Stereochemistry of E2 Elimination

Base-promoted E2 eliminations involving 1,2-dibromo-1,2-diphenylethane have been used to learn about the stereochemical preferences of this reaction. The meso starting material gives one alkene and the dl starting material gives another. 

Assuming that the breaking CH and CBr bonds must lie roughly in the same plane, then the HCCBr torsion angle must either be close to 0° (syn elimination) or close to 180° anti elimination). 

Step through the sequence of structures depicting rotation about the carbon-carbon bond in the two dibromoethane isomers l,2-dibromo-l,2-diphenylethane A andfi). For each, plot energy (vertical axis) vs. BrCCBr torsion angle (horizontal axis), and identify all minimum-energy structures. Which of these are reactive conformers , that is, conformers which are set up for either syn or anti elimination of HBr Which are non-reactive conformers , that is, which do not meet the requirements for elimination Do the reactive conformers correspond only to syn elimination, only to anti elimination, or are both pathways represented Which alkene would these reactive conformers lead to Are your results consistent with the observation that each isomer of the starting material gives only one alkene Explain. 

Do any or all of the reactive conformations correspond to the lowest-energy conformations What, if anything, does this tell you about the rate of interconversion of conformers relative to the rate of elimination  

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Table 6.8. Stereochemistry of E2 Eliminations for Some Acyclic Substrates...

This example is from biochemistry. It is a feature of biochemical reactions that enzymes almost always catalyse reactions in a completely stereospecific manner. They are able to distinguish between enantiotopic hydrogens because of the three-dimensional nature of the binding site (see Section 13.3.2). There are also occasions where chemical reactions are stereospecific refer to the stereochemistry of E2 eliminations for typical examples (see Section 6.4.1). 

Predict and explain the stereochemistry of E2 eliminations to form alkenes. Predict the products of E2 reactions on cyclohexane systems. 

There must be a stereochemical requirement in this elimination. If the Saytzeff alkene is not produced because the methyl group is trans to the leaving group, then the H and the leaving group must be trans and the elimination must be anti—the characteristic stereochemistry of E2 elimination. This evidence differentiates between the two possibilities in part (a). 

Table 5.15. Stereochemistry of E2 Elimination for Some Acyclic Systems...

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