Isomerization of Vicinal Dibromides in Conformationally Rigid Cyclohexane Systems

Ryan Acocella


Previous studies by Barton et al. have shown that vicinal bromine atoms in rigid cyclohexane systems can undergo an interesting isomerization via a diatropic shift. In this 1,2 interchange, the bromine atoms rearrange from the diaxial positions to the diequatorial positions when heated. The goal of this project was to determine if this reaction could be developed into an undergraduate laboratory experiment to demonstrate the concept of kinetic and thermodynamic products. To that end, the reaction time, the extent of isomerization, ease of analysis, and safety were evaluated. The substrate, 1,2-dibromo-4-tert-butylcyclohexane, was prepared via bromination of 4-tert-butylcyclohexene at 0 ºC to give a 9:1 mixture of diastereomers. The major diastereomer, which is the kinetic product, contains diaxial bromine atoms, whereas the minor, thermodynamic product has both bromines in the equatorial positions. This 9:1 mixture of diastereomers was heated at temperatures ranging from 100 ºC to 175 ºC in a sealed vial for approximately 3 hours. The isomerization was completed under polar and non-polar conditions by heating the substrate neat or in d6-DMSO. Samples from each reaction were analyzed by GC-MS every 30 minutes in order to monitor the change in the ratio of diaxial and diequatorial isomers, and the final reaction mixtures were analyzed by 1H NMR spectroscopy. From these experiments it was determined that the thermal rearrangement of 1,2-dibromo-4-tert-butylcyclohexene is relatively slow and stops at a 1:1 ratio of diastereomers. However when heated in d6-DMSO, there is a significant change in the ratio of diaxial to diequatorial isomers, but the analysis is complicated by a previously undescribed side reaction.


Bromination; Isomerization

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