Halogenation of dienes

Relatively few data and no systematic studies of the halogenation of dienes have been reported. Monoaddition products are usually formed through the attack on the more substituted double bond. Interhalogens add to allenes to give products in which the positive part of the reagent is attached to the central allenic carbon. Rearrangements, however, may affect product distributions. Ionic mechanisms are usually established, but very few details are available. 

PROBLEM 12.71 Look closely at the energy diagrams for the 1,2-Hydrohalogenation of diene and 1,4-Halogenation of diene reactions. Notice there is a faint pathway in the second half of the reaction in each case. Why is the product of the 1,4 addition lower in energy than the product of 1,2 addition Will the 1,4-addition always be lower Consider the HBr addition to 1,3-pentadiene. Is the product of 1,4-addition lower in this example ... 

As in the case of hydrohalogenation the bulk of the theoretical studies on the halogenation of diene polymers has been made on natural rubber with chlorine as the halogen. As with natural rubber hydrochloride the derivative has some commercial value. 

The stereochemistry of both chlorination and bromination of several cyclic and acyclic dienes has been determined. The results show that bromination is often stereospecifically anti for the 1,2-addition process, whereas syn addition is preferred for 1,4 addition. Comparable results for chlorination show much less stereospecific-ity." It appears that chlorination proceeds primarily through ion pairs, whereas in bromination a stereospecific anti-1,2-addition may compete with a process involving a carbonium ion intermediate. The latter can presumably give syn or anti product. As a result it must be expected, in general, that halogenation of dienes will not be stereospecific. 

As with addition of other electrophiles, halogenation of conjugated dienes can give 1,2- or 1,4-addition products. When molecular bromine is used as the brominating agent in chlorinated hydrocarbon solvent, the 1,4-addition product dominates by 7 1 in the case of butadiene. ... 

The reason for the different behavior of dienes like 41 and monoenes 37 or 42 is not yet established. It is hard to believe that simple steric factors should make up for the different orientation of the olefin that approaches a metal carbene intermediate. More likely is stereochemical control by an ylide-type interaction between the halogen atom of the (sterically more flexible) monoenes 37 or 42 and the electrophilic metal carbene. 

Electrophylic halogenation of 9-oxabicyclo[3.3.l]nona-2,6-diene (139), a heteroanalog of 39, in the presence of water and its epoxidation with performic acid results in ring closure to 4,8-disubstituted 2,6-dioxaada-mantane derivatives.168 4,8-Dibromo-2,6-dioxaadamantane is also prepared from the syn-dibromide of 1,5-cyclooctadiene.169... 

At variance with the earlier study58 on butadiene, the data related to halogenation of these substituted dienes reveal also that solvents have striking effects on product ratios,... 

By comparison, the direction of halogenation of monomethyl-substituted bicyclo[4.3.0] nona-3,7-dienes depends" considerably on the position of the methyl group. For a diene lacking a substituent at C(3), the reaction proceeds with retention of the initial structure (equation 83), whereas in the case of the 3-methyl substituted diene, it occurs through transannular cyclization giving a brexane type monoiodo derivative (equation 84). 

Halogenation of conjugated dienes proceeds chiefly by 1,4-addition with molecular halogens (equation 3). 1,2-Addition is favored in the presence of pyridine-halogen complexes and amine tribromide salts (equation 4)9. The stereochemistry of 1,4-bromine addition with 2,4-hexadienes and cyclopentadiene is primarily anti in the presence of amine, but syn with molecular halogen in the absence of amine. 

The thermal isomerization of dihalocyclopropanes is facilitated by the presence of aryl, alkyl and alkoxy substituents in the three-membered ring. Phenyl substituted systems may give rise to indenes, and in the alkyl series the dehydro-halogenation of 2,3-dihalocyclopropane derivatives may lead to dienes. 

Table 5. Halogenation of Butadiene and 2,3-Dimethylbuta-1,3-diene with Boron Trifluoride-Diethyl Ether Complex...

These two rubbers are prepared by halogenation of the butyl. Halogenation gives increased cure reactivity. As a result, improvement occurs in vulcanization rates resulting in improved properties over butyl. The state of cure and reversion resistance and covulcanization with other diene rubbers can also be suitably monitored. CIIR and BIIR vulcanizates have lower gas permeability,... 

The 14-electron species denoted here as Cp2M (M = Ti, Zr), generated from Group 4 metallocenes, mediate useful oxidative cyclizations of dienes 231 [103,104], enynes 233 [105] and diynes 235 [105,106] via the metallacycles 232, 234 and 236. These metallacycles are useful intermediates, which are hydrogenolysed, carbonylated, halogenated, attacked by electrophiles and converted to functionalized cyclic compounds such as 238 and 239 [107],... 

The protonolysis of the equally substituted zirconacycle 27 with weak acids such as ethanol leads to the monoprotonated dienyl zirconocene compound 28, which is subsequently converted into substituted diene 29 by a palladium crosscoupling reaction with aryl iodide (Scheme 14) [44]. Selective halogenation of such zirconacycle leads, as well, to the formation of zirconated diene 30 and then to the 1,4-dihalogenodiene 31 (Scheme 15) [45]. 

Mechanistic studies of the pyrolysis of disodiuni perfluoropentanedioate70 and derivatives71 have shown that the acid fluorides are formed during the reaction. The formation of dienes from the salts of halogenated carboxylic acids with a terminal C = C bond is also possible. The thermal decomposition of sodium (Z)-6//-perlluoro(6-chlorohex-5-enoate) gives (Z)-l//-perfluoro(l-chloropenta-l,4-diene) (7) in 75% yield.72 The decarboxylation of potassium 5H-perfluoropcnt-4-enoate, however, gives 1//-perfluorobut-2-yne (8) via the isomerization of the intermediate butadiene by potassium fluoride.72165... 

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. 

The presence of electron-withdrawing substituents such as halogen on dienes substantially increases their propagation rate constants and reduces termination rate constants. As shown in Table 11, kc0 does not increase from isoprene to chloroprene so that Cc decreases. The trend continues for 2,3-dichloroduta-diene. Although the propagation rate constant for 1,2-dichlorobutadiene is not known, its Cc is less than... 

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