Diels-Alder Reactions, Mechanistic Aspects

With the modem thermostated densimeters it is much easier to determine the temperature dependence of partial molar volumes and, hence, of reaction volumes. From these data El yanov extrapolated a generally applicable equation shown in Table 2.3 (footnote a) to describe the temperature dependence of activation and reaction volumes. The temperature dependence obtained experimentally for isoprene dimerization is in accord with the El yanov equation. 

Accordingly, the partial molar volumes of the pericyclic transition states, which 

In the thermal dimerization of chloroprene (1), the activation volumes of two [4 + 2] cycloadditions leading to 2 and 3 were found to be smaller than those of the third [4 + 2] and the [2 + 2] cycloadditions leading to 4, 5, and 6, respectively. Steward [26] explained these results in terms of concerted Diels-Alder reactions competing with stepwise [2 + 2] cycloadditions. According to its larger (less negative) activation volume, the third Diels-Alder adduct 4 should also be formed in a non-concerted process. Similarly it can be concluded from the pressure dependence of the dimerization of 1,3-cyclohexadiene (7) that the endo-Diels-Alder dimer 8 and the [6 + 4]-ene product 9 are formed concertedly while the exo-Diels-Alder adduct 10 and the [2 + 2] cyclodimers 11 and 12 arise via diradical intermediates. 

According to the activation volume data the Diels-Alder dimerization of 1,3-butadiene [39] and o-quinodimethane (Table 2.5, entries (3) and (4), respectively) fall into the same class of concerted processes as those discussed for 1 and 7, while the Diels-Alder dimerization of hexamethylbis(methylene)cyclopentane seems to occur in stepwise fashion. According to the activation volume data summarized in Table 2.6 only the Diels-Alder reaction of 1,3-butadiene with a-acetoxyacrylonitrile seems to proceed concertedly while all other Diels-Alder and homo-Diels-Alder adducts are probably formed in stepwise processes comparable to the corresponding competitive [2 + 2] cycloadditions. Stereochemical investigations of the chloroprene and 1,3-butadiene dimerization using specifically deuterated derivatives confirm the conclusions drawn from activation volume data. In the dimerization of ( )-l-deuteriochloroprene (17) the diastereomeric Diels-Alder adducts ISa-Dz and 

This is only found for reactions involving non-polar diradical intermediates and is not observed for polar zwitterionic intermediates. In the latter case the volume of activation of the zwitterionic intermediate can be more negative than that of a competing pericyclic reaction due to the solvent-dependent effect of electro- 

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This chapter introduces the experimental work described in the following chapters. Some mechanistic aspects of the Diels-Alder reaction and Lewis-acid catalysis thereof are discussed. This chapter presents a critical survey of the literature on solvent ejfects on Diels-Alder reactions, with particular emphasis on the intriguing properties of water in connection with their effect on rate and selectivity. Similarly, the ejfects of water on Lewis acid - Lewis base interactions are discussed. Finally the aims of this thesis are outlined. 

In Chapter 1 mechanistic aspects of Are Diels-Alder reaction are discussed. The literature on the effects of solvents and Lewis-acid catalysts on this reaction is surveyed. The special properties of water are reviewed and the effects of water on the Diels-Alder reaction is discussed. Finally, the effect of water on Lewis acid - Lewis base interactions is described. 

For a discussion of the mechanistic course of the reaction, many aspects have to be taken into account. The cisoid conformation of the diene 1, which is in equilibrium with the thermodynamically more favored transoid conformation, is a prerequisite for the cycloaddition step. Favored by a fixed cisoid geometry are those substrates where the diene is fitted into a ring, e.g. cyclopentadiene 5. This particular compound is so reactive that it dimerizes easily at room temperature by undergoing a Diels-Alder reaction ... 

Cyclopentadienes and cyclohexadienes are versatile dienes that are commonly used to study mechanistic, regio- and stereochemical aspects of the Diels-Alder reaction and for synthetic purposes. 

Luche and coworkers [34] investigated the mechanistic aspects of Diels-Alder reactions of anthracene with either 1,4-benzoquinone or maleic anhydride. The cycloaddition of anthracene with maleic anhydride in DCM is slow under US irradiation in the presence or absence of 5% tris (p-bromophenyl) aminium hexachloroantimonate (the classical Bauld monoelectronic oxidant, TBPA), whereas the Diels Alder reaction of 1,4-benzoquinone with anthracene in DCM under US irradiation at 80 °C is slow in the absence of 5 % TBPA but proceeds very quickly and with high yield at 25 °C in the presence of TBPA. This last cycloaddition is also strongly accelerated when carried out under stirring solely at 0°C with 1% FeCh. The US-promoted Diels Alder reaction in the presence of TBPA has been justified by hypothesizing a mechanism via radical-cation of diene, which is operative if the electronic affinity of dienophile is not too weak. 

A fundamental understanding of the mechanistic and stereochemical aspects of the Diels-Alder reaction was unfolded during the 1970s. Several theoretical approaches are available nowadays from which Fukui s Frontier Molecular Orbital theory (FMO theory) is most frequently used because of its simplicity49- 53. 

This theory proves to be remarkably useful in rationalizing the whole set of general rules and mechanistic aspects described in the previous section as characteristic features of the Diels-Alder reaction. The application of perturbation molecular orbital theory as an approximate quantum mechanical method forms the theoretical basis of Fukui s FMO theory. Perturbation theory predicts a net stabilization for the intermolecular interaction between a diene and a dienophile as a consequence of the interaction of an occupied molecular orbital of one reaction partner with an unoccupied molecular orbital of the other reaction partner. 

We will present mechanistic aspects of the Diels-Alder reaction, its selectivity and reactivity in order to explain solvent effects on the one hand, and the effects of Lewis acids on the other. Other catalytic systems like micelles will also be addressed. Some of the explanations may seem trivial or are well-known but, as we will use these in later sections, a clear terminology is desirable. 

J. Sauer, R. Sustmann, Mechanistic Aspects of Diels-Alder Reactions A Critical Survey, Angew. Chem. Int. Ed Engl. 1980,19, 779-807. 

The first Diels-Alder reaction studied under high pressure was the dimerization of cyclopen-tadiene [751]. For recent, more detailed studies of the pressure-dependence of Diels-Alder cycloaddition reactions in solvents of different polarity, as well as discussions of the corresponding mechanistic aspects, see references [857, 874]. 

Sauer J, Sustmann R (1980) Mechanistic aspects of Diels-Alder reactions a critical survey. Angew Chem Int Ed Engl 19 779-807... 

Matsuoka, T., Harano, K., Uemura, T., Hisano, T. Hetero Diels-Alder reaction of N-acyl imines. I. The reaction of N -thiobenzoyl-N,N-dimethylformamidine with electron-deficient dienophiles. Stereochemical and mechanistic aspects. Chem. Pharm. Bull. 1993, 41, 50-54. Mikami, K., Motoyama, Y., Terada, M. Asymmetric Catalysis of Diels-Alder Cycloadditions by an MS-Free Binaphthol-Titanium Complex Dramatic Effect of MS, Linear vs Positive Nonlinear Relationship, and Synthetic Applications. J. Am. Chem. Soc. 1994, 116, 2812-2820. 

Although this chapter is limited to intermolecular all-carbon reactions, the literature connected to this type of Diels-Alder reactions is still immense. The last general reviews about intermolecular Diels-Alder reactions date from nearly ten years ago - v During the past decade, several reviews were published dealing with specialized topics such as mechanistic aspects , specific dienes 2-35 dienophiles , applications in synthesis " ° and introduction of chirality by using chiral auxiliaries qj- chiral Lewis acids . ... 

The following mechanistic aspects have been found to be characteristic of the Diels-Alder reaction . ... 

The Mechanistic Aspect of the Hetero-Diels-Alder Reactions. 2... 

Addition and Cycloaddition Reactions of Tetracyanoethylene in Organic Chemistry, A. J. Fatiadi (1987). This review focusses on [2 + 2] cycloaddition, the Diels-Alder reaction, the ene reaction and higher-order cycloadditions of TCNE to carbocyclic and heterocyclic substrates. Mechanistic aspects and thermal and photocycloaddition reactions also are discussed. 406 references are given. 

The Diels-Alder reaction, a classic of synthetic organic chemistry, is theoretically a 4 + 2 cycloaddition in which a diene and a dienophile with very different electron abundances combine to form a new six-membered ring. Equation 7.1 is a typical Diels-Alder synthesis. The reaction has great synthetic usefulness and has attracted many researchers interested in its practical applications and mechanistic aspects. It is not at all certain whether any Diels-Alder reaction occurs under environmental... 

In keeping with a mechanistic emphasis, the book was reorganized. The chapter on mechanism is now Chapter 5 instead of Chapter 10. Thus the first six chapters focus on the mechanistic and structural underpinnings of organic chemistry. Synthetic aspects of organic chemistry are then discussed from a mechanistic and structural point of view. Several new sections have been added and others expanded. An expanded discussion of resonance and aromaticity is found in Chapter 1. A section on organopalladium chemistry and olefin metathesis has been added to Chapter 8 as they relate to current methods of carbon-carbon bond formation. Chapter 9 on free-radical reactions for carbon-carbon bond formation has been revised. The discussion of Diels-Alder chemistry has been moved to Chapter 10 and expanded. A number of new problems have been added which serve to further illustrate the principles developed in each chapter. Finally, thanks to input from many people who have read dris text and taught from it, the discussion has been further honed and errors corrected. 

The Diels-Alder (DA) reaction is one of the most basic and useful reactions in synthetic organic chemistry because it provides simple, efficient, and clean procedures to generate new bonds by intermolecular or intramolecular couplings. It was discovered in 1928 by Otto Diels and Kurt Alder, who were awarded the 1950 Nobel Prize in chemistry. Since then, it has received a great deal of attention, as shown by the numerous publications concerning its synthetic, mechanistic and theoretical aspects [1-5]. 

Rearrangements.—Mechanistic and stereochemical aspects of the photochemical ring expansion of cyclic ketones via oxacarbenes have been discussed. Evidence for a quantum chain process in the photoreactions of benzvalene is reported, and hydrogen-abstraction reactions and subsequent rearrangements of 1,3-diene-p-benzoquinone Diels-Alder adducts have been extensively investigated. ... 

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