Acyclic polyenes

Thomas A. Albright, Jeremy K. Burdett, and Myung-Hwan Whangbo. 

The Tt orbitals of ethylene, assembled from two CH2 units on the left side. On the right side, the n orbitals of allyl, assembled from the corresponding orbitals of ethylene and a CH2 group. 

As their energy increases, the orbitals alternate in parity with respect to a mirror plane which is perpendicular to and bisects the n system. The lowest energy orbital is always symmetric with respect to this plane. 

Nodes must always be symmetrically located with respect to the central 

The JT orbitals of the first few linear chain polyenes. No attempt has been made to represent the different orbital coefficients. The orbitals are labeled either symmetric or antisymmetric with respect to the mirror symmetry which bisects the length of the chain. 

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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. 

The organization of Part Two is according to structural type. The first section, Chapter Seven, is concerned with the synthesis of macrocyclic compounds. Syntheses of a number of heterocyclic target structures appear in Chapter Eight. Sesquiterpenoids and polycyclic higher isoprenoids are dealt with in Chapters Nine and Ten, respectively. The remainder of Part Two describes syntheses of prostanoids (Chapter Eleven) and biologically active acyclic polyenes including leukotrienes and other eicosanoids (Chapter Twelve). 

Considering their potential antiaromatic character, thiepins are also of theoretical interest. HMO calculations of -resonance energies, using reference data from 27 acyclic polyenes containing sulfur, predict antiaromatic character for thiepin and 2-benzothiepin, whereas 1- and... 

Such reactions, in acyclic polyenes, can be generalised in the form ... 

The anodic oxidation of acyclic polyenes is practically useless, since the control of the reaction site is usually difficult and hence the product is often a mixture of isomers which are not always easily isolable. 

By contrast, the Dewar resonance energy represents solely the contribution coming from the cyclic electron (bond) delocalization since the model reference structure is represented not by a system of isolated 7r-bonds, but by a hypothetical cyclic polyene with the number of tr- and tr-bonds equal to that in a given molecule. Making use of the additivity of bond energies in acyclic polyenes (65JA692), one may calculate the total energy... 

The DRE calculation scheme takes into account only two types of CC bonds, not accounting for the fact that the energy of an acyclic polyene depends on its branching. The 7r-energy of a branched acyclic polyene Ev (BP) is related to the energy of an isomeric linear polyene E (LP) by (75JCP3399)... 

This expression shows that for acyclic polyenes TRE = 0. 

Aromatic, antiaromatic, and acyclic polyenes primarily differ in bond lengths, which serves as a basis for the structural indices of aromaticity reflecting the degree of alternation of bond lengths in a ring. 

Whereas the benzene molecule possesses a structure of D6A symmetry with equal lengths of the CC bonds, for acyclic polyenes alternation of bond lengths is a characteristic [87JCS(P2)S1]. For antiaromatic molecules, alternation is even more pronounced and unlike the aromatic molecules, a high-symmetry structure of the lowest singlet state of the antiaromatic molecules does not correspond to a minimum on the PES. For... 

A similar thermal ring opening reaction has been postulated for the conversion of AT-vinyl-1,2-dihydropyridine (46) to styrene (47) and triphenylpyridine. Ring opening to an acyclic polyene was also proposed (equation 16). This appears to be a general reaction of AT-vinyl-1,2-dih dro ridines (82JOC492). 

Cyclic polyenes absorb at higher wavelengths than do acyclic polyenes. 

Isoprenoid acyclic polyene carbocations with 6, 14, and 18 tt-electrons were synthesized.138 Considering also previous data for carotenoid dications, an equation correlating /.max (ranging from 400 to 1100 nm) and the number of sp2 hybridized carbon atoms in the dication was developed. Dication (67) and an analogue with a different alkyl substitution pattern were prepared as PF6 salts.139 These dications form... 

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... 

The dimerization of acyclic polyenes in which all n bonds are lost would lead to the open structures of (54) and (55). A schematic orbital correlation diagram (Fig. 15) for process (54) shows that allyl dimerization is improbable. The cyclization of higher acyclic polyenes, e.g. to cis-or trans-7 in (55), is subject toa similar prohibition, but the formation of 8 is allowed. In general, processes in which the products retain elements of symmetry inherent in the reactants are symmetry-forbidden the argument used to demonstrate this is analogous to that used for ethylene. One dimerization of 1,3-butadiene, namely to 9, is unique this... 

Gutman, I., Ruscic, B., Trinajstic, N. and Wilcox, C.W., Jr., Graph theory and molecular orbitals. Part 12. Acyclic polyenes, J. Chemical Physics 62 3399-3405, 1975. 

A beautiful example of electrocyclic reactions at work is provided by the chemistry of the endiandric acids. This family of natural products, of which endiandric acid D is one of the simplest, is remarkable in being racemic—most chiral natural products are enantiomerically pure (or at least enantiomerically enriched) because they are made by enantiomerically pure enzymes (we discuss all this in Chapter 45). So it seemed that the endiandric acids were formed by non-enzymatic cyclization reactions, and in the early 1980s their Australian discoverer, Black, proposed that their biosynthesis might involve a series of electrocyclic reactions, starting from an acyclic polyene precursor. 

Endiandric acid A has four rings and eight stereogenic centres and yet is formed as a single diastereoisomer in one step from an acyclic polyene And it s all controlled by pericyclic reactions. 

Quite often, when calculating the delocalization energies of cyclic polyene systems, the latter are compared with isoconjugated acyclic polyenes, e.g. benzene with hexa-triene etc. 

However, when carbon atoms number 14, 18, and more, a molecule becomes a planar aromatic system again. Many years of persistent research had been spent before the compounds C14H14, CigHig and others were obtained. The authors called them [14] and [18]annulenes respectively. [18]annulene is the first (after benzene) cyclic polyene with (4A + 2) K-electrons, whose planar conformation is stable at room temperature. It is less stable than benzene, but is more stable than the corresponding acyclic polyene (nonaene) ... 

First, we shall connect only two ribbons and only at one location. The result will be a new acyclic polyene fragment. Let us call it intermediate. If the interacting orbitals of the initial ribbons had an identical pseudosymmetry, e.g. 5, they would combine to form two MOs of the intermediate ribbon with different pseudosymmetries, WS and T A. In this case, the energy WS of the intermediate MO is lower, and the energy WA is higher, than the energies of the initial MOs (Fig. 26a). The same is true when two WA MOs interact (Fig. 266). 

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