Other Facts About the Diels-Alder Reaction

14 Other Facts About the Diels-Alder Reaction 

The Diels-Alder reaction is used widely in organic synthesis, so yon mnst be able to look at a compound and determine what conjugated diene and what dienophile were nsed to make it. To draw the starting materials from a given Diels-Alder addnct  

Problem 16.27 What diene and dienophile are needed to prepare each product  

A reactive diene like 1,3-cyclopentadiene readily undergoes a Diels-Alder reaction with itself that is, 1,3-cyclopentadiene dimerizes because one molecule acts as the diene and another acts as the dienophile. 

The formation of dicyclopentadiene is so rapid that it takes only a few hours at room temperature for cyclopentadiene to completely dimerize. How, then, can cyclopentadiene be used in a Diels-Alder reaction if it really exists as a dimer  

How endo and exo products are formed in the Diels-Alder reaction 

The electron-withdrawing Z group is closer to the electron-rich diene. 

Pathway [2] With Z oriented away from the diene, the exo product is formed. 

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Other facts about the Diels-Alder reaction... 

Unlike the Diels-Alder reaction (where predictable and high regiose-lectivity is expected in a cycloaddition between an electron-rich diene and an electron-poor dienophile), little is known about the regiochemi-cal outcome of an analogous unsymmetrical HDA reaction. In fact, before our studies, reports of successful cycloadditions using substituted norbornadienes are rare in the literature.In one instance, TCNE, a symmetrical and highly reactive dienophile, reacts on the unsubstituted side of the 2-substituted NBD 17 to give the substituted cyclopropane derivative 18 (Eq. 12). Other attempts to promote the... 

It must be emphasized once again that the rules apply only to cycloaddition reactions that take place by cyclic mechanisms, that is, where two s bonds are formed (or broken) at about the same time. The rule does not apply to cases where one bond is clearly formed (or broken) before the other. It must further be emphasized that the fact that the thermal Diels-Alder reaction (mechanism a) is allowed by the principle of conservation of orbital symmetry does not constitute proof that any given Diels-Alder reaction proceeds by this mechanism. The principle merely says the mechanism is allowed, not that it must go by this pathway. However, the principle does say that thermal 2 + 2 cycloadditions in which the molecules assume a face-to-face geometry cannot take place by a cyclic mechanism because their activation energies would be too high (however, see below). As we shall see (15-49), such reactions largely occur by two-step mechanisms. Similarly. 2 + 4 photochemical cycloadditions are also known, but the fact that they are not stereospecific indicates that they also take place by the two-step diradical mechanism (mechanism... 

We have not given you much evidence to decide why it is that some thermal [2 + 2] cycloadditions occur but not others. What is special about fluoroalkenes, allenes, and ketenes in these reactions One possibility is that Mobius rather than the Hiickel transition states are involved, but the Mobius transition states are expected to suffer from steric hindrance (Section 21-10B). It is also possible that [2 + 2] cycloadditions, unlike the Diels-Alder additions, proceed by stepwise mechanisms. This possibility is strongly supported by the fact that these reactions generally are not stereospecific. Thus with tetrafluoroethene and trans,trans-2,5-bexadiene two products are formed, which differ in that the 1-propenyl group is trans to the methyl group in one adduct, 45, and cis in the other, 46 ... 

Comparing cis and trans isomers of propenyl propyl ether in their reactivity towards diphenylketene, a ratio kcislknans of about 180 has been found (in PhCN, 40°C) , confirmed by the value of 172 20 from experiments without solvent with dimethylketene the ratio is 60 10 . The opposite phenomenon has been observed in Diels-Alder reactions of fumarate/maleate and other 1,2-disubstituted ethylenes (Section 4.1.3). The fact that cis isomers are thermodynamically less stable than trans isomers seems to prevail in determining relative reactivities of these 1,2-cycloadditions. 

This proposed mechanism is essentially an elimination followed by an addition. So, it makes sense that we call this process an eUmination-addition reaction (as compared with the SNAr mechanism, which was called addition-elimination). This mechanism certainly seems a bit off the wall when you think about. Benzyne It looks like a terrible intermediate. But chemists have been able to show (with other experiments) that benzyne is in fact the intermediate of this reaction. Your textbook or instructor will most likely provide the additional evidence for the short-lived existence of benzyne (we use a trapping technique involving a Diels-Alder reaction). If you are curious about the evidence, you can look in your textbook. For now, let s make sure that we can predict the products of elimination-addition reactions. 

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