Conjugated dienes definition

What is relevant to this chapter is that a conjugated diene has higher thermodynamic stability than one would expect in the absence of conjugation. Leaving off all hydrogens and substituents in the name of simplicity, several interrelated definitions that document... 

The description of conjugated dienes as shown by equation 17 and the associated comparison with butadiene in equation 18 corresponds most closely to the conventional definition. The results are plausible in that groups on one double bond that are c/5-situated relative to the other encourage nonplanarity, cause destabilization and result in lessened conjugation energy. Or so we say. The biggest debit of this approach is that the thermochemistry of the monoenes related by single addition of H2 is often absent. An example... 

We might have thought that a-diketones would have less stabilization by this definition than conjugated dienes. After all ... 

This method of diene formation with definite E and Z structures has wide synthetic applications [518], particularly for the syntheses of natural products with conjugated polyene structures. Bombykol and its isomers (650 and 651) have been prepared by this method[5l9]. The synthesis of chlorothricolide is... 

This conjugated enone/diene difference is more definitively seen in the 184 kJ mol-1 decrease in enthalpy of formation on going from 1-pentene to butanal, in contrast to the 177 kJ mol-1 going from (E)-l,3-pentadiene to irans -crotonaldehyde. For further discussion, see J. F. Liebman and R. M. Pollack, in The Chemistry ofEnones (Eds. S. Patai and Z. Rappoport), Wiley, Chichester,... 

As just described, Zimmerman has reported one instance of a dienone rearrangement which definitely does not fit Chapman s general picture. Schuster has provided two reports410,411 of cross-conjugated cyclohexadienones which eliminate radical species. With the trichloro-methyl-substituted ketone 34, both cleavage to the cresol and rearrangement to lumiproduct are quenched by dienes.411 Stern-Volmer quenching plots indicate that the rate at which the excited triplet reacts exceeds 10° sec"1 for both reactions.412... 

This conjugated enone/diene difference is more definitively seen in the 184 kJ mol decrease in... 

Lipoxygenase enzymes are known to oxidize arachidonic acid at six possible positions. The chemical requirement for this oxidation (Eq. 4.1) appears simply to be a skipped cis diene which can produce (via the enzyme) a stabilized allylic radical, which then traps molecular oxygen to form a conjugated cis-trans diene hydroperoxide. Chart 4.1 shows the six possible mono-oxidation products as members of the HETE family. All of these compounds have been shown to be natural products. Only in the cases of 8 and 9 HETEs is the absolute stereochemistry not definitively known. Nevertheless, it is speculated that 8-HETE has the alcohol in the (5)-configuration and 9-HETE is of the (R) absolute... 

Other dienes, even cyclopentadiene, perform poorly with this dienophile 38 but the azadiene 39 that does so well here is a very special case. It is stabilised by the phenyl groups at both ends of the molecule and it cannot tautomerise into an enamine. More generally useful 1-azadienes must be stabilised by conjugating substituents that are definitely electron-donating or withdrawing so that the HOMO or the LUMO is unambiguously selected for cycloaddition. 

---