Halogen Addition to Dienes

Mixtures of 1,2- and 1,4-addition products are obtained when 1,3-butadiene reacts with chlorine or bromine. 

The tendency for conjugate addition is pronounced, and E double bonds are generated almost exclusively. 

3-Bromo-l-butene is major product when reaction is kinetically controlled 

Energy diagram showing relationship of kinetic control to thermodynamic control in addition of hydrogen bromide to 1,3-butadiene. 

At 45°C, for example, interconversion is rapid and gives an equilibrium mixture containing 85% of the 1,4-addition product and 15% of the 1,2 product. This demonstrates that the 1,4 product is more stable, presumably because it has a disubstituted double bond, whereas the double bond in the 1,2 product is monosubstituted. 

When addition occurs under conditions in which the products can equilibrate, the composition of the reaction mixture no longer reflects the relative rates of formation of the products but tends to reflect their relative stabilities. Reactions of this type are governed by thermodynamic control. 

Before leaving this section, we should point out that the numbers in the terms 1,2 and 1,4 addition refer to carbons within the C=C—C=C structural unit wherever it may be in the molecule and not to the lUPAC numbering. For example, 1,2 and 1,4 addition to 2,4-hexadiene would involve the carbons shown. 

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Generally, in bromine addition to carbon-carbon double bonds, bromine bridging, solvent dependent dissociation of the ionic intermediates, steric interactions between the counteranion and the first bonded halogen during the nucleophilic step, the possibility of carbon-carbon rotation in the carbenium ion intermediate, preassociation phenomena and nucleophilic assistance determine the stereochemical behavior of the reaction . Several of these factors have been invoked to explain the stereochemistry of bromine addition to dienes, although others have been completely ignored or neglected. Bromine addition to cyclopentadiene, 1,3-cyclohexadiene, 2,4-hexadienes and 1,3-pentadienes has been examined repeatedly by Heasley and coworkers and the product distribution has been... 

The HX compounds are electrophilic reagents, and many polyhalo and polycyano alkenes, (e.g., Cl2C=CHCl) do not react with them at all in the absence of free-radical conditions. When such reactions do occur, however, they take place by a nucleophilic addition mechanism, (i.e., initial attack is by X ). This type of mechanism also occurs with Michael-type substrates C=C—Z, where the orientation is always such that the halogen goes to the carbon that does not bear the Z, so the product is of the form X—C—CH—Z, even in the presence of free-radical initiators. Hydrogen iodide adds 1,4 to conjugated dienes in the gas phase by a pericyclic mechanism ... 

According to this mechanism, the first formed ion pair is 19a. Owing to dispersal of charge in the allylic system, the bond between halogen and C(2) is weakened so that an open carbenium ion (19c) readily forms, allowing for the possibility of front-side attack by the anion with the resulting formation of syn 1,2-adducts. This intermediate explains the formation of the cis-],2-adducts by chlorine addition to cyclic systems. However, syn 1,2-dichlorides can also result from linear dienes by rotation around the C(l)—C(2) bond in 19c to produce 19d, followed by back-side attack by the anion with respect to its position in 19d. Syn 1,4-adducts should instead arise by attack of the anion on C(4) in either 19a, 19c or 19d. Formation of anti dichlorides (1,2- or 1,4-) can only occur when there is appreciable translocation in the ion pair 19a to give 19b. Attack by the anion at C(2) in 19b yields anti 1,2-dichloride and attack at C(4) yields anti 1,4-dichloride. 

Electrophilic addition of hydrogen halides to a,/f-unsaturated aldehydes and ketones places the halogen on the /3 carbon. This orientation is opposite to that observed for related additions to conjugated dienes ... 

The photochemistry of ir-allylpalladium complexes has been studied to a limited extent. Two basic reactions have been observed. Irradiation at 366 nm of ir-allylpalladium complexes produced 1,5-diene dimers, reportedly via a radical coupling mechanism.334 333 Similar irradiations in the presence of species capable of trapping the presumed allyl radical intermediate, such as BrCCb, BrCH2Ph or allyl bromide, now yield alkylated and halogenated allyls, in addition to 1,5-diene dimer. This reaction fails for simple alkyl or aryl halides due to the instability of the associated radical (equations 130 and 131 ).336... 

How can we account for the unusual reactivity of conjugated dienes In our discussion of halogenation of the simple alkenes (Sec. 3.27), we found that not only orientation but also relative reactivity was related to the stability of the free radical formed in the first step. On this basis alone, we might expect addition to a conjugated diene, which yields a stable allyl free radical, to occur faster than addition to a simple alkene. 

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