Conjugated dienes characteristic reaction

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

As shown in (5.84b), the characteristic feature of the Diels-Alder reaction is the addition of an ethylenic double bond (dienophile) across the 1,4-positions of a conjugated diene to give a cyclohexene ring product. The ethylenic bond is usually... 

Whether or not branching occurs at an intermediate, its existence may be demonstrated by running the reaction in the presence of a reagent designed to intercept that intermediate to yield a new and characteristic product. Observations at a qualitative level can be extremely informative (e.g. formation of cyclo-adducts when aryl halides are treated with a strong base in the presence of conjugated dienes to trap a benzyne intermediate) but even more information maybe obtained from quantitative experiments, especially when product analyses are coupled with rate measurements. 

Conjugate allylation reactions, characteristics, 9, 312 Conjugated alkenes, with Pd n-complexes, 8, 334 Conjugated dienes via Pd(IV) catalysts, 8, 306 polymerization, 4, 1084... 

Isomeric (s-cis- and (i-fra/w-V-conjugated diene)zirconocene and -haf-nocene complexes exhibit pronounced differences in their characteristic structural data as well as their spectroscopic features. These differences exceed by far the consequences expected to arise simply from the presence of conformational isomers of the 1,3-diene unit. While (f-rra/u-butadiene)-zirconocene (3a) shows a behavior similar to a transition metal olefin TT-complex, the (.r-cu-diene)ZrCp2 isomer 5a exhibits a pronounced alkylmetal character (23, 45). Typical features are best represented by a tr, 7T-type structure for 5 (55). However, the distinctly different bonding situation of the butadiene Tr-system/bent-metallocene linkage is not only reflected in differences in physical data between the dienemetallocene isomers 3 and 5, but also gives rise to markedly different chemical behavior. Three examples of this are discussed in this section the reactions of the 3/5 isomeric mbcture with carbon monoxide, ethylene, and organic carbonyl compounds. 

Much attention has been directed to the chemistry of p-heteroatom-substituted allylic cations (5), especially oxyallyl cations (7), from both synthetic and mechanistic viewpoints. One of the characteristic reactions of oxyallyls (7) is their [4 -i- 3] 7 cycloadditions with conjugated dienes, which provide a... 

Cycloheptatriene and related heterocyclic analogs have been examined to some extent with regard to their reactivity with various conjugated diene partners. Characteristically, higher-order cycloaddition pathways often suffer relative to alternative modes of reaction in these systems and usually a select type of highly reactive diene partner is a prerequisite for viable cycloaddition to occur. Selectivity in these species can also be compromised by die intervention of the corresponding norcaradiene tautomer in certain circumstances. Moctest periselectivity coupled with low chemical yields limit the potential synthetic utility of these cycloadditions. 

Recall from Chapters 10 and 11 that the characteristic reaction of compounds with it bonds is addition. The n bonds in conjugated dienes undergo addition reactions, too, but they differ in two ways from the addition reactions to isolated double bonds. 

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

Unlike nickel catalysts which form cyclic dimers and trimers (1,5-cyclooctadiene and 1,5,9-cyclododecatriene), palladium compounds catalyze linear dimerization of conjugated dienes. 1,3-Butadiene itself is converted to 1,3,7-octatriene. The reaction most characteristic of palladium is the formation of various telomers. 1,3-Buta-diene dimerizes with incorporation of various nucleophiles to form telomers of the following type ... 

The oxazoles also display a number of characteristics that are typical of the furans and are explained by the structural similarity of these heterocyclic systems. The ease with which they undergo Diels-Alder reactions with dienophiles and autooxidation with singlet oxygen (see Sections IV, D and E) clearly demonstrates that oxazoles are not fully aromatic. This fact and ultraviolet data (Section III, E) suggest that oxazoles should be considered partly as conjugated dienes. 

Until now, we have been concerned with the reactions of compounds that have only one functional group. Compounds with two or more functional groups exhibit reactions characteristic of the individual functional groups if the groups are sufficiently separated from each other. If they are close enough to allow electron delocalization, however, one functional group can affect the reactivity of the other. Therefore, we will see that the reactions of isolated dienes are the same as the reactions of alkenes, but the reactions of conjugated dienes are a little different because of electron delocalization. 

Catalytic behaviors of solid base catalysts for fine chemicals synthesis as well as the fundamental reactions are described. The reactions included are double bond isomerization of olefins, addition of hydrogen and amines to conjugated dienes, dehydration, dehydrogenation, reduction, alkylation, aldol addition and condensation, Wittig-Horner and Knoevenagel reactions, dehydrocyclodimerization, and ring transformation. The characteristic features of different types of solid base catalysts, zeolites, metal oxides, solid superbases and non metal-oxides, are summarized. 

Electrophilic addition is the characteristic reaction of alkenes, and conjugated dienes undergo addition with the same electrophiles that react with alkenes, and by similar mechanisms. Hydrogen chloride, for example, adds to the diene unit of 1,3-cyclopentadiene to give 3-chlorocyclopentene. Mechanism 10.3 is analogous to the electrophilic addition of HCl to alkenes. 

In 1950, Otto Diels (1876-1954) and his student Kurt Alder (1902-1958) received the Nobel prize for their work on what has appropriately come to be known as the Diels-Alder reaction, which they discovered in 1928. (So much for timeliness at least they lived long enough to outlast the Nobel committee s renowned conservatism.) We will have more to say about its mechanism in Chapter 20, but the Diels-Alder reaction is characteristic of conjugated dienes and is of enormous synthetic utility. Therefore we must outline it here. 

The use of Ziegler-Natta catalysts in the polymerization of conjugated dienes has been widely investigated. It is characteristic of these catalysts that the resulting polymers often contain a very high proportion of one type of structural unit. By appropriate choice of catalyst, polydienes comprised almost exclusively of c -l,4-, /ra s-l,4-, 1,2-, and 3,4-units have been obtained. The mechanisms of such reactions are, at present, somewhat obscure but presumably the diene molecule co-ordinates with the metal-carbon bond of the catalyst in a manner similar to that involving the lithium-carbon bond mentioned above. 

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