Cis-l,2-Dibromocyclohexane

Display the lowest-unoccupied molecular orbital (LUMO) for cyclohexyl bromonium ion. From which side will the Br attack Will this lead to formation of cis-1,2-dibromo-cyclohexane or trans-1,2-dibromocyclohcxane Is this also the thermodynamic product Compare energies of cis-l,2-dibromocyclohexane and trans- 1,2-dibromo-cyclohexane. 

Let s consider whether cis-l, 2-dibromocyclohexane is chiral. If we did not know about chair conformations, we might draw a flat cyclohexane ring. With a flat ring, the molecule has an internal mirror plane of symmetry (cr), and it is achiral. 

Reaction of HBr with 3-methylcyc)ohexene yields a mixture of four products os- and trara-l-bromo-3-methylcyclohexane and cis- and fra/75-l-bromo-2-methylcyclohexane. The analogous reaction of HBr with 3-bromocyclohexene yields tra/TS-l,2-dibromocyclohexane as the sole product. Draw structures of the possible intermediates, and then explain wThy only a single product is formed in the reaction of HBr with 3-bromocvclohexene. 

Halogenations with dihalotriphenylphosphoranes have been reviewed briefly by Fieser and Fieser.4 Dibromotriphenylphos-phorane appears to have been studied somewhat more than the dichloro compound, but both reagents effectively convert alcohols to alkyl halides, carboxylic acids and esters to acid halides, etc. The reaction of 1,2-epoxycyclohexane with dibromotriphenylphos-phorane under conditions similar to those described here gives a mixture of cis- and trans-l,2-dibromocyclohexanes. A reagent prepared from triphenylphosphine and carbon tetrachloride has been used for similar transformations.5... 

Problem 9.39 Use cyclohexanol and any inorganic reagents to synthesize (a) /rans-l, 2-dibromocyclohexane, (b) cis-1,2-dibromocyclohexane, (c) trans-1,2-cyclohexanediol. ... 

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). ° ... 

Now construct the cis-l,2-dichloro and dibromocyclohexanes and compare their energies. Once again, explain what you find. 

Draw cis- and rares-l,2-dibromocyclohexane, along with each mirror image. 

Cyclopropane ring formation under electron transfer conditions shows no stereoselectivity. Reduction in dimethylformamide of pure meso- or ( )-2,4-dibromopentene gives the same mixture of cis- and rra s-l,2-dimethylcyclo-pentane [92], Cis- and /ranj-l,3-dibromocyclohexane are both satisfactory substrates for formation of bicyclo[3.1.0]hexane and either isomer of 1,3-dibromocyclopentane affords bicyclo[2.1.0]pentane [93]. Endo-2,endo-6-dibromobomane 16 gives a mixttire of tricyclene and bomane on electrochemical... 

In disubstituted compounds, the rule for alkyl groups is that the conformation is such that as many groups as possible adopt the equatorial position. How far it is possible depends on the configuration. In a c -l,2-disubstituted cyclohexane, one substituent must be axial and the other equatorial. In a tram-1,2 compound both may be equatorial or both axial. This is also true for 1,4-disubstituted cyclohexanes, but the reverse holds for 1,3 compounds the trans isomer must have the ae conformation and the cis isomer may be aa or ee. For alkyl groups, the ee conformation predominates over the aa but for other groups this is not necessarily so. For example, both fram-l,4-dibromocyclohexane and the corresponding di-chloro compound have the ee and aa conformations about equally populated228 and most irons-1,2-dihalocyclohexanes exist predominantly in the aa conformation.229 Note that in the... 

When cyclohexene reacts with bromine, the product is trans-isomers, but only frans-l,2-dibromocyclopentane (38) is formed in the reaction. (See Chapter 9, Section 9.5, to review diastereomers.) Only the ra. s-diastereomer formed, so this reaction is diastereospecific. This stereochemical preference has been confirmed in the reactions of many alkenes, over many years. If cyclohexene is viewed from the side, as in 41, it is clear that the initial reaction with bromine must deliver the Br of the bromonirun ion to one side of the ring or the other. It can be either on the top or the bottom because there is no facial bias in the C=C unit of cyclohexene. 

---