Reactions of Conjugated Dienes

In describing buta-1,3-diene, we say that the tt electrons are spread out, or delocalized, over the entire tt framework rather than localized between two specific nuclei. Electron delocalization and consequent dispersal of charge always leads to lower energy and greater stability. 

One of the most striking differences between conjugated and isolated double bonds is in their electrophilic addition reactions. Conjugated dienes undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. Addition of HBr to buta-1,3-diene, for instance, yields a mixture of two products (not counting cis-trans isomers). 3-Bromobut-l-ene is the typical Markovnikov product of 1,2-addition to a double bond, but l-bromobut-2-ene appears unusual. The double bond in this product has moved to a position between carbons 2 and 3, and HBr has added to carbons 1 and 4, a result described as 1,4-addition. 

How can we account for the formation of the 1,4-addition product The answer is that an allylic carbocation is involved as an intermediate, where the word allylic means next to a double bond. When buta-1,3-diene reacts with an electrophile such as H , two carbocation intermediates are possible—a primary carbocation and a secondary allylic cation. Because an allylic cation is stabilized by resonance between two forms (Section 2.4), it is more stable and forms faster than a nonallylic carbocation. 

When the allylic cation reacts with Br to complete the electrophilic addition, reaction can occur either at Cl or at C3 because both carbons share the 

FIGURE 8.15 An electrostatic potential map of the carbocation produced by protonation of buta-1,3-diene shows that the positive charge is shared by carbons 1 and 3. Reaction of Br with the more positive carbon (C3 blue) gives predominantly the 1,2-addition product. 

Reaction of conjugated dienes with aryl and alkenyl halides is treated in Section 3.2 and oxidative difunctionalization of conjugated dienes with Pd(II) is treated in Section 11.3. This chapter covers other reactions of conjugated dienes as major reactants. 

When butadiene is treated with PdCU the l-chloromethyl-7r-allylpalladium complex 336 (X = Cl) is formed by the chloropalladation. In the presence of nucleophiles, the substituted 7r-methallylpalladium complex 336 (X = nucleophile) is formed(296-299]. In this way, the nucleophile can be introduced at the terminal carbon of conjugated diene systems. For example, a methoxy group is introduced at the terminal carbon of 3,7-dimethyl-I,3,6-octatriene to give 337 as expected, whereas myrcene (338) is converted into the tr-allyl complex 339 after the cyclization[288]. 

The TT-allylpalladium complexes formed from conjugated dienes are reactive and react further with a nucleophile to give the 1,4-difunctionalized products 340. Based on this reaction, various nucleophiles are introduced into conjugated dienes to form 1,4-difunctionalized 2-alkenes. Acetoxy, alkoxy, halo, and 

4-Difunctionalization with similar or different nucleophiles has wide synthetic applications. The oxidative diacetoxylation of butadiene with Pd(OAc)i affords 1,4-diacetoxy-2-butene (344) and l,2-diacetoxy-3-butene (345). The latter can be isomerized to the former. An industrial process has been developed based on this reaction. The commercial process for l,4-diacetoxy-2-butene (344) has been developed using the supported Pd catalyst containing Te in AcOH. 1,4-Butanedioi and THF are produced commercially from 1,4-diacetoxy-2-butene (344)[302]. 

4-Diacetoxylation of various conjugated dienes including cyclic dienes has been extensively studied. 1,3-Cyclohexadiene was converted into a mixture of isomeric l,4-diacetoxy-2-cyclohexenes of unknown stereochemistry[303]. The stereoselective Pd-catalyzed 1,4-diacetoxylation of dienes is carried out in AcOH in the presence of LiOAc and /or LiCI and beiizoquinone[304.305]. In the presence of acetate ion and in the absence of chloride ion, /rau.v-diacetox-ylation occurs, whereas addition of a catalytic amount of LiCl changes the stereochemistry to cis addition. The coordination of a chloride ion to Pd makes the cis migration of the acetate from Pd impossible. From 1,3-cyclohexadiene, trans- and ci j-l,4-diacetoxy-2-cyclohexenes (346 and 347) can be prepared stereoselectively. For the 6-substituted 1,3-cycloheptadiene 348, a high diaster-eoselectivity is observed. The stereoselective cij-diacetoxylation of 5-carbo-methoxy-1,3-cyclohexadiene (349) has been applied to the synthesis of dl-shikimic acid (350). 

The diacetoxylation of E,E)- and ( ,Z)-2.4-hexadiene (351 and 353) is stereospecific, and 2,5-dimethylfurans (352 and 354) of different stereochemistry have been prepared from the isomers. Two different carboxylates are introduced with high cis selectivity by the reaction of 1,3-cyclohexadiene and 

Some synthetic applications of 1,4-difunctionalization of various 1,4-dienes are cited here. Stereoselective 1,4-difunctionalization of the 1,3-cyclohexadiene 217 using Pd(OAc)2 and O2 in DMSO afforded nearly equal amounts of 1,4-adduct 218 and addition-elimination product 219 [97]. Pd(II)-promoted intramolecular reaction of the allene-substituted 1,3-cyclohexadiene 220 gave the triene 223 regio-and stereoselectively. The reaction may be explained by nucleophilic attack of the 

It is worth mentioning that the diene 225 was obtained by the Pd(0)-catalyzed reaction of the same substrate 220 by a different mechanism, namely oxidative cyclization of 220 to generate palladacyclopentane 224 and addition of AcO-H to 224 to give 225 [98]. Also reaction of the alkyne-substituted 1,3-cyclohexadiene 226 using Pd(OAc)2 and BQ in the presence of LiCl starts by chloropalladation of the triple bond, followed by attack to the diene as shown by 227 to give the dichloro compound 228 [99], 

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I.l.IJ Reactions nitlr 1,2-, 1.3-. ami 1.4-dienes. The reaction of conjugated dienes with aryl and alkenyl halides can be explained by the following mechanism. Insertion of a conjugated 1.3-diene into an aryl or alkenylpalladium bond gives the T-allvlpalladium complex 243 as an intermediate, which reacts further... 

Several types of Pd-catalyzed or -promoted reactions of conjugated dienes via TT-allylpalladium complexes are known. The Pd(II)-promoted oxidative difunctionalization reactions of conjugated dienes with various nucleophiles is treated in Chapter 3, Section 4, and Pd(0)-catalyzed addition reactions of conjugated dienes to aryl and alkenyl halides in this chapter. Section 1.1.1. Other Pd(0)-catalyzed reactions of conjugated dienes are treated in this section. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

The most notable chemistry of the biscylopen-tadienyls results from the aromaticity of the cyclopentadienyl rings. This is now far too extensively documented to be described in full but an outline of some of its manifestations is in Fig. 25.14. Ferrocene resists catalytic hydrogenation and does not undergo the typical reactions of conjugated dienes, such as the Diels-Alder reaction. Nor are direct nitration and halogenation possible because of oxidation to the ferricinium ion. However, Friedel-Crafts acylation as well as alkylation and metallation reactions, are readily effected. Indeed, electrophilic substitution of ferrocene occurs with such facility compared to, say, benzene (3 x 10 faster) that some explanation is called for. It has been suggested that. 

After that, studies on the palladium-catalyzed reactions of conjugated dienes attracted little attention. They have only been reexamined since the late 1960 s. The scope of the reaction of butadiene catalyzed by palladium complexes has gradually been established. The catalysis by palladium is different from those of other transition metals. Although palladium is located below nickel in the periodic table, the catalytic... 

There are two main types of reactions of conjugated dienes catalyzed by palladium complexes. The first type is the linear dimerization to form 1,3,7-octatriene (16) in the absence of a nucleophile ... 

Scheme 15 Reaction of conjugated dienes with alkyl Grignard reagents and CI2TiCp2.

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