Dienes electrophilic addition

With an isolated diene, electrophilic addition of one equivalent of HBr yields one product and Markovnikov s rule is followed. The H atom bonds to the less substituted carbon— that is, the carbon atom of the double bond that had more H atoms to begin with. 

With a conjugated diene, electrophilic addition of one equivalent of HBr affords two products. 

The Adg2 is the major addition pathway of alkenes and dienes. Electrophile addition to a pi bond or protonation of a pi bond, path Ag, produces the most stable cation, which is then trapped by a nucleophile, path An. (Section 4.4.2 discusses the various addition types.) If the electrophile is a proton, this reaction is the reverse of the El reaction. The reaction commonly produces a mixture of syn and anti addition. 

Fluorination with XeF2 or C H IF2 gives both the 1,2- and 1,4-difluoro products. This reaction proceeds via the initial electrophilic addition of F to the diene (53). 

Later in this chapter we ll see how allylic carbocations are involved in electrophilic addition to dienes and how the principles developed in this section apply there as well. 

Electrophilic Additions to Conjugated Dienes Allylic Carbocations 48i... 

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 6.7). Markovnikov regiochemistry is found because the more stable carbo-cation is formed as an intermediate. Thus, addition of HC1 to 2-methylpropene yields 2-chloro-2-methylpropane rather than l-chloro-2-methylpropane, and addition of 2 mol equiv of HC1 to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

Interactive to use a web-based palette to predict products from electrophilic addition reactions to conjugated dienes. 

Conjugated dienes also undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. Addition of HBr to 1,3-butadiene, for instance, yields a mixture of two products (not counting cis-trans isomers). 3-Bromo-l-butene is the typical Markovnikov product of 1,2-addition to a double bond, but l-bromo-2-butene 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. 

Predicting the Product of sn Electrophilic Addition Reaction of a Conjugated Diene... 

Electrophilic addition of HCJ to a conjugated diene involves the formation of allylic carbocation intermediates. Thus, the first step is to protonate the two ends of the diene and draw the resonance forms of the two allylic carbocations that result. Then... 

Electrophilic addition to a conjugated diene at or below Toom temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct. When the same reaction is carried out at higher temperatures, though, the product ratio often changes and the 1,4 adduct predominates. For example, addition of HBr to 1,3-butadiene at 0°C yields a 71 29 mixture of 1,2 and 1,4 adducts, but the same reaction carried out at 40 °C yields a 15 85 mixture. Furthermore, when the product mixture formed at 0 °C is heated to 40 °C in the presence of HBr, the ratio of adducts slowly changes from 71 29 to 15 85. Why ... 

Chemical and other physical methods of ionization were also employed for the structural determination of dienes and polyenes. Such is the case for the recent investigation of aliphatic dienes and trienes by chemical ionization with nitric oxide (NO+)25. It has been known since 1975 that olefins can be chemically ionized by NO+ [CI(NO)]26. Two distinct processes may apparently occur (i) electrophilic addition of NO+ to the ene leading to [M + NO]+ ion and (ii) an oxidative cleavage (possibly catalysed by the... 

Sulfenyl chlorides and halogens react with 1,2-alkadienylphosphonic acids to afford phosphorus-containing heterocydes [72], However, the electrophilic addition of dialkyl 4-methyl-2,3,5-hexatrien-2-yl phosphonates with sulfenyl or selenyl chloride afforded 2-thienyl methylphosphonates or the seleno analogues [73, 74]. The conjugate addition of sulfenyl or selenyl chloride with the 2,4-diene moiety in the starting allene leads to the formation of the five-membered skeleton (Scheme 10.69). 

The few kinetic results and the extensive product data on the electrophilic reactions of these dienes have been mainly interpreted in terms of the simplistic mechanism described in equation 1 and postulated by analogy to that established a long time ago5 for the reactions of monoethylenic compounds. According to this naive picture, an ionic intermediate with two possible limiting structures would be formed by electrophilic addition... 

The industrial use of 1,3-dienes and of their electrophilic reactions has strongly stimulated the field in recent years. Because of the low cost of butadiene, abundantly available from the naphtha cracking process, very large scale applications in the synthesis of polymers, solvents and fine chemicals have been developed, leading to many basic raw materials of the modem chemical industry. For example, the primary steps in the syntheses of acrylonitrile and adiponitrile have been the electrophilic addition of hydrocyanic acid to butadiene24. Chlorination of butadiene was the basis of chloroprene synthesis25. 

The electrophilic addition of hydrogen sulfide and 1-butanethiol to 1,3-conjugated dienes49 in chloroform at —10 °C has been reported in a quite old paper of a Russian team. The yields were generally low, in the range of 20%, even when the reaction was catalyzed by a mixture of two Lewis acids, EtAlB /EtAlC however, polymerization of the diene was not significant. 

---