Polyenes nonconjugated—

Section 9-9B covers qualitative explanations of how the VB method is used to account for the lower-energy (longer-wavclength) radiation required for electronic excitation of conjugated polyenes compared to nonconjugated polyenes. Thus 1.3-butadiene has a max for ultraviolet light at 217 nm, whereas 1,5-hexadiene has a corresponding max at 185 nm. 

Oxidative coupling of organometallic precursors with alkenes, alkynes, conjugated and nonconjugated polyenes and polyalkynes remains the main method of synthesizing various classes of five-membered rings with other elements. However, some alternative methods do exist. 

Figure 3.5 Major site of reaction for nonconjugated polyenes in AD reactions.

Intramolecular carbaluminations of olefins are facile (in the absence of ether) and show synthetic potential . This combines monohydroalumination (see 5.3.3.4.1) of nonconjugated polyenes and carbalumination leading to the synthesis of various mono- ... 

Rotational isomerism normally complicates the study of gaseous nonconjugated dienes and polyenes because many conformers appear simultaneously, and hence only few structures of free molecules in this category have been studied. 

TABLE 1. Structural parameters for 1-butene (1-BU) and nonconjugated acyclic dienes and polyenes... 

Table 3 lists all polyenes whose radical cations have been investigated by one or other of the above-described techniques and some of the structures listed are shown below the table. Note that some nonconjugated dienes do not retain their structure upon ionization [e.g. semibullvalene 104 (equation 61) or the cyclopentadiene dimers 126 and 294 (equation 62)] but break a bond to form a bisallylic radical cation, a rather common tendency of radical cations that have this possibility. 

Nonconjugated dienes and polyenes undergo double-bond migration to form the most stable conjugated system in the presence of basic catalysts. Dienes in which the two double bonds are separated by only one carbon atom exhibit the highest reactivity. Double-bond shifts in dienes with more separated double bonds take place in consecutive steps under catalytic conditions that is, only the monoanion is formed.122... 

Nonconjugated dienes and polyenes have triplet photochemistry which may be considered to arise from intramolecular interaction of one excited double bond with an isolated ground-state double bond. For example, the photocyclization of enrfo-dicyclopentadiene can be effected using acetone as a sensitizer.286 Other more flexible 1,5-dienes, when sensitized to triplet states, cross couple to yield bicyclo[2.1.1]-hexane structures. For instance, triplet mercury atoms convert both 1,5-hexadiene and 1,5-cyclooctadiene to such structures.267 Irradiation of the cyclooctadiene in the presence of cuprous chloride produces the tricyclo derivative in good yield266 but recent evidence again indicates that this latter reaction may proceed via free-radical intermediates.269... 

Numerous dienes and polyenes have been subjected to hydroxymercuration-demercuration.311 With nonconjugated dienes, the products can usually be predicted by applying what one has learned from the corresponding simple alkenes. Isolated double bonds are more reactive than conjugated double bonds and frequently one of the double bonds is sufficiently more reactive than the others that monohydroxy-lated products can be obtained. Improvements in selectivity have been reported by using mercury(II) tri-fluoroacetate339 or by adding sodium lauryl sulfate.340... 

Nonconjugated dienes or polyenes can be monomercurated if the diene or polyene is employed in excess, but more frequently these compounds are allowed to react with an excess of the mercury salt and di- or poly-mercurated products are obtained. The relative reactivity of isolated double bonds is basically that expected from studies on simple alkenes. 

Dienes and polyenes undergo acyloxymercuration. Nonconjugated dienes afford either mono- or di-mercurated species, depending on the stoichiometry of the reaction (equation 288).511-513 Products involving carbon-carbon bond formation are occasionally observed during this process.512 Allenes afford vinylmercurials (equation 289).514,515... 

Figure 2 Planar 7r-systems of representative conjugated molecules and polymers, and cr-conjugated polysilanes. Polyenes, acenes and rylenes have variable length. Polydiacetylenes, polythiophenes, polyparaphenylenevinylenes and polysilanes give families with similar backbones and different nonconjugated side groups.

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