Some Conjugated Cyclic Polyenes

Furthermore, although delocalisation of (4n + 2) TV-electrons provides an important contribution to the overall stability of a conjugated cyclic polyene, it should be borne in mind that other contributing factors may in some circumstances nullify or override its effect as, for example, do steric factors in the case of cyclo-decapentaene, [10]annulene. 

The material in this chapter is composed almost entirely of concepts. There are few new reactions or synthetic procedures. We concentrate here on the special stability of some planar, cyclic, and fully conjugated polyenes. The special stability called aromaticity is encountered when the cyclic polyene has a molecular orbital system in which all degenerate bonding molecular orbitals are completely fiUed. 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look hrst at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cyclization of a conjugated acyclic polyene. One tt bond is broken, the other tt bonds change position, a new a bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclo-hexadiene, and a conjugated diene can be converted into a cyclobutene. 

Some cyclic conjugated polyenes do not satisfy the Hiickel rule and are not aromatic. Two examples are cyclobutadiene and cyclooctatetraene. Both are cyclic polyenes with alternating single and double bonds, but neither is aromatic. 

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. 

The middle part is always a conjugated polyene (symmetrical or unsymmetrical) and can be prepared by a number of established methods which are discussed in detail in Chapter 3 Part I. Some of these middle parts are readily available more common ones, e.g. the Cio-dialdehyde, are manufactured on a ton scale as industrial intermediates for the technical syntheses of (3,(3-carotene (3) and astaxanthin (406). For the synthesis of unsymmetrical carotenoids, the Cio-dialdehyde can be converted into a monoacetal derivative. The free aldehyde moiety is coupled with one end group, and the intermediate product is deprotected and then combined with the second end group. In these reactions, there are some positions which permit a coupling in high yield [C(9)-C(10) and C(11)-C(12)] and others [C(7)-C(8)] which, for cyclic carotenoids, give products in only low yield because of steric hindrance due to the adjacent methyl groups. The choice of the middle part and its synthesis has become a simple matter today. 

In odd polyenes, the sequence of conjugation nuy be interrupted at some places. These kinds of defects are referred to as soliions [21]. Defects are also formed by introducing electron-donor (Li, Na, K) or -acceptor (O, Rr. 0 dopants to influence electrk and magnetk properties. Cyclic, highly conjugated structures may be prepared by pyrolysis of polyacrylonilrile, CHr-CH—Cw N. for examine, resulting in an increase of... 

The special stability and reactivity associated with cyclic delocalization is not unique to benzene and polycyclic benzenoids. Thus, we shall see that other cyclic conjugated polyenes can be aromatic, but only if they contain (An + 2) tt electrons (n = 0, 1, 2, 3,. . . ). In contrast, An tt circuits may be destabilized by conjugation, or are antiaromatic. This pattern is known as Hiickel s rule. Nonplanar systems in which cyclic overlap is disrupted sufficiently to impart alkene-like properties are classified as nonaromatic. Let us look at some members of this series, starting with 1,3-cyclobutadiene. 

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