Stereochemistry of Diels—Alder reaction

Stereochemistry of Diels-Alder Reactions. Thermodynamic vs. Kinetic Control... 

The first organoaluminium complex that catalysed a Diels Alder reaction was formed from menthol and ethylaluminium dichloride. This finding was complemented by work of Corey who showed that the aluminium diamine complex (49) was effective for controlling the stereochemistry of Diels-Alder reactions involving cyclopentadiene and acryloyl and crotonyl amides (e.g. 

The rates, regiochemistry, and stereochemistry of Diels-Alder reactions are affected by the solvent, and are often correlated to solvent polarity scales. In Chapter 15, we will cover orbital interactions that dictate the dominant regio-isomers of Diels-Alder reactions similar to that given below. The diene A is considered to be a nucleophile and... 

Chiral sulfinyldicncs have also been used in Diels-Alder reactions [88] interest in the synthesis of 1- and 2-sulfinyldienes in enantiopure form [531] and in their use to control the stereochemistry in Diels-Alder reactions is growing fast (see [532] and references therein). 

There are exceptions to favored endo stereochemistry of Diels-Alder additions. Some of these exceptions arise because the addition reaction is reversible, dissociation being particularly important at high temperature. The exo configuration is generally more stable than the endo and, given time to reach equilibrium (cf. Section 10-4A), the exo isomer may be the major adduct. Thus endo stereospecificity can be expected only when the additions are subject to kinetic control. 

Q Predict the products of Diels-Alder reactions, including the orientation of cycloaddition with unsymmetrical reagents and the stereochemistry of the products. 

Regiochemistry and stereochemistry When both components of a cycloaddition reaction are unsymmetrically substituted, two regioisomeric cycloadducts are possible. In the case of Diels-Alder reactions, these are shown in the reactions of both C-1 and C-2 substituted dienes and monosubstituted dienophiles. Isomeric adducts can be referred as ortho, meta and para in reference to similar disubstitution isomers of benzene. 

Mechanistic studies in this area are essentially nonexistent. Recently, the regiochemistry of some [4 + 2] cycloadditions of thioaldehydes has been rationalized based on FMO theory.2 It is difficult to generalize the resulting stereochemistry of this type of Diels-Alder reaction since systematic studies are lacking and since many structurally diverse kinds of thiocarbonyl dienophiles have been used as 277 components. 

Cycloadditions. In fact, cycloadditions like the Diels-Alder reaction are syn additions wherein the maximum overlap of the interacting n bonds eventually further governs the stereochemistry. In Diels-Alder reaction the 1, 4-addition of the double bond of a dienophile I i.e., 2, 4-cyclo hexene-1, 4-dione) to a conjugated diene II to generate a 6-membered ring, such that up to four new stereocenteres (i.e., chiral centres) may be created simultaneously at one go, as depicted below ... 

Maintaining an eye for ciarity With every edition we improve the presentation of topics, reactions, and diagrams where the opportunity arises. In this edition some examples include improved discussion and diagrams regarding endo and exo Diels-Alder transition states, the effect of diene stereochemistry in Diels-Alder reactions (Section 13.1 OB), and improved mechanism depictions for aromatic sulfonation and thionyl chloride substitution. 

Sol 15. The majority of Diels—Alder reactions yield endo products. In most of the cases, the exo product is thermodynamically more stable, but the endo adduct forms much more rapidly (kinetic control). However, [4 + 2] cycloaddition of furan and maleic anhydride gives predominantly exo adduct. Such a stereochemistry of the furan-maleic anhydride adduct is due to the fact that the initially formed endo compound readily reverses into the reactants whereas the exo cycloaddition gives a relatively stable adduct that is the product of the thermodynamic control. 

Now let us look more closely at the scope and limitations, stereochemistry, and mechanism of Diels-Alder reactions. 

Draw the products of each reaction. Indicate the stereochemistry of Diels-Alder products. 

Recent advances in the control of absolute stereochemistry in Diels-Alder reactions of 2-pyrones with dienophiles have been reviewed. The position of the electron-withdrawing group on simple y-pyrones is a decisive factor in the 4 - - 2-cycloaddition of y-pyrones with Danishefsky s diene and with azomethine ylides. The solid-state photo-4 -I- 2-cycloadditions of 2-pyrone-5-carboxylates with maleimides give only the... 

Endo-Exo Stereochemistry Sometimes Diels-Alder reactions give two types of products, viz, endo and exo isomers. The product in which substituent on dienophiie are cis to double bond formed in diene after Diels-Alder reaction is known as endo-isomer and the other product in which substituent on dienophiie are trans-to double bond is known as exo-isomer. Alteranatively, the product in which substituent is on the side away from bridge is known as endo-isomer and the product having substituent on the same side of the bridge is known as exo-isomer. 

The stereochemistry of the 1,3-dipolar cycloaddition reaction is analogous to that of the Diels-Alder reaction and is a stereospecific syn addition. Diazomethane, for example, adds stereospecifically to the diesters 43 and 44 to yield the pyrazolines 45 and 46, respectively. 

Figure 15.6 FMO rationalizes the stereochemistry of substituted Diels-Alder reactions...

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