Conjugated Diene Systems

The parent diene of conjugated diene systans is C=C-C=C. This system in a hexane solvent absorbs at 217 nm. If the conjugated system is increased, the wavelength of the absorption 

Absorption of parent diene system C=C-C=C Shift to longer X by 

Double bond extension to diene system Diene system within a ring 

Exor clic nature of double bond in conjugated system Each alkyl substituent or ring residue Auxochrome is O-acyl O-alkyl S-alkyl N-alkyl2 Cl, Br 

Source Scott, A.I., Interpretation of the Ultraviolet Spectra of Natural Products, Peigamon, Oxford, U.K., 1964. With permission. 

---

The Diels-Alder Reaction consists in the direct combination of a compound containing a conjugated diene system u ith a reagent which possesses a double or triple bond activated bj suitable adjacent groups. Examples of such reagents are maleic anhydride, p-benzoquinone, acraldehyde and acetylene dicarboxylic esters. Combination always occurs at the 1,4 positions of the diene system ... 

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... 

When butadiene is treated with PdCU the l-chloromethyl-7r-allylpalladium complex 336 (X = Cl) is formed by the chloropalladation. In the presence of nucleophiles, the substituted 7r-methallylpalladium complex 336 (X = nucleophile) is formed(296-299]. In this way, the nucleophile can be introduced at the terminal carbon of conjugated diene systems. For example, a methoxy group is introduced at the terminal carbon of 3,7-dimethyl-I,3,6-octatriene to give 337 as expected, whereas myrcene (338) is converted into the tr-allyl complex 339 after the cyclization[288]. 

Section 10 10 Protonation at the terminal carbon of a conjugated diene system gives an allylic carbocation that can be captured by the halide nucleophile at either of the two sites that share the positive charge Nucleophilic attack at the carbon adjacent to the one that is protonated gives the product of direct addition (1 2 addition) Capture at the other site gives the product of conjugate addition (1 4 addition)... 

Myrcene with its conjugated diene system readily undergoes Diels-Alder reactions with a number of dienophiles. For example, reaction with 3-meth.5i-3-pentene-2-one with a catalytic amount of AlCl gives an intermediate monocyclic ketone, which when cyclized with 85% phosphoric acid produces the bicycHc ketone known as Iso E Super [54464-57-2] (49). The product is useful in providing sandalwood-like and cedarwood-like fragrance ingredients (91). 

Reaction of myrcene and sulfur dioxide under pressure produces myrcene sulfone. This adduct is stable under ordinary temperatures and provides a way to stabilize the conjugated diene system in order to hydrate it with sulfuric acid. The myrcene sulfone hydrate produced is pyrolyzed in the vapor phase in order to regenerate the diene system to produce myrcenol [543-39-5] (50). 

When Diels and Alder published their famous paper in 1928, Diels had been working with related reactions for several years [6]. In 1925, Diels reported the reaction of azodicarboxylic ester (Et0C(0)2CN=NCC(0)0Et) with compounds containing a conjugated diene system. He found that addition of the azodicarboxylic ester occurs at the 1,4-position of the conjugated system as with cyclopentadiene and with butadiene. This work probably led to the famous Diels-Alder reaction. In 1927, Diels and his student Alder published a paper on the reaction of azodicarboxylic ester with styrene. 

No other data on substituted conjugated dienes are extant in the literature. Of the five nonequivalent positions on a conjugated diene system, no study has been made of the remaining four. Thus the vinylidene (X ), vinylene (X ), vinylidenevinylene (X ), and cw,trans-divinylene positions of 40 remain unexamined. No studies of... 

We focused our attention on Tall oil, a by-product of the paper industry, whenever this is prepared according to the KRAFT process. Said material consists of a mixture of highly unsaturated fatty acids (many of which with conjugated diene systems) and terpene derived rosin acids. The rosin acids have the molecular formula C20H30O2 and thus belong to the diterpenes (pimaric and abietic acids). Tall Oil has an iodine number equal to approximately 170 gl2/100 g. 

Dienes bonded to silicones to give products stable to hydrolysis are available in a wide variety of combinations, from the distillable l,3-bis(isoprenyl)-l,l,3,3-tetramethyldisiloxane to high polymers with dozens of diene groups attached [23], Diels-Alder cycloadditions may be performed with dieno-philes. Maleic anhydride adds almost quantitatively to the conjugated diene system, giving easy access to a siloxane species of considerable industrial importance [24]. 

Table 5 Base peaks in the El-mass spectra of Type I compounds including a conjugated diene system [163,164]a... 

For the components with a conjugated diene system, derivatization with MTAD is useful to determine the unsaturated positions [168]. Adducts of the di-enophile MTAD, which is formed with conjugated dienes via Diels-Alder cyclo-... 

Other conjugated diene systems readily react with either H4Ru4-(CO)i2 or Ru3(CO)i2, to yield 7r-allyl complexes. Thus, butadiene yields the crotyl derivative HRu3(CO)9C4H5. The X-ray structure of the related adduct formed from cis-trans- or trans-trans-2,4-diene is shown in Fig. 12. The hydride is considered to bond to the Ru(l)-Ru(2) edge, as this is the longest metal-metal bond in the structure and in the... 

Lipoxygenase (linoleate oxygen oxidoreductase) catalyzes the hydroperoxidation of fatty acids containing a methylene-interrupted conjugated diene system. The degradation of the hydroperoxides results in the formation of numerous secondary products (46-48). 

Doubts remain in the absolute identification of the individual olefinic carbon atoms of the (9Z,11Z) (38) and (9 ,11 ) (42), expect for the fact that the shifts of the inner (CIO and Cll) and outer (C9 and Cl2) olefinic carbon atoms of the conjugated diene system are distinguishable. 

In order to assign the chemical shifts of the carbon atoms of the conjugated diene system of each CLA isomer, it was necessary to conduct INADEQUATE, HMBC (heteronuclear multiple bond correlation) and two-dimensional 1H-13C correlation spectroscopy (COSY) techniques on the carbon signals of the diene system of the ,Z-isomers. The results of these experiments for the CLA isomers are summarized in Table 13. 

Less complex non-conjugated diene systems also lead to cubane-like derivatives as in the diene 175. Here the outcome of the reaction is dependent upon the excited state. Thus, direct irradiation brings about fragmentation with the formation of 1,4-difluorobenzene and excited-state naphthalene while triplet-sensitized irradiation follows a different path with the formation of the cage compound 17682. 

The complete shift of a conjugated diene to a new conjugated diene system has also been observed in the -ionone system (33). Such diene rearrangements require a stoichiometric quantity of Fe(C0)5. The rearranged diene ligand is conveniently liberated from the Fe by oxidation of the complex with FeCls-... 

From the practical viewpoint, the more important products from 1,3-dienes are those that occur by 1,4-polymerization of the conjugated diene system, for example, for isoprene, we obtain... 

C. Conjugated Diene Systems as Substrates of Photooxygenation Reactions... 

At higher temperatures the tertiary. chloride undergoes a rearrangement under the influence of CuCl giving a chloride with a conjugated diene system. This isomer has a considerably higher refractive index. 

The Diels-Alder reaction is in fact a [4 -i- 2] cycloaddition reaction, where C-1 and C-4 of the conjugated diene system become attached to the double-bonded carbons of the dienophile to form a six-membered ring. For example, 1,3-butadiene reacts with maleic anhydride to produce tetrahy-drophthalic anhydride on heating. 

Conjugated diene systems (572) have been protected temporarily through their cycloaddition reaction with PTAD (equation 9) (78CC727). Chemical operations can be performed on another portion of the molecule and then the diene unmasked by LAH treatment or by reaction with a mineral base (K2C03) in a solvent like DMSO. 

Study of the photochemistry of myrcene, 23, a triene containing both a conjugated diene system and a remote, unconjugated double bond, has been exceptionally informative. Direct excitation produces a cyclobutene derivative 24, /S-pinene, and a host of minor products287 including 25. In contrast, indirect excitation using a number of sensitizers gives only a derivative of bicyclo[2.1.1]hexane, 25.288... 

Confalone and Ko have reported formation of the macrocyclic INOC product (104) in 30% yield from cyclization of an aryl-bridged nitrotriene containing a 17-atom intervening bridge.48 Cyclization to the conjugated diene system did not occur, presumably because of geometric constraints and the fact that the more accessible double bond is trisubstituted. Isoxazoline (104) is a potential maytansine precursor. 

---