Polyene 1,5-hexadiene

The metathesis of acyclic alkadienes and higher polyenes may involve both inter- and intramolecular processes. An example of an intermolecular reaction is the conversion of 1,5-hexadiene into 1,5,9-decatriene and ethene ... 

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,... 

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... 

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. 

Fig. 4.1 illustrates the first few members of the series of neutral polyenes the equilibria between butadiene 4.1 and cyclobutene 4,2, between hexadiene 4,3 and cyclohexadiene 4.4, and between octatetraene 4.5 and cyclooctatriene 4,6. There are of course heteroatom-containing analogues, with nitrogen or oxygen in the chain of atoms, and the systems can be decked out with substituents and other rings. To appreciate what the fundamental reaction is, it is only necessary to tease out the components—the longer conjugated... 

Ozonolysis as used below is the oxidation process involving addition of ozone to an alkene to form an ozonide intermediate which eventually leads to the final product. Beyond the initial reaction of ozone to form ozonides and other subsequent intermediates, it is important to recall that the reaction can be carried out under reductive and oxidative conditions. In a general sense, early use of ozonolysis in the oxidation of dienes and polyenes was as an aid for structural determination wherein partial oxidation was avoided. In further work both oxidative and reductive conditions have been applied . The use of such methods will be reviewed elsewhere in this book. Based on this analytical use it was often assumed that partial ozonolysis could only be carried out in conjugated dienes such as 1,3-cyclohexadiene, where the formation of the first ozonide inhibited reaction at the second double bond. Indeed, much of the more recent work in the ozonolysis of dienes has been on conjugated dienes such as 2,3-di-r-butyl-l,3-butadiene, 2,3-diphenyl-l,3-butadiene, cyclopentadiene and others. Polyethylene could be used as a support to allow ozonolysis for substrates that ordinarily failed, such as 2,3,4,5-tetramethyl-2,4-hexadiene, and allowed in addition isolation of the ozonide. Oxidation of nonconjugated substrates, such as 1,4-cyclohexadiene and 1,5,9-cyclododecatriene, gave only low yields of unsaturated dicarboxylic acids. In a recent specific example... 

Torsion about one of the formal double bonds is invariably the most efficient excited singlet state decay process of acyclic polyenes, and also often occurs efficiently in cyclic systems of moderate-to-large ring size- . E.Z-isomerization in the excited singlet state manifold takes place about only one of the double bonds per photon, as was initially demonstrated for 2,4-hexadiene (5) by Saltiel and coworkers and has since been shown to be quite general. Table 1 contains a summary of quantum yields for the direct E,Z-photoisomerization, in solution, of acyclic and cyclic polyenes 1, 42, 43, 5-18 bearing various substituents. For the most part, quantum yields for direct E,Z-photoisomerization of aliphatic dienes are not highly dependent on the structure of the system (i.e. acyclic, cyclic or exocyclic). 

As mentioned earlier, conformational isomerization about the formal single bonds of polyene systems is facile in the ground state, where it occurs with activation barriers on the order of 2-4 kcalmol in acyclic systems . The process also occurs in acyclic dienes upon direct excitation, as was shown by SquiUacote and coworkers using low temperature matrix isolation techniques, at temperatures where thermal conformational reequilibration is suppressed (10-20 K) . Thus, direct irradiation of trans-1,3-butadiene in an argon matrix at 15 K results in the efficient formation of the c/s-conformer, distinguishable from the trans-conformer by its distinct UV absorption and infrared spectra . The process is quite general, at least for aliphatic dienes such as isoprene (2), 2-isopropyl-1,3-butadiene (24), 2,4-hexadiene (5) and 2,3-dimethylbutadiene... 

A few non-conjugated polyenes have been studied by the force-field method. It was calculated that 1,4-cyclohcxadiene is planar (D2 ), although one electron diffraction study (Oberhammer and Bauer, 1969) on the molecule indicated a boat form (C2 ). An independent electron diffraction work indeed gave a planar structure (Dallingaand Toneman, 1967). The problem with the former electron diffraction study seems to have been a misinterpretation of the observed 3,6-distance, a problem related to what is sometimes referred to as shrinkage (see Bartell and Kohl, 1963 and references therein). This effect occurs because the atomic nuclei are undergoing vibrational motion. Thus, one mode of vibration of 1,4-cyclo-hexadiene is as shown in eqn (11). 

The most recent application of olefin metathesis to the synthesis of polyenes has been described by Tao and Wagener [105,117], They use a molybdenum alkylidene catalyst to carry out acyclic diene metathesis (ADMET) (Fig. 10-20) on either 2,4-hexadiene or 2,4,6-octatriene. The Wagener group had earlier demonstrated that, for a number of nonconjugated dienes [118-120], these polymerizations can be driven to high polymer by removal of the volatile product (e. g., 2-butene). To date, insolubility limits the extent of polymerization of unsaturated monomers to polyenes containing 10 to 20 double bonds. However, this route has some potential for the synthesis of new substituted polyacetylenes. Since most of the monomer unit is preformed before polymerization, it is possible that substitution patterns which cannot be incorporated into an alkyne or a cyclic olefin can be built into an ADMET monomer. 

Intramolecular eyclizations of polyenes. When l,S-hexadiene is heated in mineral oil at 115° (N2) with a catalytic amount of diisobutylaluminum hydride, methylenecyclopentane is formed in 81% yield (equation I). ... 

Correlation diagrams can be constructured in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclo-hexadiene. They lead to the prediction that the disrotatory mode is an allowed process while the conrotatory process is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclization reactions can be analyzed by the same process. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases where the substitution pattern does not correspond in symmetry to that of the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

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. 

---