Isomerism of Diene Systems

Conjugated olefins have played a key role in the development of theoretical models in organic chemistry. In this section we will examine in detail conformational isomerism in 1,3 dienes and related molecules. 

On the basis of a previous discussion, we predicted that pi nonbonded interactions will produce an order of stability of gauche trans cis, assuming appreciable Cl—C4 pi overlap in 1,3-butadiene. However, if sigma nonbonded interactions dominate pi interactions the order of stability will be cis gauche trans. Finally, in the event that steric effects are the controlling factor the order of stability will become trans gauche cis. 

Experimentally, 1,3-butadiene is known to exist in a planar trans form but the identity of the second energy minimum has been a matter of controversy  

Ab initio results are sununarized in Table 19 They seem to suggest the existence of two energy minima, a trans and a gauche, the trans being the more stable. 

In other words, the apparent order of stability of 1,3-butadiene is trans gauche cis. We conclude, therefore, that the preferred conformation of 1,3-butadiene is dictated by steric effects which overwhelm attractive pi and sigma nonbonded interactions. 

---

IS. Conformational Isomerism of Diene Systems Table 19. Computed relative energies of the conformers of 1,3-butadiene... 

Deprotection can be effected at 25° in 90-95% yield by cleavage with the FeCl,/ AC2O system in ether as the solvent. Under these conditions no isomerization of diene or enyne units is observed. After cleavage, addition of Na2HP04 converts the FeCl, to insoluble FePO . 

The radical cations of diene systems in cyclic molecules are also capable of reaction as demonstrated by Demnth, Roth and their coworkers. They have studied the influence of phase on the photochemical reactivity of some naturally occurring dienes. Thns the irradiation of the diene 10 in homogeneous solution (acetonitrile/water) in the presence of an electron-accepting sensitizer such as cyanonaphthalene (CN) or DCB brings about fraws,cw-isomerization only. However, when the electron transfer reaction is carried out in the presence of sodinm dodecyl sulphate, transannular hydrogen abstraction reactions yield the two prodncts 11 and 12. Similar reactivity is observed with frans-geranyl acetate 13 and all-trawi-famesyl acetate 14. The authors report that these cyclizations are the flrst examples of biomimetic processes brought about under SET conditions. 

Tertiary acetylenic halides give unsatisfactory results owing to further isomerization of the allenic halide into a conjugated diene system under the influence of the copper salt. Bromo- and iodoallenes with the structures R R2C=C=CH-X can also be synthesized by an aqueous procedure, consisting of reaction between... 

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

In principle, thermal valence isomerization of the 3,4-benzo-2-thiabicyclo[3.2.0]hepta-3,6-diene skeleton 1 should lead to the 1-benzothiepin ring system. 

The facile, photoinduced valence isomerization of ethyl 1//-azepine-l-carboxylate to ethyl 2-azabicyclo[3.2.0]hepta-3,6-diene-2-carboxylatehas been studied as a potential solar energy storage system.101102 Unfortunately, the system proved to be inefficient due to build up of polymeric material during the thermally induced, exothermic retro-reaction. 

Theoretically, valence isomerization of 3//-azepincs can give rise to 2-azabicyclo[3.2.0]hepta-2,6-dienes 20 and/or the isomeric fused azetines, 6-azabicycIo[3.2.0]hepta-2,6-dienes 21. Practically, however, 2-alkoxy- and 2-amino-3//-azepincs 19, on photolysis, yield the 2-azabicyclic systems 20 exclusively, since such structures are stabilized by imidate or amidine resonance.238... 

The first examples of transition metal-catalyzed [5 + 2]-cycloadditions between vinylcyclopropanes (VCPs) and 7r-systems were reported in 1995 by Wender and co-workers.10 This [5 + 2]-reaction was based conceptually on the Diels-Alder reaction, replacing the four-carbon, four-7r-electron diene with a five-carbon, four-electron VCP (Scheme 1). Although the [5 + 2]-reaction of VCPs and 7r-systems can be thought of as a homolog of the Diels-Alder [4 +21-reaction, the kinetic stability of VCPs (activation barrier for the thermal isomerization of VCP to cyclopentene has been reported as 51.7 kcal mol-1)11 makes the thermal [5 + 2]-reactions involving VCPs and 7r-systems very difficult to achieve. A report of a thermal [5 + 2]-cycloaddition between maleic anhydride and a VCP has been published,12 but this reaction has not been reproduced by others.13 14 Based on the metal-catalyzed isomerization of VCPs to cyclopentenes and dienes,15-20 Wender and co-workers hypothesized that a metal might be used to convert a VCP to a metallocyclohexene which in turn might be trapped by a 7r-system to produce a [5 + 2]-cycloadduct. Based on its previous effectiveness in catalyzed [4 + 2]-21 and [4 + 4]-cycloadditions (Section 10.13.2.4), nickel(0) was initially selected to explore the potential of VCPs as four-electron, five-carbon components in [5 + 2]-cycloadditions. 

In summary, we expect that 2,3-substitution will not greatly alter the conformational preference of the diene system, L e. conformational isomerism of 2,3-difluoro-butadiene will be subject to the same electronic factors as that of the unsubstituted diene and these factors operate in the same direction in both cases. It is expected that, by analogy to 1,3-butadiene, the order of conformer stability of 2,3-difluoro-butadiene will be dictated by steric effects, L e. it will be tram > gauche > cis. 

This conversion is directed so as to create the most favorable conditions for the delocalization of the nnpaired electron within the aromatic nucleus. It is worth noting here that thermal treatment (150—190°C) also initiates isomerization of the initial neutral molecule of norcara-diene into the benzotropylidene system. At the same time, the reductive transformation of Scheme 6.33 proceeds smoothly even at negative temperatures. Under comparable reaction conditions (25°C), the rate of conversion of the neutral molecule is 15 orders lower than that of the anion-radical. 

The thermal isomerization of dihalocyclopropanes is facilitated by the presence of aryl, alkyl and alkoxy substituents in the three-membered ring. Phenyl substituted systems may give rise to indenes, and in the alkyl series the dehydro-halogenation of 2,3-dihalocyclopropane derivatives may lead to dienes. 

---