Diels hydrocarbon 867 -Alder reaction

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 reactants as shown in Scheme 1.1 can consist of only hydrocarbon fragments (homo-Diels-Alder reaction) but can also contain one or more heteroatoms on any of the positions... 

Firestone at al. " demonstrated the importance of solvent density in the special case of intramolecular Diels-Alder reaction in highly viscous media. Efficient packing of the hydrocarbon solvent was... 

In the case of the retro Diels-Alder reaction, the nature of the activated complex plays a key role. In the activation process of this transformation, the reaction centre undergoes changes, mainly in the electron distributions, that cause a lowering of the chemical potential of the surrounding water molecules. Most likely, the latter is a consequence of an increased interaction between the reaction centre and the water molecules. Since the enforced hydrophobic effect is entropic in origin, this implies that the orientational constraints of the water molecules in the hydrophobic hydration shell are relieved in the activation process. Hence, it almost seems as if in the activated complex, the hydrocarbon part of the reaction centre is involved in hydrogen bonding interactions. Note that the... 

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

Sulfur dioxide acts as a dienophile ia the Diels-Alder reaction with many dienes (253,254) and this reaction is conducted on a commercial scale with butadiene. The initial adduct, sulfolene [77-79-2] is hydrogenated to a solvent, sulfolane [126-33-0] which is useful for selective extraction of aromatic hydrocarbons from... 

Hydrocarbon oxadiazoles do not readily undergo Diels-Alder reactions, but 2,5-bis(trifluoromethyl)-l, 3,4-oxadiazole reacts with a number of strained or elec-... 

A domino reaction,in this case consisting of an inter- and an intramolecular Diels-Alder reaction, is a key step in the synthesis of the hydrocarbon pago-dane 30, reported by Prinzbach et al When the bis-diQnQ 27 is treated with maleic anhydride 4, an initial intermolecular reaction leads to the intermediate product 28, which cannot be isolated, but rather reacts intramolecularly to give the pagodane precursor 29 ... 

Rideout and Breslow first reported [2a] the kinetic data for the accelerating effect of water, for the Diels Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile and the cycloaddition of anthracene-9-carbinol with N-ethylmaleimide, giving impetus to research in this area (Table 6.1). The reaction in water is 28 to 740 times faster than in the apolar hydrocarbon isooctane. By adding lithium chloride (salting-out agent) the reaction rate increases 2.5 times further, while the presence of guanidinium chloride decreases it. The authors suggested that this exceptional effect of water is the result of a combination of two factors the polarity of the medium and the... 

The Diels-Alder reaction is one of the most important methods used to form cyclic structures and is one of the earliest examples of carbon-carbon bond formation reactions in aqueous media.21 Diels-Alder reactions in aqueous media were in fact first carried out in the 1930s, when the reaction was discovered,22 but no particular attention was paid to this fact until 1980, when Breslow23 made the dramatic observation that the reaction of cyclopentadiene with butenone in water (Eq. 12.1) was more than 700 times faster than the same reaction in isooctane, whereas the reaction rate in methanol is comparable to that in a hydrocarbon solvent. Such an unusual acceleration of the Diels-Alder reaction by water was attributed to the hydrophobic effect, 24 in which the hydrophobic interactions brought together the two nonpolar groups in the transition state. 

Much effort has been directed at developing aqueous Diels-Alder reactions toward the syntheses of a variety of complex natural products. Grieco employed micellar catalysis and pure water as the solvent for the Diels-Alder reaction of dienecarboxylate with a variety of dienophiles. For example, when the Diels-Alder reaction in Scheme 12.3 was carried out in water, a higher reaction rate and reversal of the selectivity were observed, compared with the same reaction in a hydrocarbon solvent (Scheme 12.3).81 Similarly, the reaction of 2,6-dimethylbenzoquinone with sodium ( )-3,5-hexadienoate (generated in situ by the addition of 0.95 equiv sodium bicarbonate to a suspension of the precursor acid in water) proceeded for 1 hour to give a 77% yield of the adduct... 

More interesting is the use of nonpolar solvents (e. g. xylene, toluene, carbon tetrachloride, hydrocarbons), because these are transparent to MW and absorb them only weakly. They therefore enable specific absorption by the reactants. If these reactants are polar, energy transfer occurs from the reactants to the solvent and the results might be different under the action of MW and A. This effect seems to be clearly dependent on the reaction and is, therefore, the subject of controversy. In xylene under reflux, for example, no MW-specific effects were observed for the Diels-Alder reaction [5] whereas important specific effects were described for aryldiazepinone synthesis [33]. 

Chemical separation of conjugated dienes and other polyunsaturated hydrocarbons is based on the availability of tt delocalized electrons. The use of a strong dienophile (e.g. tetracyanoethylene, TCNE) will derivatize only conjugated dienes, thus separating the polyunsaturated compounds into two groups. However, such derivatization is not always reversible since a retro-Diels-Alder reaction may require a high temperature. Hence, the retrieved compounds may be the thermostable ones and not those present in the initially analysed mixture. 

The reasons for the ewrfo-selectivity of Diels-Alder reactions are only useful for the reactions of dienophiles bearing substituents with lone pairs without a Lewis basic site no secondary orbital interactions are possible. But even in reactions of pure hydrocarbons the ewrfo-selectivity is observed, requiring alternative explanations. For example, the ewrfo-preference of the reactions of cyclopropene with substituted butadienes have been rationalized on the basis of a special type of secondary orbital interactions70. Apart from secondary orbital interactions which are probably the most important reason for the selec-tivities of Diels-Alder reactions, recent literature also advocates other interpretations. 

At first glance the Diels-Alder reaction represents an organic transformation which is relatively insensitive to solvent effects (Table 9). For the dimerization of cyclopentadiene, the second-order rate constants in a broad range of organic solvents are quite similar5. The data of Table 9 refer to the special case of a Diels-Alder reaction between two pure hydrocarbons. Usually, Diels-Alder reactions only proceed at an appreciable rate when either the diene or the dienophile is activated by electron-donating or electron-withdrawing... 

All the chlorinated hydrocarbons belonging to this second group of compounds, once used in large amounts, have been banned for use in the U.S. since 1974. They are made by the Diels-Alder reaction, named after two chemists who won the Nobel Prize in 1950 for the discovery of this important reaction. The synthesis of the important insecticides chlordane, heptachlor, aldrin, dieldrin, and endrin are summarized in Fig. 20.4. 

Polycyclic aromatic hydrocarbons are moderately reactive as the diene component of Diels-Alder reactions. Anthracene forms adducts with a number of reactive dienophiles. The addition occurs at the center ring. There is no net loss of resonance stabilization, because the anthracene ring (resonance energy = 1.60 eV) is replaced by two benzenoid rings (total resonance energy = 2 x 0.87 = 1.74 eV).48 49... 

Retro-Diels-Alder reactions have long been studied and discussed with an emphasis on whether they should be considered concerted or step-wise processes. Femtosecond real time studies of representative retro-Diels-Alder reactions of simple hydrocarbons have helped to provide an answer and to sharpen the nature... 

Replacing the hydrogen in 68 with a phenyl group leads to the secondary acetylenic monomer 70. It was believed that this disubstituted acetylene would suppress the reaction of acetylene with itself and insure that there was an acetylene functionality available for reaction with the o-quinodimethane at 200 °G The DSC of 68 showed the presence of a single exothermic peak at 263 °C which the authors felt was adequate evidence for the occurrence of a Diels-Alder reaction between the acetylene and benzocyclobutene. Unfortunately they did not report on any control experiments such as that between diphenylacetylene and simple benzocyclobutene hydrocarbon or a monofunctional benzocyclobutene in order to isolate the low molecular weight cycloaddition product for subsequent characterization. The resulting homopolymer of 68 had a Tg of 274 °C and also had the best thermooxidative stability of all of the acetylenic benzocyclobutenes studied (84% weight retention after 200 h at 343 °C in air). 

By far the most important property of benzo[c] furans is their capacity to act as 471-components in cycloaddition reactions. Whereas the reactions described before 1969 were almost always of the Diels-Alder type, more recent investigations have shown that they can also participate in [7 4 + 714]-and [714 + TCgj-addition (Section IV,C). In this chapter Diels-Alder reactions will be discussed. Benzo[c]furans have been used for two main purposes. First, Diels-Alder adducts with olefinic compounds can conveniently be dehydrated to naphthalene derivatives or higher condensed hydrocarbons not easily accessible by other methods second, benzo[c]furans are excellent... 

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

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. 

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