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 1,2-dibromocyclohexane Is this... 

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. ... 

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. 

Hydrogen bromide undergoes photoaddition to 1-bromocyclohexene to produce czj-1, 2-dibromocyclohexane as the sole product/11 Studies of the photoaddition of HBr to other olefins indicate that essentially stereospecific trans addition occurs[Pg.272]

The case is interesting in 1,2 dibromocyclohexane. The moment for the diaxial isomer is assumed to be zero, while the calculated and observed values for the diequatorial form are respectively 3.09 D and 3.12 D. The trans diaxial form is more stable than the trans diequatorial, although the latter is favoured sterically. The reason for the lesser stability of the trans diequatorial form is that it is destabilized by dipole repulsion. 

In addition to substitution N-bromosuccinimide also forms addition compounds although in small amounts. Such addition reactions are catalysed by tetra-alkyl ammonium salts. Thus while with cyclohexene allylic substitution occurs, in presence of tetramethylammonium bromide, 1 2 dibromocyclohexane is obtained as the main product. 

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. 

Catalysis by various low-valent metalloporphyrins of the type already depicted in Section 3.7.2 (see reference lb for a precise list) is represented in Figures 4.3 and 4.4 for several cyclic and acyclic 1,2-dibromides. A striking example of the contrast between redox and chemical catalyses is shown in Figure 4.3a, with fluorenone anion radical on the one hand and iron(I) octaethylporphyrin on the other. Starting with the oxidized, inactive form of the catalyst, in each case—the active form is produced at a reversible wave. Addition of the same amount of 1,2-dibromocyclohexane triggers a catalytic increase in the current that is considerably less in the first... 

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... 

Addition of the second electron is probably concerted with elimination of bromide ion so that a carbanion intermediate is never formed. Thus, reduction of 1,2-dibromocyclohexanes gives a quantitative yield of the cyclohexene in both aprotic... 

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 ... 

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