Diels-Alder reaction cyclohexene production

The C7-C12 and C10-C11 bonds can be made by a Diels-Alder reaction (cyclohexene product). This observation simplifies the problem considerably. 

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... 

Diels-Alder Reactions. The important dimerization between 1,3-dienes and a wide variety of dienoplules to produce cyclohexene derivatives was discovered in 1928 by Otto Diels and Kurt Alder. In 1950 they won the Nobel prize for their pioneering work. Butadiene has to be in the j -cis form in order to participate in these concerted reactions. Typical examples of reaction products from the reaction between butadiene and maleic anhydride (1), or cyclopentadiene (2), or itself (3), are <7 -1,2,3,6-tetrahydrophthaHc anhydride [27813-21 -4] 5-vinyl-2-norbomene [3048-64-4], and 4-vinyl-1-cyclohexene [100-40-3], respectively. 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

Another stereochemical feature of the Diels-Alder reaction is addressed by the Alder rule. The empirical observation is that if two isomeric adducts are possible, the one that has an unsaturated substituent(s) on the alkene oriented toward the newly formed cyclohexene double bond is the preferred product. The two alternative transition states are referred to as the endo and exo transition states ... 

The alkene that adds to the diene is called the dienophile. Because the Diels-Alder reaction leads to the formation of a ring, it is termed a cycloaddition reaction. The product contains a cyclohexene ring as a strarctural unit. 

In the Diels-Alder transition state, the two alkene carbons and carbons 1 and 4 of the diene rehybridize from sp2 to sp 5 to form two new single bonds, while carbons 2 and 3 of the diene remain sp2-hybridized to form the new double bond in the cyclohexene product. We ll study this mechanism at greater length in Chapter 30 but will concentrate for the present on learning more about the characteristics and uses of the Diels-Alder reaction. 

One of the most useful features of the Diels-Alder reaction is that it isstaeo-specific, meaning that a single product stereoisomer is formed. Furthermore, the stereochemistry of the reactant is maintained. If we carry out the cycloaddition with a cis dienophile, such as methyl ds-2-butenoate, only the cis-substituted cyclohexene product is formed. With methyl tmtts-2-butenoate, only thetrans-substituted cyclohexene product is formed. 

The Diels-Alder cycloaddition reaction (Section 14.4) is a pericvclic process that takes place between a diene (four tt electrons) and a dienophile (two tr electrons) to yield a cyclohexene product. Many thousands of examples of Diels-Alder reactions are known. They often take place easily at room temperature or slightly above, and they are stereospecific with respect to substituents. For example, room-temperature reaction between 1,3-butadiene and diethyl maleate (cis) yields exclusively the cis-disubstituted cyclohexene product. A similar reaction between 1,3-butadiene and diethyl fumarate (trans) yields exclusively the trans-disubstituted product. 

Cycioaddition reaction (Sections 14.4, 30.6) A peri cyclic reaction in which two reactants add together in a single step to yield a cyclic product. The Diels-Alder reaction between a diene and a dienophile to give a cyclohexene is an example. 

Harano and colleagues [48] found that the reactivity of the Diels-Alder reaction of cyclopentadienones with unactivated olefins is enhanced in phenolic solvents. Scheme 6.28 gives some examples of the cycloadditions of 2,5-bis-(methoxycar-bonyl)-3,4-diphenylcyclopentadienone 45 with styrene and cyclohexene in p-chlorophenol (PCP). Notice the result of the cycloaddition of cyclohexene which is known to be a very unreactive dienophile in PCP at 80 °C the reaction works, while no Diels-Alder adduct was obtained in benzene. PCP also favors the decarbonylation of the adduct, generating a new conjugated dienic system, and therefore a subsequent Diels-Alder reaction is possible. Thus, the thermolysis at 170 °C for 10 h of Diels-Alder adduct 47, which comes from the cycloaddition of 45 with 1,5-octadiene 46 (Scheme 6.29), gives the multiple Diels-Alder adduct 49 via decarbonylated adduct 48. In PCP, the reaction occurs at a temperature about 50 °C lower than when performed without solvent, and product 49 is obtained by a one-pot procedure in good yield. 

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... 

Figure 5.60 The transition-state region of the reaction profile (along the IRC) for the model butadiene + ethylene Diels-Alder reaction. (The zero of energy corresponds to the cyclohexene product.)...

Figure 5.60 displays a portion of the Diels-Alder reaction profile in the neighborhood of the TS complex. In this case the IRC profile is rather unsymmetric around s = 0, ascending slowly from the reactant side toward the TS summit, then plummeting rapidly toward the product cyclohexene limit, which lies about 29 kcal mol-1 below the reactants (and 54 kcal mol-1 below the TS). 

The Diels-Alder reaction is the reaction of a diene with a mono-ene to form a cyclohexene derivative, an important reaction for the construction of organic intermediates. One of its attractions is the atom efficiency of 100%, no by-products being formed. The mono-ene, or dienophile which may also be an alkync, has a LUMO of low energy while the diene is usually electron rich with a high lying HOMO. The interaction of these two orbitals starts the reaction between the two molecules (Figure 2.33) [26],... 

Fig. 10.11 The stepwise and concerted mechanisms for the Diels-Alder reaction between butadiene and ethylene. The reactants (lower left) proceed to the product, cyclohexene (lower right) either through a two step, two transition state mechanism involving the formation of a diradical intermediate (top center), or more directly through the symmetric synchronous transition state (bottom center) (Storer, J. W., Raimondi, L., and Houk, K. N., J. Am. Chem. Soc. 116, 9675 (1994))...

Diels-Alder reactions of 2-pyrones are efficient methods towards construction of bridged cyclohexene derivatives for natural product syntheses. Early studies by... 

Photochemical [2 + 2] cycloadditions followed by vinylcyclobutane to cyclohexene rearrangements often yield the same products as the thermal Diels-Alder reactions. Strongly directing substituents on unsymmetrical dienes, however, may lead to constitutionally different isomers depending on the reaction path.19 21... 

The Diels-Alder reaction is one of the most important carbon-carbon bond forming reactions,521 522 which is particularly useful in the synthesis of natural products. Examples of practical significance of the cycloaddition of hydrocarbons, however, are also known. Discovered in 1928 by Diels and Alder,523 it is a reaction between a conjugated diene and a dienophile (alkene, alkyne) to form a six-membered carbo-cyclic ring. The Diels-Alder reaction is a reversible, thermally allowed pericyclic transformation or, according to the Woodward-Hoffmann nomenclature,524 a [4 + 2]-cycloaddition. The prototype reaction is the transformation between 1,3-butadiene and ethylene to give cyclohexene ... 

This and other similar cycloadditions, however, when unactivated hydrocarbons without heteroatom substituents participate in Diels-Alder reaction, are rarely efficient, requiring forcing conditions (high temperature, high pressure, prolonged reaction time) and giving the addition product in low yield. Diels-Alder reactions work well if electron-poor dienophiles (a, p-un saturated carbonyl compounds, esters, nitriles, nitro compounds, etc.) react with electron-rich dienes. For example, compared to the reaction in Eq. (6.86), 1,3-butadiene reacts with acrolein at 100°C to give formy 1-3-cyclohexene in 100% yield. 

Although ion-molecule processes would be possible in some electron impact reactions, this process would not lead to the formation of dimeric products formed in the present experiments (20). Ethylene was the major gaseous product. Ethylene and butadiene may be produced from the reverse Diels-Alder reaction of cyclohexene (10) since they were also... 

The Diels-Alder reaction is highly stereospecific. The diene reacts in an unfavorable conformation in which its double bonds lie in a plane on the same side (cis) of the single bond connecting them. This s-cis (or cisoid) conformation is required to give a stable product with a cis double bond. Addition of ethene to the alternate and more stable (transoid) conformation would give an impossibly strained Pimv-cyclohexene ring. Possible transition states for reaction in each conformation follow, and it will be seen that enormous mo-... 

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