Cyclohexane, 1,2-dibromo-, trans

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. 

It is believed that equatorial substituents such as chlorine or bromine would increase the guest diameter beyond the allowed values (assuming that the guest molecules stack roughly parallel to the canal68)). Support for this comes from the study of fluorocyclohexane where the population of the axial conformer is not enhanced to any major extent70. Nitro-71) and cyano-cyclohexane, trans-l,2-dichloro-, trans-1,2-dibromo-, tram-1,4-dichloro-, trans-1,4-dibromo-, and trans-l-bromo-4-chloro-cyclohexane all pack most efficiently in the thiourea canals as the axial or diaxial conformer 68,72. Tram-2,3-dichloro-1,4-dioxane behaves similarly73. In contrast isocyanato-, tram-1,4-diiodo-, trans-1 -bromo-4-iodo-, and tram-1 -chloro-4-iodo-cyclohexane are present as mixtures of the axial/equatorial or diaxial/diequatorial conformations as appropriate 68,72). The reason for this anomalous behaviour of the iodosubstituted cyclohexanes is not clear. 

We now focus on the effect of replacing fluorine by another halogen of a higher period, e.g. Br. The trans and cis conformations of 1,2-dibromo-cyclohexane are shown below along with the dominant orbital interactions ... 

AH68 CeHxoBr2 trans-1.2-dibromo-cyclohexane EtOH 50 PY Et4NI 0.05 - - - DME/SCE 3-0 C-0.5... 

It is well known that in cyclohexan series (26, n = 6) this elimination is very difficult to perform with classical bases26). For example, to our knowledge, no satisfactory method has so far been found for preparation of 1-bromo cyclohexene from trans 1,2-dibromo cyclohexane. This apparently very simple reaction, leads only with great difficulties to the desired compound (yields are low and tedious purifications are required to obtain a pure product27)). 

Fig. 6. Plots of /nc- H against concentration in mole % for (a) dichloro- and (b) dibromo-ethylenes in dimethylformamide (DMF) and cyclohexane (CH) solutions. Values for cis isomers are denoted by circles, those of trans isomers by squares. (From Watts et...

CloHigBr2O, 2,6-Dibromo-3,3,5,5-tetramethylcyclohexanone, 29, 493 CloHi5CI2O, 2,2-Dichloro-4-t-butylcyclohexanone, 39B, 96 Cl Hi 6 2 f 3-Acetyl-2-vinyl-cyclohexan-1-ol, 45B, 115 Cl Hi6O3, trans-2,4-Dihydroxy-2,4-dimethylcyclohexane-trans-1-acetic acid 7-lactone, 37B, 78... 

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