Bromination conjugated dienes

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. ... 

Most methods used for analysis of alkenes, such as bromination and hydrogenation, can be employed to determine the number of double bonds in polyenes. These methods were also employed to classify various petroleums ( bromine number ). However, these classical methods are employed less in analysis of conjugated dienes and polyenes mostly because the products produce a less informative mixture than in the alkene case. 

Later on, product distribution studies15 of the ionic addition of chlorine to conjugated dienes, and in particular to cyclopentadiene, 1,3-cyclohexadiene, cis,cis-, trans,trans-and c ,fraws-2,4-hexadienes, and cis- and trans-1,3-pentadienes have supplied the first stereochemical data, showing that the stereochemistry of 1,4-addition is predominantly syn, although to an extent smaller than that of bromine addition. Moreover, the 1,2-addition is generally non stereoselective, except for the addition to the 3,4-bond of cis-and trans-1,3-pentadienes where the attack is 89-95% anti. Finally, appreciable amounts of cis- 1,2-dichlorides were obtained from the two cyclic dienes, whereas 2,4-hexadienes showed a preference for anti 1,2-addition, at least in the less polar solvents (carbon tetrachloride and pentane). On the basis of all these results the mechanism shown in equation 29 was proposed. 

Bromine addition to conjugated dienes gives 1,2- and 1,4-addition products (equation 38), with a stereochemical outcome which is strongly dependent on the diene structure and the reaction conditions. 

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. 

Radical addition of HBr to an alkene depends upon the bromine atom adding in the first step so that the more stable radical is formed. If we extend this principle to a conjugated diene, e.g. buta-1,3-diene, we can see that the preferred secondary radical will be produced if halogenation occurs on the terminal carbon atom. However, this new radical is also an allylic radical, and an alternative resonance form may be written. 

Bromination will provide a mixture as the conjugated diene will react faster than the isolated double bond, leading to 1,2- and 1,4-addition of Br2 across the diene. 

More recently, the use of benzoyl peroxide catalyst or light (or both) has extended the scope of the reaction. Thus, previously unsuccessful brominations of conjugated diene systems and terminal methyl groups can now be accomplished. ... 

Conjugated dienes (and compounds that behave like conjugated dienes in the Diels-Alder reaction) react with singlet oxygen to form cyclic peroxides as if molecular oxygen acted as a dienophile. The yields of the peroxides, prepared by photochemical oxidation [13, 55] or by chemical oxidations with hydrogen peroxide and sodium hypochlorite, alkaline hydrogen peroxide and bromine, alkaline salts of peroxy acids [14, 26], or the ozonide of triphenyl phosphite [29], are comparable. 

Dihydrothebaine- was originally given the structure [n], which was in accord with most of its chemical properties [3-4] except its failure to condense with benzoquinone or maleic anhydride and its failure to yield a derivative of 14-bromothebainone [iii] on treatment with bromine [5], However, the production of a conjugated diene as final product in a sodium-alcohol or sodium-ammonia reduction is contrary to general experience and examination of the infra-red and ultra-violet absorption spectra of dihydrothebaine-, thebaine, /3-dihydrothebaine, several 1 4-dihydroanisole derivatives, and 1-alkoxy-l 3-dienes led to modification of the structure of dihydrothebaine- to [iv] [6], which structure is more in accord with its properties than is [n], and in particular explains why the hydrogenation of this base proceeds with absorption of only one mole of hydrogen (see Chap. XIV). 

However, the production of a conjugated diene as final product in a sodium-alcohol or sodium-ammonia reduction is contrary to general experience and moreover structure [n] fails to acoount for the failure of dihydrothebaine- to oondense with maloio anhydride or benzo-quinono, or to roaot with bromine to give a derivative of 14-bromotho-... 

The diene-Br2 complex is again in equilibrium with the reagents, and nucleophilic attack at carbon can be carried out either by the bromide of the ammonium bromide ion pair, formed at the moment of the electrophilic attack, or by the less nucleophilic pyridine added in excess in the reaction medium. It is noteworthy that this mechanism is characterized by a rate- and product-limiting nucleophilic step which should be quite insensitive to steric hindrance around the double bond. In agreement with a weak influence of the steric effects, pyridinium perbromide reacts in chloroform and tetrahydrofuran with substituted conjugated and non-conjugated dienes to give selectively (>95%) bromine addition to the more alkylated double bond (equation 44). 

Palladium(II)-promoted alkenylation involving a-bromo sulfonamide has been utilized to construct the bridgehead bicyclic sultam 193 <04OL1313>. Treatment of 192 with palladium acetate in DMF containing K2CO3, tri-2-furylphosphine and 4A molecular sieves at 100 C furnishes 193. Subsequent bromination with NBS and elimination with DBU give rise to conjugated diene 194. When irradiated at 350 nM, 194 is isomerized via a two-photon process to the structurally novel spiro heterocycle 198. 

Allylic bromination by NBS can be applied with success not only to olefins but also to <%,/ -unsaturated ketones, carboxylic esters, nitriles, and lactones. Radical-formers must be added when NBS is used for allylic bromination of conjugated dienes, for side-chain bromination of aromatic or heterocyclic compounds (see p. 198), or for replacement of tertiary hydrogen atoms next to a C=C bond. 

Just prior to the 20th century Johannes Thiele (1865-1918) studied additions to conjugated dienes and discovered a tendency toward 1,4-addition, for example, the bromination of 1,3-butadiene to form 1,4-dibromo-2-butene (part A in the accompanying figure). There were apparently unsatisfied "residuai vaiences" on carbons 1 and 4 of 1,3-butadiene that rendered these positions reactive. On the other hand, the "residuai vaiences" on carbons 2 and 3 of 1,3-butadiene were adjacent and "saturated each other" rendering these positions unreactive. The Lewis-Langmuir eiectron-pair structures, deveioped during the... 

The bromination of allylic compounds (Wohl-Ziegler reaction) is generally used in the conversion of aUcenes into conjugated dienes, which are obtained from the allylic bromides by base-catalyzed elimination (Scheme 4.22). 

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