Dibromides, vicinal

Step 2 of the mechanism m Figure 6 12 is a nucleophilic attack by Br at one of the carbons of the cyclic bromonium ion For reasons that will be explained m Chapter 8 reactions of this type normally take place via a transition state m which the nude ophile approaches carbon from the side opposite the bond that is to be broken Recall mg that the vicinal dibromide formed from cyclopentene is exclusively the trans stereoisomer we see that attack by Br from the side opposite the C—Br bond of the bromonium ion intermediate can give only trans 1 2 dibromocyclopentane m accordance with the experimental observations... 

Lithium aluminum hydride has also been shown to reduce vicinal dibromides to olefins efficiently, but the scope is limited by the reactivity of this reagent towards many functional groups. 

Ambrettolide is obtained from hibiscus and has a musk-like odor. Its preparation from a compound A is outlined in the table that follows. Write structural formulas, ignoring stereochemistry, for compounds B through G in this synthesis. Hint Zinc, as used in step 4, converts vicinal dibromides to alkenes.)... 

Vicinal (Section 8.2) A term used to refer to a 1,2-disubstitution pattern. For example, 1,2-dibromoethane is a vicinal dibromide. 

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

No j3-bromo- or iodoalkyl complexes have yet been isolated. The reaction of vicinal dibromides or diiodides with [Co (CN)j] , [Co(CN)5H], or a Co(I)-DMG complex merely gives the olefin 32, 75,105,109,161), though kinetic evidence was obtained for the intermediate formation of the j8-bromo complex in the reaction of [Co"(CN)5] with a,j8-dibromopropionate and a,/3-dibromosuccinate (75). It is interesting that the pentacyanide produced is the bromo or iodo, and not the aquo, complex 32, 75), which suggests that the decomposition may involve a cis rather than a trans elimination of Co—X. The /3-chloroethyl complex can be prepared by tbe reaction of [Co(CN)5H]3- with CICH2CH2I 105). 

The stereospeciflcity of dehalogenation of vicinal dibromides to olefins was examined for reducing agents including Cr(II), iodide, and Fe° (Totten et al. 2001). For dibromostilbene, the ( )-stilbene represented >70% of the total olefin that was produced, and for threo-dibromopentane reduction by Cr(Il) produced ca. 70% of ( )-pent-2-ene, whereas values for iodide and Fe° were <5% of this. 

As vicinal dibromides are usually made by bromination of alkenes, their utility for synthesis is limited, except for temporary masking of a double bond. Much more frequently it is desirable to convert a diol to an alkene, and several useful procedures have been developed. The reductive deoxygenation of diols via thiono carbonates was... 

This nucleophilic attack results in the formation of a vicinal dibromide by opening the three membered ring. V i 1... 

The Reduction of Vicinal Dibromides. Outer- and Inner-Sphere Catalysts. Rates and Stereoselectivity... 

Figure 4.3 summarizes the reduction characteristics of an example of a vicinal dibromide, trans-1,2 dibromocyclohexane, directly at an electrode,... 

Cobalt(I) salen has been employed as a catalyst for the reduction of the following species benzal chloride [159] benzotrichloride [160] 1-bromobutane, 1-iodobutane, and 1,2-dibromobutane [161] iodoethane [162], benzyl chloride [163], and ethyl chloroacetate [164]. Rusling and coworkers have investigated the use of cobalt(I) salen, as well as vitamin Bi2s and cobalt(I) phthalocyanine, in both homogeneous phase and bicontin-uous microemulsions for the catalytic reduction of vicinal dibromides [165] and... 

Method C (general procedure) The vicinal dibromide (1-2 mmol) and the diaryl ditel-... 

In some useful synthetic procedures, the carbanionic character results from a reductive process. A classical example of the -elimination reaction is the reductive debromination of vicinal dibromides. Zinc metal is the traditional reducing agent.209 A multitude of other... 

Certain convenient brominating agents, such as pyridine hydrobromide perbro-mide (PyHBr3) and tetramethylammonium tribromide, may be used to transform alkenes to vicinal dibromides. They often give better yields than does liquid bromine, but may react by different mechanisms. Isomeric 1-phenylpropenes, for instance, react nonstereoselectively with bromine, but exhibit near-exclusive anti addition with the other reagents 244... 

Several groups have reported reactions giving thiazoloquinazoline compounds, probably the most interesting of which is the cyclization-ring expansion of the aziridine thiourea derivative 225 to the dihydro-thiazolo[2,3-6]quinazoline 226 in hydrochloric acid of Howard and Klein.395 Other preparations of this ring system involve the treatment of l-(2-carbethoxyphenyl)-2-thioureas with a-chloro 396-398 or a-bromo396,397 ketones or with vicinal dibromides.398 Tetrahydro salt... 

Control of the catalyzed reaction, the reductive transformation of a vicinal dibromide to an alkene, was achieved by formation of the doubly reduced, neutral viologen which migrated to the organic phase and functioned as a redox catalyst. After reaction, its dicationic oxidized form migrated rapidly to the aqueous phase where cyclic photoactivation could be attained. 

Another approach in the study of the mechanism and synthetic applications of bromination of alkenes and alkynes involves the use of crystalline bromine-amine complexes such as pyridine hydrobromide perbromide (PyHBts), pyridine dibromide (PyBn), and tetrabutylammonium tribromide (BiMNBn) which show stereochemical differences and improved selectivities for addition to alkenes and alkynes compared to Bn itself.81 The improved selectivity of bromination by PyHBn forms the basis for a synthetically useful procedure for selective monoprotection of the higher alkylated double bond in dienes by bromination (Scheme 42).80 The less-alkylated double bonds in dienes can be selectively monoprotected by tetrabromination followed by monodeprotection at the higher alkylated double bond by controlled-potential electrolysis (the reduction potential of vicinal dibromides is shifted to more anodic values with increasing alkylation Scheme 42).80 The question of which diastereotopic face in chiral allylic alcohols reacts with bromine has been probed by Midland and Halterman as part of a stereoselective synthesis of bromo epoxides (Scheme 43).82... 

An effect of the ring-size in addition to that observed in the elimination taking place in the reduction of vicinal dibromides, has been observed for medium-sized ring compounds which carry the electro-active group either in the side-chain or directly attached to the ring. The half-wave potentials of cycloalkyl bromides can be correlated with the half-wave potentials of cyclic ketones and their derivatives (Fig. 29). The observed relation indicates that the relative effect of ring-size on the reactivity is similar in all the reaction series compared. 

Ce) Bromine adds across the double bond of alkenes to give vicinal dibromides. 

The best approach is to reason backward. 1,2-Dibromopropane is a vicinal dibromide, and we prepare vicinal dibromides by adding elemental bromine to alkenes. 

Reaction (c) is an elimination reaction of a vicinal dibromide to give a conjugated diene and requires E2 conditions. Sodium methoxide in methanol was used. 

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