Nonconjugated dienes Interactions

Conjugated enones are more stable than nonconjugated enones for the same reason that conjugated dienes are more stable than nonconjugated dienes (Section 14.1). Interaction between the tt electrons of the C=C bond and the tt electrons of the C=0 group leads to a molecular orbital description for a conjugated enone that shows an interaction of the tt electrons over all four atomic centers (Figure 23.3). 

Despite the fact that the electrochemical oxidation of most of the nonconjugated dienes generally does not give products which result from interaction of the double bonds with one another, the anodic oxidation l-acetoxy-l,6-heptadienes gives intramolecularly cyclized products, that is, the cyclohexenyl ketones (equation 15)13. The cyclization takes place through the electrophilic attack of the cation generated from enol ester moiety to the double bond. 

Jordan, K.D., Paddon-Row, M.N. Long-range interactions in a series of rigid nonconjugated dienes. 1. Distance dependence of the ti+, tu and k +, k splittings determined by ab initio calculations. J. Phys. Chem. 1992, 96, 1188. 

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... 

The problem of long range Interactions between widely separated subunits In molecules Is of Importance In many areas of chemistry and biology. Although many studies have appeared characterizing these Interactions In excited states and cationic species, very little has been done In gas-phase anions. For these reasons we have undertaken a study of such Interactions In nonconjugated dienes and dlones. Several of the dienes we have examined (35) are shown below ... 

The initial stage involves the formation of a highly reactive anion Fe(CO)4 , which interacts with isolated double bonds, followed by double bond isomerization and the formation of n-complexes with the iron tricarbonyl residues. The final product is composed of the ri" -(butadienyl)irontricarbonyl units with both trans-trans- and cis-trans-tetramethylene groups. Since the iron tricarbonyl complexes with two noncon-jugated double bonds are unstable, no complexes between two polymer chains are formed. The reaction of iron carbonyls with low-molecular-weight nonconjugated dienes is accompanied by the double bond migration. 

The unsaturated compounds we looked at in Chapters 6 and 7 had only one double bond, but many compounds have numerous sites of unsaturation. If the different unsaturations are well separated in a molecule, they react independently, but if they re close together, they may interact with one another. In particular, compounds that have alternating single and double bonds—so-called conjugated compounds—have some distinctive characteristics. The conjugated diene 1,3-butadiene, for instance, has some properties quite different from those of the nonconjugated 1,4-pentadiene. 

Again, one may take the difference of the enthalpies of hydrogenation of the diene and the sum of those for the two monoenes. Doing this separately for 14 and 15, we find the reaction enthalpies for the Z- and -dienes are —1.9 1.2 and —1.8 1.1 kJmol-1. These values are effectively zero. A stabilizing—or destabilizing—interaction was not expected for nonconjugated acyclic dienes and none was found. 

The above type of bonding is assumed to occur in other metal-olefin and metal-acetylene complexes (172). Acetylenes have two mutually perpendicular sets of ir-orbitals and are therefore capable of being bonded to one or to two metal atoms both types of complexes are known. When the hydrocarbon is a nonconjugated polyolefin e.g., cyclo-octa-1,5-diene, each C C bond interacts independently with the metal atom. In complexes of conjugated polyolefins, e.g., cyclopentadiene, infrared and nuclear magnetic resonance studies (99) indicate that it is not yet possible to distinguish between structure (IV), in which each C C bond independently contributes two --electrons to the metal-olefin bonding, and structure (V), in which... 

In contrast to norbornadiene, bicyclo[2.2.2]octa-2,5-diene does not always give the transan-nularly bridged products in normal solution reactions, whereas in the anodic electrooxidation this nonconjugated octadiene showed the formation of products, mainly tricyclo-[2.2.2.0 ]octane derivatives 5 and 6 from transannular interaction, exclusively. ... 

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