which consist exclusively of endocyclic double bonds , the radialenes 3, polyolefins which are characterized by semicyclic double bonds, the fulvenes 4, hybrids containing endo-and semicyclic double bonds, and finally, the dendralenes 5 [2] which are acyclic cross-conjugated polyenes... 

To produce more reliable predictions of aromaticity, Hess and Schaad (following a suggestion of Dewar) calculated delocalization (resonance) energies of cyclic hydrocarbons by comparing the compounds Htickel-theory with a value calculated for a hypothetical acyclic conjugated polyene with the same number and kinds of bonds as in a localized structure of the cyclic hydrocarbon. [B. A. Hess and L. J. Schaad, J. Am. Chem. Soc., 93, 305, 2413 (1971) 94, 3068 (1972) 95, 3907 (1973) B. A. Hess, L. J. Schaad, and C. W. Holyoke, Tetrahedron, 28, 3657, 5299 (1972) Schaad and Hess,... 

The degree of unsaturation in the molecule, i.e. the length of the conjugated polyene chromophore, has a substantial effect on retention times. Thus the acyclic hydrocarbons of the biosynthetic desaturation series, namely phytoene, phytofluene, tetrahydrolycopene, neurosporene and lycopene are well resolved, as are compounds having the same substituent group but different levels of desaturation, e.g. the 1-hydroxy-derivatives of these hydrocarbons. The 1,2-dihydrocarotenes, found in Rps. viridis are eluted later than their parent hydrocarbons e.g. 1,2-dihydroneurosporene after neurosporene. 

Dendralenes [24] (acyclic cross-conjugated polyenes) have been used as dienes in tandem Diels-Alder reactions, and a methodology for the synthesis of highly functionalized angularly anel-lated aromatic compounds has been developed (Scheme 16.23) [25]. A tandem double Diels-Alder reaction of DMAD with [3]dendralene followed by oxidation with DDQ gave the tetramethyl ester... 

Dendralenes, acyclic cross-conjugated polyenes having a nonplanar structure, containing DT rings (DT[ ]dendralenes) have received considerable attention as novel multistage redox systems. The [3]- and [4]-dendralenes containing... 

Scheme 4-156. Stereospecific 1,3-migration of an tricarbonyliron fragment along acyclic conjugated polyenes.

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. 

Sterically strained conjugated acyclic dienes and polyenes. 

We first consider benzene, 17, the prototypical aromatic molecule. From the entries in Table 3.21 and comparisons analogous to Example 3.15, one can recognize that conjugative stabilizations in benzene are significantly stronger than those of comparable species in Table 3.19. Thus, on a per-pi-bond basis, the estimated stabilizations in benzene are 40.8 kcal mol-1, more than three times those of diene 2 (12.8 kcal mol-1), twice those of the acyclic triene 11 (20.3 kcal mol-1), and about 58% greater than the most strongly stabilized polyene, 16 (25.8 kcal mol-1). 

The it energy of a non-classical conjugated hydrocarbon can be compared directly with that of a classical analogue by the PMO method.14 Consider an even monocyclic polyene. This can be formed by fusion of methyl with an odd AH with one atom less. These components can also be fused to form an acyclic polyene. Comparison gives the aromatic energy of the cyclic system by difference. In this way we find that rings with An + 2 atoms are more stable, and those with An atoms less stable, than analogous acyclic compounds. The same method can be used for the bicyclic systems XVII, XIX, XXI, XXII, XXIII. The procedure is indicated below... 

---