Cyclohexene anti bromination

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

Cyclohexene reacts with bromine to give a bromonium ion, which will react with any available nucleophile. The most abundant nucleophiles in saturated aqueous sodium chloride solution are water and chloride ions. Attack by water gives the bromohydrin, and attack by chloride gives the dihalide. Either of these attacks gives anti stereochemistry. 

When cyclohexene is treated with bromine in carbon tetrachloride, the racemic anti-1,2 -dibromocy-clohexane is obtained exclusively. 

Because only anti addition occurs, addition of Br2 to a cyclohexene forms only the enantiomers that have the added bromine atoms on opposite sides of the ring. 

This section will begin with a simple method for producing epoxides that uses the reaction of alkenes and hypohalous acids (generated by the reactions CI2 + H2O HOCl or Br2 + H2O HOBr) to give the trans-(or anti-) halohydrin as the major product. An example is the conversion of cyclohexene to bromohydrin 149 (sec. 2.10.C). Subsequent treatment with a base such as sodium hydride generates the alkoxide (150), which is anti- to the adjacent bromine, which anti- orientation leads to displacement of halide... 

Bromination of alkenes is stereospecific because the geometry of the starting alkene determines which product diastereoisomer is obtained. We couldn t demonstrate this with cyclohexene because only a Z double bond is possible in a six-membered ring. But bromination or chlorination of Z and -2-butene in acetic acid produces a single diastereoisomer in each case, and they are different from each other. Anti addition occurs in both cases—more evidence that a bromonium ion is the intermediate. 

Addition of bromine to an alkene is highly stereospecific and gives products with stereochemistry consistent with an anti addition mechanism. For example, addition of bromine to cyclohexene produces fraMS-l,2-dibromocyclohexane. Addition of bromine to cis-2-butene produces ( )-2,3-dibromobutane (equation 9.2), while addition to the trans isomer gives the meso product (equation 9.3). ° ... 

The reaction of bromine with cyclohexene involves anti addition, which generates, initially, the diaxial conformation of the addition product that then undergoes a ring flip to the diequatorial conformation of rn r-l,2-dibromo-cyclohexane. However, when the unsaturated bicyclic compound I is the alkene, instead of cyclohexene, the addition product is exclusively in a stable diaxial conformation. Account for this. (You may find it helpful to build handheld molecular models.)... 

The addition of bromine and chlorine to a cycloalkene gives a trans dihalocycloal-kane. For example, the addition of bromine to cyclohexene gives trawfrl,2-dibromocyclo-hexane the cis isomer is not formed. Thus, the addition of a halogen to a cycloalkene is stereoselective. A stereoselective reaction is a reaction in which one stereoisomer is formed or destroyed in preference to all others that might be formed or destroyed. We say that addition of bromine to an alkene occurs with anti stereoselectivity. 

The addition of chiorine or bromine to cyclohexene and its derivatives gives a trans diaxiai product because oniy axial positions on adjacent atoms of a cyciohexane ring are anti and copianar.The initial frans diaxiai conformation of the product is in equiiibrium with the trans diequatorial conformation, and, in simpie derivatives of cyciohexane, the iatter is the more stable conformation and predominates. 

If this mechanism is correct, we can predict that the required inversion in the opening of the cyclic ion must produce a trans stereochemistry in the products. So, we have the opportunity to test the mechanism, and it turns out that addition of Br2 (or CI2) to cyclohexene in the solvent water exclusively produces the compound resulting from anti addition in which the bromine (or chlorine) and... 

FIGURE 10.16 The bromonium ion mechanism demands that the products be formed by anti addition, as they are. This stereochemical experiment uses cyclohexene to show that the bromine and solvent molecule become attached from different sides of the ring through the anti addition enforced by the presence of the bromonium ion. 

The reaction of bromine with cyclohexene involves anti addition, which generates, initially, the diaxial conformation of the addition product that thai imdergoes a ring flip to the diequatorial conformation of trans-1,2-dibromocyclohexane. 

What is the stereochemistry of bromination Do the two bromine atoms add from the same side of the double bond (syn, as in catalytic hydrogenation) or from opposite sides (see margin) Let us examine the bromination of cyclohexene. Addition on the same side should give ciA-1,2-dibromocyclohexane the alternative would result in trans-1,2-dibromocyclohexane. The second pathway is borne out by experiment—only anti addition is observed. Because anti addition to the two reacting carbon atoms can take place with equal probability in two possible ways—in either case, from both above and below the TT bond—the product is racemic. 

The formation of diequatorial and diaxial dibromides from 3- and 4-sub-stituted cyclohexenes has prompted examination of the factors which may influence the steric course of the reactions. All additions of bromine are anti and all adducts are formed under kinetic control. The diequatorial diastereo-isomer (207) content of the reaction product decreases when a basic solvent such as diethyl ether is used the same result pertains in the presence of tertiary amines or when the brominating agent is either pyridine perbromide or pyri-dinium hydrobromide perbromide. Of the olefins examined, (205b) yields the highest amount of diequatorial adduct. 

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