The Diels-Alder Reaction of Conjugated Dienes

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. 

When energy is supplied to a mixture of a conjugated diene and an alkene, a ring-forming reaction takes place to produce a cyclohexene. The alkene in this reaction is called a dienophile, because it has demonstrated an affinity for the diene. The product of the reaction is often called the adduct, which is another word for product in a reaction between two molecules. Our discussion of the Diels-Alder mechanism begins with the arrow formalism, which points out which bonds are broken and shows where the new bonds are made in the forward reaction (Rg. 12.51). 

Which orbitals are involved The Diels-Alder reaction begins with the overlap of the n systems of the dienophile and the diene. The two reaction partners approach in parallel planes and as the bonds form, the end carbons of both the diene and alkene rehybridize from s/P to (Rg. 12.52). 

FIGURE 12.52 In the Diels-Alder reaction, the two participants approach each other in parallel planes. As the reaction occurs, the end atoms of the diene and the dienophile rehybridize from s- to s.  

PROBLEM 12.24 Write an arrow formalism for the reverse Diels-Alder reaction. 

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It is found that chiral dienes form better performing cationic Rh complexes than diphosphines, for use in catalyzing intramolecular Diels-Alder reaction of conjugate diene and alkyne units.A cationic Ru(I) catalyst 140 operates on the basis of one-point association of the dienophile prior to establishment of the transition state for the Diels-Alder reaction. The most effective case demonstrated thus far is an intramolecular process. ... 

Cycloaddition of 2-cyanoalk-2-enones with several conjugated dienes proceeded under zinc chloride catalysis.636 Zinc halides have also shown reactivity with phenylacetylenes.637 Zinc chloride is an effective Lewis acid catalyst in the Diels Alder reactions of the keto esters and the effects on stereochemistry of catalysts used have been examined.638... 

The formation of heterocycles by cycloaddition reactions of conjugated dienes is the subject of this chapter. Almost the entire account is devoted to the Diels-Alder reaction of dienes with heterodienophiles to yield six-membered ring compounds (equation 1). Many such reactions have been reported and there is a plethora of reviews. Somela p are general others are cited at appropriate places in the text. This account is highly selective, concentrating on recent work with particular regard to the stereochemistry of these processes. 

Certain heterocyclic aromatic rings (among them furans)862 can also behave as dienes in the Diels-Alder reaction. Some hetero dienes that give the reaction are —G=C—0=0, 0=C—0=0, and N=C—C=N.854 For both all-carbon and hetero systems, the diene can be a conjugated enyne. If the geometry of the molecule is suitable, the diene can even be nonconjugated, e.g.,863... 

Although the Diels-Alder reaction of a conjugated diene, such as butadiene or isoprene, with maleic anhydride, has been known to yield tetrahydrophthalic anhydride, it has recently been shown (81, 85) that alternating copolymers are prepared under the influence of ionizing radiation (81) or free radical initiators (81, 85). The participation of the charge transfer complex as a common intermediate in both adduct... 

Another way to prepare dibasic acids for the preparation of polyamides would be to oxidize cyclohexene from the Diels-Alder reaction of 1,3-butadiene and ethylene or cy-clooctene made from 1,3-butadiene by cyclic dimerization, followed by a reduction that might involve conjugation of the double bonds in situ (12.12). The latter may be preferable because the former requires forcing conditions.37 It may be possible to run the former reaction under high pressure or with ultrasound or with a metal complex catalyst (such as a metal triflate) to reduce the electron density of the diene by complexation. 

Raasch showed that 3,4-dichloro-, tetrabromo-, and tetrachlorothiophene 1,1-dioxides react with a range of cyclic and acyclic non-conjugated dienes to give polycyclic compounds in good yields by [4+2] cycloaddition followed by intramolecular [4+2] cycloaddition [35]. For example, the Diels-Alder reaction of these dioxides with 1,5-cyclooctadiene produces the adducts 80, which then undergo an intramolecular Diels-Alder reaction to give polycyclic compounds 81 in good yields in one pot (Scheme 47). In this way, a variety of polycyclic compounds, shown below, were synthesized (Table 10). 

In a Diels-Alder reaction, a conjugated diene reacts with a dienophile to form a cyclic compound in this concerted [4 + 2] cycloaddition reaction, two new a bonds are formed at the expense of two tt bonds. The conjugated diene must be in an s-cis conformation. The reactivity of the dienophile is increased by electron-withdrawing groups attached to the sp carbons. The HOMO of one reactant and the LUMO of the other are used to show the transfer of electrons between... 

Diels-Alder cycloaddition is an organic chemical reaction between a conjugated diene and a substituted alkene to form a substituted cyclohexene system [62]. Polymer-supported chloroaluminates, poly(l-butyl-4-vinylpyridinium) chloroaluminate, poly(l-butyl-3-vinylimidazolium) chloroaluminate, and poly(l-ethyl-3-vinylimidazolium) chloroaluminate were synthesized, and their activities were tested for the Diels-Alder reaction of cydopentadiene with methyl methacrylate [63]. These polymer-supported heterogeneous catalysts exhibited activities comparable to their corresponding monomeric homogeneous analogs and could be reused with a moderate loss of initial activity up to five runs. 

Perhaps the simplest example of a Diels-Alder reaction is between the conjugated diene 1,3-butadiene (1) and the simple alkene ethene. When they are heated to >200°C in a reaction bomb, the product is cyclohexene. Therefore, the Diels-Alder reaction between a diene and an alkene leads to a... 

In a Diels-Alder reaction, a conjugated diene reacts with a dienophile to form a cyclic compound in this concerted [4 + 2] cycloaddition reaction, two new (T bonds and a tt bond are formed at the expense of two TT bonds. 

Although 2,3-di-iert-butyl-l,3-butadiene has four adjacentp orbitals, the bulky tert-butyl groups prevent the diene from adopting the s -cis conformation needed for the Diels-Alder reaction. Thus, this diene does not undergo a characteristic reaction of conjugated dienes. 

CH = CH — CH = CH — are said to have conjugated double bonds and react somewhat differently from the other diolefins. For instance, bromine or hydrogen is often added so that a product of the type -CHBr-CH=CH-CHBr- is formed. Also, these hydrocarbons participate in the Diels-Alder reaction see diene reactions). They show a tendency to form rubber-like polymers. Hydrocarbons not falling into these two classes are said to have isolated double... 

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 ... 

The Diels-Alder Reaction usually occurs readily it is of great value (a) for diagnosing the presence of a conjugated diene grouping, (6) for synthetic purposes in the preparation of cyclic systems. 

Compounds containing a double or triple bond, usually activated by additional unsaturation (carbonyl, cyano, nitro, phenyl, etc.) In the ap position, add to the I 4-positions of a conjugated (buta-1 3-diene) system with the formation of a ax-membered ring. The ethylenic or acetylenic compound is known as the dieTwphile and the second reactant as the diene the product is the adduct. The addition is generally termed the Diels-Alder reaction or the diene synthesis. The product in the case of an ethylenic dienophile is a cyctohexene and in that of an acetylenic dienophile is a cyctohexa-1 4-diene. The active unsaturated portion of the dienophile, or that of the diene, or those in both, may be involved in rings the adduct is then polycyclic. 

A particular kind of conjugate addition reaction earned the Nobel Prize in chemistry for Otto Diels and Kurt Alder of the University of Kiel (Germany) m 1950 The Diels-Alder reaction is the conjugate addition of an alkene to a diene Using 1 3 buta diene as a typical diene the Diels-Alder reaction may be represented by the general equation... 

Section 10 12 Conjugate addition of an alkene (the dienophile) to a conjugated diene gives a cyclohexene derivative in a process called the Diels-Alder reaction It is concerted and stereospecific substituents that are cis to each other on the dienophile remain cis m the product... 

The most notable chemistry of the biscylopen-tadienyls results from the aromaticity of the cyclopentadienyl rings. This is now far too extensively documented to be described in full but an outline of some of its manifestations is in Fig. 25.14. Ferrocene resists catalytic hydrogenation and does not undergo the typical reactions of conjugated dienes, such as the Diels-Alder reaction. Nor are direct nitration and halogenation possible because of oxidation to the ferricinium ion. However, Friedel-Crafts acylation as well as alkylation and metallation reactions, are readily effected. Indeed, electrophilic substitution of ferrocene occurs with such facility compared to, say, benzene (3 x 10 faster) that some explanation is called for. It has been suggested that. 

The Diels-Alder reaction,is a cycloaddition reaction of a conjugated diene with a double or triple bond (the dienophile) it is one of the most important reactions in organic chemistry. For instance an electron-rich diene 1 reacts with an electron-poor dienophile 2 (e.g. an alkene bearing an electron-withdrawing substituent Z) to yield the unsaturated six-membered ring product 3. An illustrative example is the reaction of butadiene 1 with maleic anhydride 4 ... 

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