1,2 Dibromocyclohexane, preparation

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... 

Another common method for the preparation of benzene and naphthalene epoxides is by introduction of double bonds into the dihydro precursors. This is generally achieved by bromination-dehydrobromination. Thus unsubstituted benzene oxide-oxepin (86 96) can be obtained by dehydrohalogenation of the dibromocyclohexane epoxide 99, using a basic amine or sodium methoxide in ether.53... 

The 1,2-dibromocyclohexane was prepared by the method of Snyder and Brooks.2 If the cyclohexene is cooled to ca. —30° with a dry ice-isopropyl alcohol bath and the bromine is not diluted, it is possible to run this preparation on a threefold scale in one-third of the recorded time. The product was always purified by the recommended procedure. 

Dibromocyclohexane has been prepared by the treatment of cyclohcxcnc with bromine in solution in chloroform,1 ether,2 and aqueous sodium bromide.3... 

Scheme 15 Preparation of supraminol complexes 29 35 and 29 36 from the (R,R)-trans-1,2-diaminocyclohexane (29) and (/ ,/ )-4-cyclohexene-l,2-diol (35) and (R,R,R,R)-4, 5-dibromocyclohexane-1,2-diol (36).

Relative to tertiary alkyl halides, secondary derivatives react considerably slower. At room temperature and long reaction periods ( 24h) cyclohexyl chloride is almost quantitatively methylated with dimethyltitanium dichloride (prepared in situ from dimethylzinc and catalytic amounts of TiQ4)137>, but other cyclic or acyclic halides tend to undergo competing rearrangements prior to C—C bond formation 77). The same applies to 1,2-dihalides such as 1,2-dibromocyclohexane which affords 1,1-dimethylcyclohexane instead of the 1,2-dimethyl derivative137. In complete contrast, activated secondary chlorides behave much like tertiary derivatives, i.e., methylation is fast and position specific at low temperatures. Examples are shown in Equation 86137>. It should be noted that in such cases cuprate chemistry affords less than 40 % of methylation products138). 

An interesting preparation of 2-cyclohexenylmalonates involves alkylation of malonic esters with 1,2 dibromocyclohexane. Dehydrohalogenation accompanies alkylation to give the olefinic malonates in 55-65% yields. " "... 

A filtered soln. of MgBrg prepared from Mg and ethylene bromide in ether added dropwise at 0-5° to a soln. of benzoyl peroxide in cyclohexene and benzene, allowed to warm to room temp., and stirred for 2 additional hrs. fra/15-1,2-dibromocyclohexane. Y 83%. S.-O. Lawesson and N. G. Yang, Am. Soc. 81, 4230 (1959). 

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