Bromination of dienes

Thus, fluorination of 1,3-dienes proceeds through an allylic ion, while weakly bridged halonium ions are the intermediates in chlorination and bromination of dienes (vide infra). Furthermore, starting from the experimental evidence that 13 is produced under kinetic conditions and not from subsequent rearrangement of the 1,2- and 1,4-adducts, the authors suggested that 13 arose from rearrangement of the allyl cation intermediate, 17. Consistent with an open ion pair intermediate is also the stereoselective formation of the threo isomer from both 1,3-pentadienes, as well as the preference for the addition to the 1,2-bond observed in the reaction of both isomeric pentadienes. This selectivity may indeed... 

A mechanistic scheme involving weakly bridged intermediates, liable to undergo carbon-carbon bond rotation and counteranion translocation, analogous to that proposed for chlorination (see above), has been reported also for the bromination of dienes in order to rationalize the product stereochemistry. 

Other Methods. Some immobilized Ti" species, supported on silica or polystyrene [e.g. (68)], have been examined as catalysts for the hydroalumination of alkenes with LiAlH4 (cf. 3, 152). They show more selectivity than soluble Ti" species for example mono-hydroalumination-bromination of dienes (69 n = 1, 2, or 4) (Scheme 35) can be accomplished more easily. 

The stereochemistry of both chlorination and bromination of several cyclic and acyclic dienes has been determined. The results show that bromination is often stereo-specifically anti for the 1,2-addition process, whereas syn addition is preferred for 1,4-addition. Comparable results for chlorination show much less stereospeciftcity. It appears that chlorination proceeds primarily through ion-pair intermediates, whereas in bromina-hon a stereospecific anfi-l,2-addition may compete with a process involving a carbocation mtermediate. The latter can presumably give syn or anti product. 

Bromination of the enolate anion from the reaction of 3j -acetoxypregna-5,16-dien-20-one (1) with methylmagnesium bromide in the presence of cuprous chloride affords (after treatment with sodium iodide to dehalo-genate any 5,6-dibromide) a mixture of 17a-bromo- and 17)5-bromo-16a-methyl compounds (11) and (12) in a ratio 9 1. The 17a-iodides can be obtained in an analogous reaction. 

Allylic bromination of pregnenolone acetate with dibromodi-methylhydantoin affords the 7-bromo compound (155) of undefined stereochemistry. Dehydrobromination by means of collidine followed by saponification affords the 5,7 endocyclic cis,cis-diene, 156. This compound contains the same chromophore as ergosterol, a steroid used as a vitamin D precursor. The latter displays a complex series of photochemical reactions among the known products is lumisterol, in which the stereochemistry at both C9 and Cio is inverted. Indeed, irradiation of 156 proceeds to give just such a product (158). This reaction can be rationalized by... 

Dibromo-7-oxabicyclo[4.1.0]heptanes can be obtained by bromination of the respective 7-oxabicyclo[4.1.0]hept-3-ene, the monoepoxide of cyclohcxa-1,4-diene, and when submitted directly to the dehydrohalogenation reaction give products 3 and 4.2 149 150... 

Substituted and benzo-annulated oxepins readily undergo addition of bromine across the nonaromatic double bond. Bromination of 3,6-bridged oxepins can occur in two different ways, either as a 1,2-addition164 or as formal 1,4-addition to the diene system of the corresponding benzene oxide to give products i.129 138-140 164... 

The rate of bromination of ll-oxatricyclo[4.4.1.01,fi]undeca-3,8-diene is markedly higher in ether than in dichloromethane or ehloroform. The former solvent thus permits the reaction to be carried out at relatively low temperatures. 

After treatment of zirconacyclopentadienes with 2 equivalents of CuCl at room temperature, the addition of a halogenating agent such as NBS at —78 °C leads to the formation of octatetraenes 57a [7m]. This reaction involves slow bromination of the diene-dicopper compounds and coupling with the alkenyl bromide moiety (Eq. 2.40). 

Bromination of bicyclopropenyl system 369 at ambient temperature in absolute CHCI3 leads either to diene 372 (15%) and trienes 374-376 (15%, 35% and 10%, respectively) when R = H, or to the stable cyclopropenium salt 371 (95%) when R = Ph (equation 134)188. The electrophilic attack of bromine on compounds 369 creates the cationoid intermediates 370 which undergo either fragmentation to salt 371 (path a) or an electrocyclic ring opening (path b). When diene 372 is heated at about 150 °C in the solid state it rearranges to 1,2,3,5-tetraphenylbenzene 373 with concomitant loss of bromine. 

Bicyclo[2.2.1]hepta-2,5-diene, reaction with acetic acid to yield nortii-cyclyl acetate, 46, 74 Bicj clohexyl, 45, 61 2-Biphenylylmagnesium iodide, 46, 94 Bomyl chloride 46, 56 Bromination of y-butyrolactone, 46, 22 Bromine, reaction with i-butyrolactone in presence of red phosphorus, 46, 22... 

Based on bis-silylated dienes another approach to quinoxaline derivatives such as 80 (Scheme 4.10) was found [97]. Fast [4+2] cycloaddition takes place by treatment of Cgo with 2,3-bis(trimethylsilyloxy)butadiene 98, yielding the acyloin-fused fullerene derivative 100 in good yields (Scheme 4.16). The silylated diene is formed in situ by treatment of 98 at 180 °C in o-dichlorobenzene. Controlled bromination of the intermediate 99 leads to the transient diketone 101, which reacts readily in a one-pot reaction with various o-diaminoarenes to yield the quinoxaline-fused fullerenes 102. 

Bromination of the enol ether product with two equivalents of bromine followed by dehydrobromination afforded the Z-bromoenol ether (Eq. 79) which could be converted to the zinc reagent and cross-coupled with aryl halides [242]. Dehydrobromination in the presence of thiophenol followed by bromination/dehydrobromination affords an enol thioether [243]. Oxidation to the sulfone, followed by exposure to triethylamine in ether, resulted in dehydrobromination to the unstable alkynyl sulfone which could be trapped with dienes in situ. Alternatively, dehydrobromination of the sulfide in the presence of allylic alcohols results in the formation of allyl vinyl ethers which undergo Claisen rearrangements [244]. Further oxidation followed by sulfoxide elimination results in highly unsaturated trifluoromethyl ketonic products (Eq. 80). 

The syntheses described in this subsection rely upon the availability of o-diaroylbenzenes. A few methods seem to be of broader applicability. As Adams and co-workers have shown, the Diels-Alder reaction of dienes and diaroylethylenes gives 4,5-diaroylcyclohexenes, which on subsequent treatment with catalytic amounts of acid (sulfuric or phosphoric) form 4,7-dihydrobenzo[c]furans. Subjecting these compounds to the action of 2 moles of bromine in acetic acid in the presence of sodium acetate resulted in diaroylbenzenes (Eq. fi). - - ... 

In accordance with an earlier suggestion by Olah and Hockswender,240 the initial 1 1 adducts have been shown to be n complexes572 and a second species, a 2 1 bromine-alkene complex was directly observed.573 In the course of the bromination of crowded alkenes, such as ( )-2,2,3,4,5,5-hexamethylhex-3-ene (46), the bro-monium ion intermediate was shown to be reversibly formed.574 However, steric hindrance prevents the formation of the usual dihalogenated product. Instead, it loses a P proton to give an allylic bromo derivative, which is then dehydrobromi-nated to diene 47 ... 

Room-temperature ionic liquids may be used as green recyclable alternatives to chlorinated solvents for stereoselective halogenation.577 The bromination of alkenes and alkynes in [bmim][Br] is a/m -stereospecific, whereas that of 1,3-dienes gives selectively the 1,4-addition products. The reactions of arylacetylenes, however, are not selective when carried out in [bmim][PF6]. Tetraethylammonium trichloride, a stable crystalline solid may be used in the chlorination of alkenes and alkynes to afford the products with exclusive anti stereoselectivity.578 It has... 

Ring-C ketones do not interfere with bromination at C-21. However, if a 3-ketone is also present during the reaction, the A-ring can be brominated simultaneously. This can be followed by dehydrobromination of the A-ring without removal of the 21-bromide. Thus, bromination of a 5a- or 5j -pregnane-17a-ol-3,20-dione gives a 2,4,21-tribromo compound which can be dehydrobrominated to a 21-bromo-17a-hydroxypregna-l,4-dien-3-one.173,199... 

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