Self-Diels-Alder reactions

In contrast to those unreactive dienes that can t achieve an s-cis conformation, other dienes are fixed only in the correct s-cis geometry and are therefore highly reactive in the Diels-Alder cycloaddition reaction. 1,3-Cyclopentadiene, for example, is so reactive that it reacts with itself. At room temperature, 1,3-cycIopentadiene dimerizes. One molecule acts as diene and a second molecule acts as dienophile in a self Diels-Alder reaction. 

The use of zeolites is particularly advantageous for self-Diels-Alder reactions of gaseous dienes because it reduces the polymerization of the reactant. An example is the cyclodimerization of 1,3-butadiene to 4-vinylcyclohexene [20a] carried out at 250 °C with satisfactory conversion when non-acidic zeolites, such as large-pore zeolites Na-ZSM-20, Na- S and Na-Y, are used. 

Indium trichloride [30] and methylrhenium trioxide [31] catalyze the aqueous Diels-Alder reaction of acrolein and acrylates with cyclic and open-chain dienes. Some examples of the cycloaddition of methyl vinyl ketone with 1,3-cyclohexadiene are reported in Scheme 6.18. MeReOs does not give satisfactory yields for acroleins and methyl vinyl ketones with substituents at the jS-position and favors the self-Diels-Alder reaction of diene. 

Cyclobutadiene is highly reactive and shows none of the properties associated with aromaticity. In fact, it was not even prepared until 1965, when Rowland Pettit of the University of Texas was able to make it at low temperature. Even at -78°C, however, cyclobutadieni is so reactive that it dimerizes by a self-Diels-Alder reaction. One molecule behaves as a diene and the other as a dienophile ... 

At low monomer concentrations (10-20%), the polymerization mixture after 24 hrs contained polymer 45% and 81%, respectively, and unreacted monomer. At higher concentration (>40%), all the monomer appeared to have been totally reacted in less than 5 hrs. The reaction mixture, however, was found to contain 5-10% of a dimer formed by a self Diels-Alder reaction of the monomer. 

Pyrrolidine has been the common organocatalyst used by Boger and coworkers for the synthesis of highly substituted pyridines based on the inverse-electron-demand Diels-Alder reaction of in situ formed enamines and 1,2,4-triazines. Barbas and coworkers have described the amine-catalysed direct self-Diels-Alder reaction of a,p-unsaturated ketones. The inverse-electron-demand hetero-Diels-Alder reaction of nitrosoalkenes and in situ formed enamines as dienophiles was also shown to be possible. ... 

In 2002, the Barbas group reported the first example of a self- Diels-Alder reaction or double Michael reaction of a,(3-unsaturated ketones providing access to acyl-substituted cyclohexanones. This reaction employed a combination of enamine and iminium ion chemistry, allowing for the HOMO and LUMO orbital to be raised and lowered respectively, promoting reactivity. Using L-proline, the ketone undergoes a self-Diels-Alder reaction, and produces access to exo.endo products in moderate yields and modest diastereoselectivities (Scheme 5.35). 

Scheme 5.35 Proline-catalysed self-Diels-Alder reaction of a,P-unsaturated ketones.

Scheme 5.36 Self-Diels-Alder reaction of ester-containing a,p-unsaturated ketones.

An intense purple crystalline solid m.p. 219-220 C. One of the few monomeric cyclo-pentadienone derivatives, most of which spontaneously undergo self Diels-Alder type dimerization. Used as a diene in many studies of various aspects of the Diels-Alder reaction. ... 

Sorbic acid is oxidized rapidly in the presence of molecular oxygen or peroxide compounds. The decomposition products indicate that the double bond farthest from the carboxyl group is oxidized (11). More complete oxidation leads to acetaldehyde, acetic acid, fumaraldehyde, fumaric acid, and polymeric products. Sorbic acid undergoes Diels-Alder reactions with many dienophiles and undergoes self-dimerization, which leads to eight possible isomeric Diels-Alder stmctures (12). 

The dimer acids [61788-89-4] 9- and 10-carboxystearic acids, and C-21 dicarboxylic acids are products resulting from three different reactions of C-18 unsaturated fatty acids. These reactions are, respectively, self-condensation, reaction with carbon monoxide followed by oxidation of the resulting 9- or 10-formylstearic acid (or, alternatively, by hydrocarboxylation of the unsaturated fatty acid), and Diels-Alder reaction with acryUc acid. The starting materials for these reactions have been almost exclusively tall oil fatty acids or, to a lesser degree, oleic acid, although other unsaturated fatty acid feedstocks can be used (see Carboxylic acids. Fatty acids from tall oil Tall oil). 

An ab initio MO calculation by Jorgensen revealed enhanced hydrogen bonding of a water molecule to the transition states for the Diels-Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile, which indicates that the observed rate accelerations for Diels-Alder reactions in aqueous solution arise from the hydrogenbonding effect in addition to a relatively constant hydrophobic term.7,76 Ab initio calculation using a self-consistent reaction field continuum model shows that electronic and nuclear polarization effects in solution are crucial to explain the stereoselectivity of nonsymmetrical... 

Diels-Alder reaction Self-micellization of the diene affects yield Grieco etal., 1983... 

J. Kang, J. Rebek, Jr, Acceleration of a Diels-Alder reaction by a self-assembled molecular capsule , Nature 1997, 385, 50-52. 

Extensive reviews of Diels-Alder reactions and hetero-Diels-Alder reactions in aqueous media have been presented. " " " " Micelles in the presence of catalytically active transition-metal ions catalyse the Diels-Alder reaction between 3-(/ -substituted phenyl)-l-(2-pyridyl)prop-2-en-l-ones with cyclopentadiene by a factor of 1.8 x 10 compared with the uncatalysed reaction in MeCN. " Diels-Alder reactions have been shown to be accelerated by encapsulation of both reactants by pseudospherical capsules assembled from self-complementary molecules (103). " ... 

The combination of modem valence bond theory, in its spin-coupled (SC) form, and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field (CASSCF) wavefunction, is demonstrated to provide quantitative and yet very easy-to-visualize models for the electronic mechanisms of three gas-phase six-electron pericyclic reactions, namely the Diels-Alder reaction between butadiene and ethene, the 1,3-dipolar cycloaddition of fulminic acid to ethyne, and the disrotatory electrocyclic ringopening of cyclohexadiene. 

Recently, the first examples of catalytic enantioselective preparations of chiral a-substituted allylic boronates have appeared. Cyclic dihydropyranylboronate 76 (Fig. 6) is prepared in very high enantiomeric purity by an inverse electron-demand hetero-Diels-Alder reaction between 3-boronoacrolein pinacolate (87) and ethyl vinyl ether catalyzed by chiral Cr(lll) complex 88 (Eq. 64). The resulting boronate 76 adds stereoselectively to aldehydes to give 2-hydroxyalkyl dihydropyran products 90 in a one-pot process.The diastereoselectiv-ity of the addition is explained by invoking transition structure 89. Key to this process is the fact that the possible self-allylboration between 76 and 87 does not take place at room temperature. Several applications of this three-component reaction to the synthesis of complex natural products have been described (see section on Applications to the Synthesis of Natural Products ). 

Kang, J., Santamaria, J., Hilmersson, G. and Rebek, f. fr. (1998) Self-assembled molecular capsule catalyzes a Diels-Alder reaction. J. Am. Chem. Soc.. 120. 7389-7390. Kang, J., Hilmersson, G., Santamaria, J. and Rebek, f. fr. (1998) Diels—Alder reactions through reversible encapsulation. J. Am. Chem. Soc., 120, 3650—3656. 

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