Characteristics of the Diels-Alder Reaction

Kurt Alder (1902-1958) was born in Konigshutte, Prussia, and moved to Germany after World War I. 

He received his Ph.D. in 1926 at Kiel working with Otto Dieis. He worked first at I. G. Farbert on the manufacture of plastics but then became professor at the University of Cologne (1940-1958). He snared the 1950 Nobel Prize in chemistry with his mentor, Otto Diels. 

1950 Nobel Prize in chemistry was awarded to Diels and Alder in recognition of the importance of their discovery. 

The mechanism of the Diels-Alder cycloaddition is different from that of other reactions we ve studied because it is neither polar nor radical. Rather, the Diels-Alder reaction is a pericyclic process. Pericyclic reactions, which we ll discuss in more detail in Chapter 30, take place in a single step by a cyclic redistribution of bonding electrons. The two reactants simply join together through a cyclic transition state in which the two new carbon-carbon bonds form at the same time. 

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. 

The Diels-Alder cycloaddition reaction occurs most rapidly if the alkene component, or dienophile ( diene lover ), has an electron-withdrawing substituent group. Thus, ethylene itself reacts sluggishly, but propenal, ethyl propenoate, maleic anhydride, benzoquinone, proiicnenitrile, and similar compounds are highly reactive. Note also that alkyncs, such as methyl propynoate, can act as Diels-Alder dienophiles. 

One of the most useful features of the Diels-Alder reaction is that it is stereo-specific, meaning that a single product. stereoisomer is formed. Furthermore, the stereochemistry of the reactant is maintained. If we carrv out the axloaddition 

Another stereochemical feature of the Dicls-Alder reaction is that the diene and dienophile partners orient so that the endo product, ratlier than the alternative exo product, is formed. The words endo and exo are used to indicate relative stereochemistry when referring to bicyciic structures like substituted norbornanes (Section 4.9). A substituent on one bridge is said to be exo if it is anti (trans) to the larger of the other two bridges and is said to be endo if it is syn (cis) to the larger of the other two bridges. 

Endo products result from Diels-Alder reactions because the amount of orbital overlap between diene and dienophile is greater when the reaclants lie directly on top of one another so that the electron-withdrawing. substituent on the dienojihile is underneath the diene. In the reaction of 1,3-cyciopentadiene with maleic anhydride, for instance, the following result is obtained  

Predict the product of the following Diels-Alder reaction  

Electrostatic potential maps of ethylene, propenal, and propenenitrile show that electron-withdrawing groups make the doublebond carbons less negative. 

Mechanism ot the Diels-Alder cycloadditron reaction. The reaction occurs in a single step through a cyclic transition state in which the two new carbon-carbon bonds form simuitaneously. 

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

There is a second type of stereoselectivity that is characteristic of the Diels-Alder reaction. The addition of a dienophile such as maleic anhydride to a cyclic diene like 1,3-cyclopentadiene could provide two products, the endo-adduct 2 and the exoadduct 3 (Eq. 12.5). However, only the e do-cycloadduct 2, in which the two boldfaced hydrogens are syn to the one-carbon bridge, is observed experimentally, and its preferential formation follows what is now commonly termed the Alder rule. The basis for this result is believed to be stabilization of the transition state 4 by secondary orbital interactions that occur through space between the p-orbitals on the internal carbons of the diene and the carbonyl carbon atoms of the dienophile, as shown by the dashed lines in 4. Analogous stabilization is not possible in transition state 5. Structure 4 is thus characterized as the one being stabilized by maximum orbital overlap. It should be noted that not all Diels-Alder reactions are as stereoselective as the one between l,3maleic anhydride mixtures of endo-and cxo-products are sometimes obtained. 

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