Cross-Conjugated Polyenes

Poor correlations are obtained for cross-conjugated polyene systems such as... 

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 simplest cross-conjugated polyene is 122, 3-methylene-1,4-pentadiene or 1,1-divinylethylene itself. Accepting the analysis in Reference 2 that was made using Roth s data, we find this species to be some 23 kJ mol 1 less stable than the simplest conjugated polyene, 80, (li)-, 3,5-hexatriene or 1,2-divinylethylene. The next simplest cross-conjugated polyenes are 3-methylene-l,4,6-heptatriene, 123, and 3,4-dimethylene-1,5-hexadiene, 124, that would naturally be compared with ( , )- ,3,5,7-octatetraene,... 

In spite of the continued interest in cross-conjugated polyenes ( dendralenes ), of which more than 100 are known307, surprisingly few of these have been investigated by PE or radical ion spectroscopy. ... 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

Methods have been presented, with examples, for obtaining quantitative structure-property relationships for alternating conjugated and cross-conjugated dienes and polyenes, and for adjacent dienes and polyenes. The examples include chemical reactivities, chemical properties and physical properties. A method of estimating electrical effect substituent constants for dienyl and polyenyl substituents has been described. The nature of these substituents has been discussed, but unfortunately the discussion is very largely based on estimated values. A full understanding of structural effects on dienyl and polyenyl systems awaits much further experimental study. It would be particularly useful to have more chemical reactivity studies on their substituent effects, and it would be especially helpful if chemical reactivity studies on the transmission of electrical effects in adjacent multiply doubly bonded systems were available. Only further experimental work will show how valid our estimates and predictions are. 

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... 

Similar reactions lead to dehydroflexixanthin 566 272) and p,(3-carotene-2,2 -diol 567 273). Polyenes like 570 used in the combat against tumor, can be obtained from the phosphonium salt 568 and the formylpolyene ester 569 274). Applying the Wittig reaction, also branched unsaturated cross-conjugated carotenals 571 275) and the polyene derivative 572 276> can be prepared (Scheme 96). 

Other polyenes that have been synthesized using Wittig reactions include the sequiterpenes a- and /S-sinesal (81) and (82), cross-conjugated carotenals (83), and (84). ... 

Haeck, H.H., Kralt, T., and van Leeuwen, P.H., Syntheses of carotenoidal compounds. Part 1. Preparation of some substituted polyenes with a cross-conjugation. Reel. Trav. Chim. Pays-Bas, 85, 334, 1966. 

We have shown [13,79, 80] that the electronic spectra of c -l,3-buta-diene [81], cyclopentadiene, aromatic five-membered heterocycles [48,79], and norbornadiene [80] can be understood on the basis of a model with two interacting double bonds. Cyclopentadiene (CP) is the prime example of a ring-shaped molecule with a conjugated rr-electron system, and its structure can be related to that of short polyenes such as cM-1,3-butadiene (CB) and the simplest heterocycles, such as pyrrole (PY), furan (FU), and thiophene (TP). In the series, ci5-l,3-butadiene, cyclopentadiene, norbornadiene (NB), the latter is the most complex system, with the two ethylenic units coupled through indirect conjugation and 77, a interaction. One more system will be added here to the set of molecules with two interacting double bonds methylenecyclopropene (MCP) [(1) in Fig 3] the simplest cross-conjugated 77-electron system. 

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... 

The DTHDA reaction is a special case of the DTDA (diene-transmissive Diels-Alder) reaction in which one or more heteroatoms are contained within a cross-conjugated triene/polyene (heterodendralene) it-system, a dienophile it-system, or both. The DTHDA protocol is an efficient and attractive method for the stereoselective synthesis of a variety of heterocyclic systems. 

The merit and potential of the DTHDA methodology for heterocyclic synthesis can be widely extended by structural variations of both diene and dienophile the kind of heteroatoms, their number, and their positions in either the cross-conjugated triene/polyene ([3/ ] heterodendralene) or the dienophile framework, as well as the inter- or intramolecular cycloaddition mode, and control of regio-and stereoselectivities. 

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... 

For many, the true measure of DTDA sequences of cross-conjugated polyenes lies with their application in target-orientated synthesis. At present, only four examples have been reported in this area, each featuring a substituted [3]dendra-lene (or masked [3]dendralene) (Scheme 12.46). No doubt, as we advance our understanding and control of DTDA chemistry, many more examples will follow. 

Yet, compared to the other x-systems, the acyclic linear polyenes, and the cyclic oligoenes (i.e., the annulenes), cross-conjugated molecules have lived in the shadows so far. Acyclic linear polyenes are encountered widely in fatty acid, polyketide, and terpenoid natural product domains and the ultimate polyene, polyacetylene is a conductive polymer that was the focus of the 2000 Chemistry Nobel Prize. The annulenes played crucial roles in the development of practical and theoretical organic chemistry (aromatic and antiaromatic compounds), and serve as pivotal bulk chemicals in industry (benzene, toluene, the xylenes, styrene, etc.). 

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