Diene double

Further evidence for the importance of mesomeric ion, 166b, comes from a careful examination of the solvolytic behavior of the cyclic bromodienes, 167 (153). In these systems, the planarity of the diene double bonds precludes... 

The mechanism of this new reaction is shown in Scheme 14. Coordination of the diene to palladium(II) makes the diene double bond electrophilic enough to be attacked by the allylsilane. The attack by the allylsilane takes place on the face of the diene opposite to that of the palladium (anti). This is the first example of an anti attack by an allylsilane on a 7T-(olefin)metal complex. Benzoquinone (BQ)-induced anti attack by chloride ion produces the product 58. 

In practice, the adduct with the endo2 configuration usually is the major product. As a general rule, Diels-Alder additions tend to proceed to favor that orientation that corresponds to having the diene double bonds and the unsaturated substituents of the dienophile closest to one another. This means that addition by Equation 13-3 is more favorable than by Equation 13-4, but the degree of endo-exo stereospecificity is not as high as the degree of stereospecificity of suprafacial addition to the diene and dienophile. 

Addition of It to the isomeric 1,3-pentadienes and other nonsymmetrically substituted dienes occurs selectively at the more substituted diene double bond (106). For example, the reaction of 1-t with cis-l,3-pentadiene yields adducts 51, 52, and 53 (mixture of stereoisomers) in the ratio 3.3 1.2 1. 

Vinylation of dienes in the presence of piperidine or morpholine yields aminodienes as major products. Sometimes trienes are minor products. The reaction is believed to proceed by way of a ir-allylpal-ladium complex formed by addition of the vinylpalladium halide to the least-substituted diene double bond. Nucleophilic attack of the amine upon the ir-allylic complex gives the aminodienes, while hydridopalladium halide elimination yields trienes (Scheme 6).97... 

In the series of hydroxycyclohexadienylperoxyl radicals, one encounters the competition between the H02-/02- elimination leading to phenol [reactions (9) and (14)/(15)] and fragmentation of the ring (Pan et al. 1993). That latter has been attributed to an intramolecular addition of the peroxyl radical function to a diene double bond [reaction (24)]. This reaction is reversible [reaction (-24)], but when 02 adds to the newly created carbon-centered radical the endoperoxidic function is locked in [reaction (25)]. In analogy to reaction (24), the first step of the trichloromethylperoxyl-radical-induced oxidation of indole is its addition to the indole C(2)-C(3) double bond (Shen et al. 1989). 

The stereochemistry of the cycloadducts in intramolecular Diels-Alder reactions depends upon the different geometry of the possible transition states 37—40 whose nomenclature can be explained as follows The orientation with the chain connecting the diene and dienophile lying under or above the diene is called endo. The opposite means exo. E and Z mark the geometry of the diene double bond which is connected with the chain. Syn and anti describe the arrangement of the hydrogen atoms (or substituents) at the prestereogenic centers which are involved in the C-C bond formation.12... 

The Re(III) complex Re(PPh3)2(MeCN)Cl3 (2 mol%) catalyzes the ATRA of tetrachloromethane or bromotrichloromethane to terminal alkenes in 39-76% yield [303]. p-Pinene suffered a cyclobutylcarbinyl radical ring opening, thus supporting the free radical mechanism. With l, -dienes double addition was found, while 1,3-dienes gave the 1,4-addition product. Internal alkenes were almost inert under the reaction conditions. 1,6-Dienes 158 underwent a tandem radical addition/ cyclization reaction to cycles 159 in 64—87% yield with 3-6 1 c/s-diastereos-electivity (cf. Fig 43). This compares well to the results obtained with the most frequently used catalyst Ru(PPh3)3Cl2 (see Part 2, Sects. 3.3.1 and 3.3.2). 

Unconjugated dienes can produce an even more complicated range of macro-molecular structures. Homopolymers of such monomers are not of current commercial importance but small proportions of monomers like 1,5-cyclooctadicne are copolymerized with ethylene and propylene to produce so-called EPDM rubbers. Only one of the diene double bonds is enchained when this terpolymeriza-tion is carried out with Ziegler-Natta catalysts (Section 9.5). The resulting small amount of unsaturation permits the use of sulfur vulcanization, as described in Section 1.3.3. 

Parent heteroannular diene Double bond extending the conjugation Alkyl substituent or ring residue Exocyclic location of double bond to any ring Groups OAc, OR... 

Intramolecular reactions of allylic acetates with conjugated dienes catalyzed by Pd(0) lead to a 1,4-addition of a carbon and an oxygen nucleophile to the diene. The reaction, which is formally an isomerization, involves tw different yr-allyl complexes (Scheme 8-4) [44]. Reaction of 22 in the presence of the Pd(0) catalyst Pd2(dba)3-CHCl3 (dba = dibenzyl-ideneacetone) and LiOAc/HOAc in acetonitrile at reflux produces the cyclized isomer 25 in 62% yield. The double bond was exclusively of E stereochemistry, while the ring stereochemistry was a mixture of cis and tram isomers. Oxidative addition of the Pd(0) to the allylic acetate gives the intermediate jr-allyl complex 23. Subsequent insertion of a diene double bond into the allyl-palladium bond produces another jr-allyl intermediate (24), which is subsequendy attacked by acetate to give the product 25. 

Two subtypes of G protein-coupled receptors for cannabis s psychotropic component, A -tetrahydrocannabinol (THC), have been cloned to date, the cannabinoid CBi and CB2 receptors (Howlett et al. 2004). Yet, five different types of endogenous agonists for these cannabinoid receptors have been identified so far (Fig. 1). These compounds, named endocannabinoids by analogy with THC (Di Marzo and Fontana 1995), are all derived from long-chain polyunsaturated fatty acids. In particular (1) the anandamides are amides of ethanolamine with polyunsaturated fatty acids with at least 20 carbon atoms and three 1,4-diene double bonds. The C20 4 homologue in this series, V-arachidonoylethanolamine (AEA) (Devane et al. 1992), also known simply as anandamide, has been most studied. 

Diene double-bond substituents10 11 Anomeric configurations on the glucose moiety12 13 Substituents on the glucose moiety12 13. 

Polyolefin terpolymers contain three olefinic monomers. A well-known example is ethylene propylene diene monomer (EPDM). The diene (double bond) monomer is... 

In the most general terms, a cycloaddition reaction is one that forms a ring from multiple jr-bonds. To illustrate some of the concepts and ways that these reactions are classified, let s look at the cycloaddition reaction shown below. This reaction, known as the Diels-Alder reaction, is between a conjugated diene (double alkene) and a another edkene that likes to react with dienes (hence dienophile, or diene lover). It is classified as a [4 -l- 2] cycloaddition, referring to the number of atoms directly involved in bond-forming events. 

The diene double bond in equation (9.94) may be either cis or trans. The cis products all have lower glass transition temperatures and/or reduced crystallinity, and they make superior elastomers. A random copolymer of butadiene and styrene is polymerized to form SBR (styrene-butadiene rubber). This copolymer forms the basis for tire rubber (see below). The trans materials, such... 

Early work bearing on organolithium-initiated diene polymerization was reported by Ziegler and co-workers in 1934. It was demonstrated that the organolithium adds to the diene double bond to give an intermediate capable of reacting further with the diene. 

As indicated earlier, one of the diene double bonds can be substituted by a triple bond. The resulting cyclic product then has an additional double bond. When these new bonds are conjugated with other olefinic linkages, a further adduct is formed. Thus, 2,5-dimethyl-l,5-hexadiene-3-yne 23 reacts with 2 moles of MA to give product 24. ... 

Vanadium oxide dichloride Cyclic peroxides from dienes Double ring closure... 

Endo products result from Diels-Alder reactions because the amount of orbital overlap between diene and dienophile is greater when the reactants lie directly on top of one another so that the electron-withdrawing substituent on the dienophile is underneath the diene double bonds. In the reaction of 1,3-cyclopenta-diene with maleic anhydride, for instance, the following result is obtained ... 

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