Dibromocyclohexanes

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... 

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. 

Table 2. Percentages of Dibromocyclohexane 2h, Dibromocyclopentanone UHi, trans-1-Acetoxy-2-bromocyclohexane 2a, (OS=OAc) and trans-l-Acetoxy-2-bromo-cyclopentane 10a (OS=OAc) obtained from Solvolysis of 4 in HOAc containing 0.5 M Cyclopentene and Varying [Br ], n=0.1(LiC104), ... 

Table 3. Percentages of Dibromocyclohexane 2h, Dibromocyclopentane IQh, trans-l-Methoxy- ...

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. ... 

A facile synthesis of carba- -D-glucopyranose (99) and its l antipode (104) was accomplished by means of resolution of the starting compound DL-( 1,3,5/2,4)-2,3-diacetoxy-4,5-dibromocyclohexane-1 -carboxylic... 

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. 

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 tra 5-l,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. 

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]

Before the explanation of ladder poly silanes is started, ladder compounds of other Group 14 elements are briefly mentioned. Hydrocarbons with a ladder-shaped carbon framework are known as ladderanes. The study on ladderanes goes back to 1927, when bicyclo[2.2.0]hexane ([2]ladderane) was synthesized by the reduction of dy-l,4-dibromocyclohexane with sodium.19 Ladderanes so far reported are tricyclo[4.2.0.02,5]octane ([3]ladderane),20 tetracyclo[4.4.0.02,507,1°]decane21 ([4]ladderane), and a number of their derivatives (Fig. 3). 

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... 

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... 

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... 

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