2,4-Hexadiene Diels-Alder reactions

A Dimethyl butynedioate undergoes a Diels-Alder reaction with (2 ,4 )-hexadiene. Show the structure and stereochemistry of the product. 

What stereochemistry would you expect for the product of the Diels-Alder reaction between (2 ,4 )-2,4-hexadiene and ethylene What stereochemistry would you expect if (2 ,4Z)-2,4-hexadiene were used instead ... 

Two other important sigmatropic reactions are the Claisen rearrangement of an allyl aryl ether discussed in Section 18.4 and the Cope rearrangement of a 1,5-hexadiene. These two, along with the Diels-Alder reaction, are the most useful pericyclic reactions for organic synthesis many thousands of examples of all three are known. Note that the Claisen rearrangement occurs with both allylic aryl ethers and allylic vinylic ethers. 

The first studies on cation-radical Diels-Alder reactions were undertaken by Bauld in 1981 who showed [33a] the powerful catalytic effect of aminium cation radical salts on certain Diels-Alder cycloadditions. For example, the reaction of 1,3-cyclohexadiene with trans, iraw5-2,4-hexadiene in the presence of Ar3N is complete in 1 h and gives only the endo adduct (Equation 1.14) [33]. 

This section is devoted to cyclizations and cycloadditions of ion-radicals. It is common knowledge that cyclization is an intramolecular reaction in which one new bond is generated. Cycloaddition consists of the generation of two new bonds and can proceed either intra- or intermolecularly. For instance, the transformation of 1,5-hexadiene cation-radical into 1,4-cyclohexadienyl cation-radical (Guo et al. 1988) is a cyclization reaction, whereas Diels-Alder reaction is a cycloaddition reaction. In line with the consideration within this book, ring closure reactions are divided according to their cation- or anion-radical mechanisms. 

Diets-Alder catalysis.2 This cation radical enhances the reactivity of a neutral or electron-rich eis-1,3-diene in Diels-Alder reactions. Thus 1,3-cyclohexadiene undergoes Diels-Alder dimerization only at temperatures around 200°. The presence of 5-10 mole % of this salt effects dimerization even at —78°, with the usual endo/ exo selectivity (5 1). It also permits facile condensation of 1,3-cyclohexadiene with a hindered dienophile such as 2,5-dimethyl-2,4-hexadiene (equation 1) the dimer of the former diene is a minor product (20% yield). 

Diels-Alder reactions. Although 2,4-hexadiene shows only slight regioselec-tivity in Diels-Alder reactions, the 1-acetoxy derivative (1) undergoes regioselective cycloaddition with a variety of dienophiles.1... 

Diels-Alder reactions.1 Nitrosoarenes undergo Diels-Alder reactions at 25° with cis- and/or tra/is-hexadienals 2 to give unstable adducts that can be identified by IR and H-NMR as 3 or the hemiacetals 4. On standing or warming to 40° these primary products rearrange to pyridinium betaines (5) or pyrroloindoles (6) as the... 

In 1989, the irradiation of (E,E)-2,4-hexadiene S3 sensitized by meso-porphyrin IX dimethyl ester led to the formation of cis-3,6-dimethyl-l,2-dioxene (62), which was the major product detected at — 78 °C in Freon 11 [69]. Endoperoxide 62 was purified under vacuum at 0.75 mmHg, and collected in a trap (98% isolated yield). Dienes that can adopt a cisoid conformation, such as 53 or ( , )-l,4-di phenyl butadiene, were photooxidized by the corresponding endoperoxides in high or quantitative yield in a suprafacial Diels-Alder reaction [60, 70], Dienes that cannot readily adopt cisoid conformations, such as (fc, Z)-2,4-hexadienes and (Z, Z)-2,4-hexadienes, lose their stereochemistry in the singlet oxygen [2 + 4]-cyclo-addition [60], (E,Z)- and (Z,Z)-dienes give a complex mixture of hydroperoxides and aldehydes, which suggests the intervention of intermediate zwitterions or 1,4-diradicals [71]. 

Explain which of these isomers of 2,4-hexadiene is more reactive as a diene in the Diels-Alder reaction ... 

Why does a stable bis-allyl analog exist on the Cope reaction surface of 27 In the prototype Diels-Alder reaction of 1,5-hexadiene, the possible bis-allyl intermediate (i.e., two isolated allyl radicals) is about 26 kcal mol higher in energy than the Cope transition state. Only with significant radical stabilization might one expect a bis-aUyl intermediate to occur. One can consider 28 as composed of two bridged phenalenyl radicals (29), which affords a rather dramatic stabilization of each radical. 

In general, die influence of diene conformation on the rate of silico Diels-Alder reaction speaks for a concerted mechanism of the [4+2] cycloaddition. This mechanism is confirmed by the small influence of solvent polarity and radical trt s on the reaction rate, which speaks against a pronotmced ionic or radical multiple step mechanism. On the other hand, trans-piperylene reacts over 1000 times faster with the standard silaethene than does tra s,tra s-hexadiene, wh eas the opposite is true for ethenes [27]. This fact contradicts dramatically the carbon-analogy of silaethenes, fiom which either trans-piperylene reacts too fast, or trans.trans-hexadiene too slow with the silaethene. 

Two groups have recently examined enantioselective A -sulfinyl dienophile Diels-Alder reactions. In one case, Whitesell et al. found that a phenylmenthol derived N-sulfinyl carbamate adds to ( , )-2,4-hexadiene under Lewis acid catalysis to afford a single diastereomer (equation 52). If the Lewis acid was omitted, a complex mixture of cycloaddition products was obtained. Attack of the diene on the N-sulBnyl dienophile as shown in the equation would rationalize the observed results. The site of Lewis acid complexation, however, is unknown. 

Problem 16.20 Why is (Z,Z)-2,4-hexadiene considerably less reactive than (E, )-2,4-hexadiene in a Diels-Alder reaction ... 

The uncatalyzed Diels-Alder reaction is well known to be highly stereospecific, preferentially occurring via syn addition to both the diene and dienophilic components. Stereochemical studies of the cation radical Diels-Alder reaction have confirmed an analogous stereospecificity in two distinctly different systems. The initial study was carried out using the cycloaddition of the three geometric isomers of 2,4-hexadiene as dienophilic components and 1,3-cyclohexadiene as the diene component [39]. Each of the three isomers of the acyclic diene was found to add stereo-specifically to cyclohexadiene. In a more recent study, the cis and trans isomers of 1,2-diaryloxyethenes were found to add stereospecifically to 1,3-cyclopentadiene (Scheme 17) and also to 2,3-dimethyl-l,3-butadiene [46]. 

Example 6.6. The Diels-Alder reaction of (2E, 3E)-2,4-hexadiene and dimethyl maleate. 

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