Preparation of Dienes

Chiral allenes are another example of molecules that are chiral, but that do not contain a chirality center. Like the chiral biaryl derivatives that were described in Section 7.9, chiral allenes contain an axis of chirality. The axis of chirality in 2,3-pentadiene is a line passing through the three carbons of the allene unit (carbons 2, 3, and 4). 

Is 2-methyl-2,3-pentadiene chiral What about 2-chloro-2,3-pentadiene  

The Cahn-Ingold-Prelog R-S notation has been extended to include molecules with a chirality axis. See the article by Mak in the November 2004 issue of the Journal of Chemical Education for a brief discussion of assigning / or Sto chiral molecules that do not contain a chirality center. 

The conjugated diene 1,3-butadiene is used in the manufacture of synthetic rubber for automobile tires and is prepared on an industrial scale in vast quantities. Production in the United States is currently 5 X 10 Ib/year. One industrial process is similar to that used for the preparation of ethylene In the presence of a suitable catalyst, butane undergoes thermal dehydrogenation to yield 1,3-butadiene. 

The use of 1,3-butadiene in the preparation of synthetic rubber is discussed in the boxed essay Diene Polymers that appears on page 406. 

Laboratory syntheses of conjugated dienes involve elimination reactions of unsaturated alcohols and alkyl halides. In the two examples that foUow, the conjugated diene is produced in high yield even though an isolated diene is also possible. 

As we saw in Chapter 5, dehydrations and dehydrohalogenarions are typically regioselective in the direction that leads to the most stable double bond. Conjugated dienes are more stable than isolated dienes and are formed faster via a lower energy transition state. 

What dienes containing isolated double bonds are capable of being formed, but are not observed, in the two preceding equations describing elimination in 3-methyl-5-hexen-3-ol and 4-bromo-4-methyl-l-hexene  

We begin with two trees, both cultivated on plantations in Southeast Asia. One, Hevea brasiliensis, is a source of natural rubber and was imported from Brazil in the nineteenth century. The other, Isonandragutta, is native to Sumatra, Java, and Borneo and gives a latex from which gutta-percha is obtained. 

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Block and Aslam441 reported a novel variant of the Ramberg-Backlund reaction in which a,/J-unsaturated a -bromoalkyl sulfones 368 afforded 1,3-dienes upon treatment with base. Since a -bromoalkyl sulfones 368 can be obtained readily by the initial treatment of olefins with bromomethanesulfonyl bromide under photoirradiation and subsequent treatment with triethylamine, this reaction was utilized for preparation of dienes which contain one additional carbon over that of the corresponding olefin. Starting from the cyclic olefins 369-372 the 1,3-dienes 373-376 were obtained442. 

Table 3.22. Examples of the preparation of dienes by ring-opening cross metathesis.

The preparation of unsaturated phosphonates and phosphine oxides represents another class of reagents that are extensively used in organic synthesis. For example, allylic phosphonates are commonly employed in the preparation of dienes and polyenes via Horner-Emmons olefmation. Vinyl phosphonates have numerous uses as both synthetic intermediates" and bioactive compounds. 

Synthetic methods for the preparation of dienes and enynes abound, yet the use of alkenyl iodonium salts offers distinct advantages. Their coupling reactions with electron-deficient alkenes and alkenyl- or alkynylstannanes constitute a valuable extension to the previously existing methodology, because of mild conditions, ease of operation, high stereoselectivity and good yields. The simplest reaction of this category is with unsaturated carbonyl compounds and requires palladium catalysis. 

Preparation of dienes is accomplished by dehydration of diols or ole-finic alcohols. Pinacol, (CHjljCOHCOHfCHjlj, is converted to 2,3-di-methyl-1,3-butadiene by heating with 48% hydrobromic acid or by passing the vapors over activated alumina at 420-470°, Yields of the diene are 60% and 86%, respectively. Aniline hydrobroniide is used as a catalyst in the dehydration of 3-methyl-2,4-pentanediol to 3 methyl-l,3-penta-diene (42%). An excellent laboratory preparation of isoprene from acetone in 65% over-all yield has been described. The last step involves catalytic dehydration of dimethylvinylcatbinol over aluminum oxide at 300° to give isoprene in 88% yield. ... 

Equation 54 describes the preparation of dienes by a similar cyclopropane cleavage followed by proton transfer and elimination of sulfinate. The weak donor ability of a (trimethylsilyl)methyl group also assists Lewis acid-promoted ring cleavage according to equation 55, which allows synthesis of y, -unsaturated ketones Interestingly, the related ester opens smoothly only if fluoride reagents are employed (equation 56) ... 

As is the case for oxycyclopropanes extra activation and extended product functionality is achieved by using sulphur-substituted cyclopropanes with a vicinal acceptor group. According to equation 116 synthesis of a variety of y-oxothioesters and j -oxoketene dithioacetals is possible. A formylcyclopropane of this type has been converted to a furan derivative " . Transformation of the carbonyl to a hydroxymethyl group and acid treatment allows preparation of dienes " ... 

Amongst numerous methods available, the Nef reaction appears to be the most flexible procedure for the synthesis of 4-oxo-3-alkylbutanoates (228) (Scheme SS), intermediates in the preparation of dienes used for cycloaddition reactions. 

The value of the metal complexation results from control of the reaction, rather than any activation, Lewis acids being excellent catalysts for diene polymerization. Friedel-Crafts acylations of diene complexes have been used for the preparation of dienes, with decomplexation following carbonyl reduc-tion. 5 Decomplexation to afford dienones has been less explored. The intermediate cationic o -complex on treatment with triethyl phosphite or triphenylphosphine affords metal-free. y-unsaturated phospho-nates or phosphonium salts (Scheme 19). The initial s-cis conformation of the diene fragment of the... 

Other examples of the preparation of dienes from olefins using bromomethanesulfonyl bromide (82) are shown in equations 49-5249. 

The ethyl ester of (.S l-pyroglutamic acid (,S)-32 was applied for the preparation of dien-amines used as chiral dienes in enantioselective Diels-Alder reactions (Section D.l.6.1.1.1.1.5.1.). Thus, acrolein or methacrolein reacted with the auxiliary under acidic catalysis to form the corresponding dienamines 31. 

Copper-catalyzed asymmetric synthesis of nonracemic allylic esters. Preparation of diene... 

A very important method of preparation of diene compounds is the condensation reaction of dienes with metal vapors. The reaction is carried out at low temperature and at low pressure in a specifically designed flask containing the diene either neat or in solution. The metal atoms vaporize from the electrically heated metal wire and react with dienes which are present in the liquid phase(Figure 8.7). Often, such condensations of metal vapors with dienes are carried out in matrices at liquid nitrogen temperature. In this manner, the air-stable compounds [M(C4H6)3] (M = Mo, as... 

Condensation of metal vapors with various 6e ligands (cf. preparation of diene complexes, Chapter 8) allows the synthesis of many complexes which would be difficult or impossible to obtain by other methods owing to the reactivity of substituents. The following bisarene complexes were obtained in this manner [Cr(arene)2], where arene = PhX (X = F, Cl, CF3, COOMe, Me, /-Pr) or (X = F, Cl, CF3,... 

One of the limiting factors in the application of the intramolecular Diels-Alder reaction has been the development of methods for the preparation of the requisite trienes. The first chapter of this volume summarizes methods for the preparation of dienes and dienophiles. Examples representative of every general approach to 1,3-dienes and to dienophilic functional group combinations have been included. 

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