Electrophilic addition reactions of conjugated dienes

The amount of 1,2- and 1,4-addition products formed in the electrophilic addition reactions of conjugated dienes depends greatly on the reaction conditions. 

Section 8.6 Electrophilic Addition Reactions of Conjugated Dienes... 

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

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 ... 

The reactions of a conjugate diene reflect the fact that a conjugated diene should be viewed as a functional group in its own right, rather than as two separate alkenes. Electrophilic addition to a conjugated diene results in a mixture of two possible products arising from 1,2-addition and 1,4 addition ... 

Conjugated dienes undergo two-step electrophilic addition reactions just like simple alkenes (Section 6.3). However, certain features are unique to the reactions of conjugated dienes. 

Conjugated dienes coordinated to a transition metal can readily be transformed into a rr-allylmetal complex by functionalization at the 4-position.f This makes dienes useful substrates for catalytic transformations since the 7r-allyl complex formed can undergo further reaction. t A number of Pd-catalyzed reactions of conjugated dienes are known that proceed via rr-allylpalladium intermediates and lead to useful 1,4- or 1,2-funtional-ization of the diene. There are two types of reactions of this kind (i) Pd(0)-catalyzed reactions that involve initial oxidative addition and (ii) Pd(II)-catalyzed reactions that involve electrophilic activation of the diene by the metal followed by nucleophilic attack. This section deals with C— N and C—O bond formation via these two types of reactions. This topic has previously been reviewed in connection with Pd-catalyzed additions to conjugated dienes. [5] [n] -jijg present review will focus mainly on the work published since 1997 but will also briefly discuss previous work. 

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. 

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. 

As in the case of addition reactions of carbon nucleophiles to activated dienes (Section HA), organocopper compounds are the reagents of choice for regio- and stereoselective Michael additions to acceptor-substituted enynes. Substrates bearing an acceptor-substituted triple bond besides one or more conjugated double bonds react with organocuprates under 1,4-addition exclusively (equation 51)138-140 1,6-addition reactions which would provide allenes after electrophilic capture were not observed (cf. Section IV). 

Similar to the addition reactions of acceptor-substituted dienes (Scheme 16), the outcome of the transformation depends on the regioselectivity of the nucleophilic attack of the organocopper reagent (1,4- vs. 1,6-addition) and of the electrophilic capture of the enolate formed. The allenyl enolate obtained by 1,6-addition can afford either a conjugated diene or an allene upon reaction with a soft electrophile, and thus opens up the possibility to create axial chirality. The first copper-mediated addition reactions to Michael acceptors of this type, for example, 3-alkynyl-2-cyclopentenone 75,... 

Cyclopropenes, even unactivated ones, exhibit extraordinarily high reactivity in both electrophilic and nucleophilic addition reactions. They are also good dienophiles and react with a variety of conjugated dienes including acyclic 1,3-dienes, alicyclic 1,3-dienes, anthracenes and furans. An endo selectivity is usually observed. An alkyl or aryl substituent at the 3-position of cyclopropene sterically hinders the Diels-Alder addition and thus 3,3-dialkyl- and 3,3-diarylcyclopropenes exhibit a reduced dienophilicity (equation 131) . On the other hand, numerous Diels-Alder reactions have been reported for 3,3-dicyano- and 3,3-dihalocyclopropenes. The reactions of 3-monosubsti-tuted cyclopropenes with the diene take place stereoselectively from the less crowded side of the substrates (equation 132) °. 

---