Trans-1,2-Dibromocyclohexane

When a solution of 10a and 11 in ether was kept at room temperature for 12 h, a 2 1 inclusion complex of 10a and 11 was obtained as colorless prisms (m.p. 134-137 °C) in 72% yield. When a powdered mixture of the inclusion crystal and 7 was kept at room temperature in the solid state for 3 days, (+)-trans-1,2-dibromocyclohexane (12) of 12% ee was obtained in 56% yield by distillation of the reaction mixture [3]. Much more efficient enantioselective reactions in the solid state are described in Sect. 6. 

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

FIGURE 4.3. Redox and chemical homogeneous catalysis of trans-1,2 dibromocyclohexane. a cyclic voltammetry in DMF of the direct electrochemical reduction at a glassy carbon electrode (top), of redox catalysis by fhiorenone (middle), of chemical catalysis by an iron(I) porphyrin, b catalysis rate constant as a function of the standard potential of the catalyst couple aromatic anion radicals, Fe(I), a Fe(0), Co(I), Ni(I) porphyrins. Adapted from Figures 3 and 4 of reference lb, with permission from the American Chemical Society. 

A microemulsion made from dodecane, water, and didodecyldimethylammonium bromide has proven to be a good medium for the catalytic reduction of trans-1,2-dibromocyclohexane and for Sn2 reactions... 

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-1,2-dibromocyclohexanes. A reagent prepared from triphenylphosphine and carbon tetrachloride has been used for similar transformations.5... 

The simple carbocation intermediate of Equation 10-1 does not account for formation of the antarafacial-addition product. The results with SN1 reactions (Section 8-6) and the atomic-orbital representation (see Section 6-4E) predict that the bonds to the positively charged carbon atom of a carbocation should lie in a plane. Therefore, in the second step of addition of bromine to cyclo-alkenes, bromide ion could attack either side of the planar positive carbon to give a mixture of cis- and trans-1,2-dibromocyclohexanes. Nonetheless, antarafacial addition occurs exclusively ... 

The trans-addition of bromine to C=C double bonds follows the onium mechanism generally formulated in Figure 3.42. Cyclohexene reacts stereoselectively to give racemic trans-1,2-dibromocyclohexane ... 

Due to the same compulsory anti-selectivity the potassium alkoxide-mediated HBr eliminations from trans-1,2-dibromocy clohexane ultimately results in the formation of 1,3-cyclohexa-diene rather than 1-bromocyclohexene (Figure 4.25). The reason is that an anh -selective elimination occurs initially, trans-1,2-Dibromocyclohexane—except in very polar solvents— prefers a chair conformation with axial C-Br bonds (as it is only in this conformation that the... 

Note that according to the foregoing definition, chirality occurs only in molecules that do not have a rotation/reflection axis. However, if the molecule has only ( ) an axis of rotation, it is chiral. For example, both trans-1,2-dibromocyclohexane (D in Figure 3.3) and the dibromosuccinic acid E have a two-fold axis of rotation (C2) as the only symmetry element. In spite of that, these compounds are chiral because the presence of an axis of rotation, in contrast to the presence of a rotation/reflection axis, is not a criterion for achirality. 

Because of the same necessity for anf/-selectivity, trans-1,2-dibromocyclohexane does not react to form 1-bromocyclohexene in the KOfBu-promoted HBr elimination (Figure 4.23). Instead 3-bromocyclohexene is produced through a fairly regioselective E2 elimination. In a subsequent fast 1,2-elimination, it loses a second equivalent of HBr. In this way 1,3-cyclohexadiene is produced. 

We can draw two nonsuperimposable mirror images of the most stable chair conformation of trans-1,2-dibromocyclohexane with both bromines equatorial. These structures cannot interconvert by ring-flips or other rotations about bonds, however. They are mirror-image isomers enantiomers. 

Cyclohexene is dissolved in a solution of lithium chloride in chloroform. To this solution is added one equivalent of bromine. The material isolated from this reaction contains primarily a mixture of trans- 1,2-dibromocyclohexane and trans-l-bromo-2-chlorocyclohexane. Propose a mechanism to show how these compounds are formed. 

Problem 9.39 Use cyclohexanol and any inorganic reagents to synthesize (a) trans-1,2-dibromocyclohexane, (b) cis-1,2-dibromocyclohexane, (c) trans-1,2-cyc ohexanediol. M... 

French chemists2 report that treatment of trans-1,2-dibromocyclohexane (1) with the complex base sodium amide-sodium t-butoxide produces cyclohexene (2, 36% yield) and 1-bromocyclohexene (3, 60% yield). Use of either base (separately) leads... 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39], This latter system has important applications in electrochemical synthesis and catalysis [88-92], Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of trans-1,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

However, since the bromine atom is electron-rich, two bromine atoms in the diequatorial conformation will repel each other. This dipole-dipole repulsion will be relieved as the bromine atoms become axially oriented in the diaxial conformation. Thus at equilibrium, trans 1,2-dibromocyclohexane will exist in both the diequatorial and the diaxial conformations in approximately equal parts, since the two forms are of about equal energy. 

Por, 5,10,15,20-tetraphenyl-210H,23H-porphine DBCH, trans-1,2-dibromocyclohexane DBl 8C6, dibenzo-18-crown-6. 

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