The Stereochemical Outcome of an E2 Reaction

The examples in the previous section focused on regiochemistry. We wiU now focus our attention on stereochemistry. For example, consider performing an E2 reaction with the following substrate  

This substrate has two identical 3 positions so regiochemistry is not an issue in this case. Deprotonation of either p position produces the same result. But in this case, stereochemistry is relevant, because two stereoisomeric alkenes are possible  

Both stereoisomers (cis and trans) are produced, but the trans product predominates. This specific example is said to be stereoselective, because the substrate produces two stereoisomers in unequal amounts. 

EXERCISE 10.4 Draw the expected product(s) when the following substrate is treated with a strong base to give an E2 reaction  

ANSWER Let s first consider the expected regiochetnical outcome of the reaction. The reaction does not employ a sterically hindered base, so we expect formation of the more substituted alkene (the Zaitsev product)  

---

You need to get into the habit of drawing Newman projections so that you can determine the stereoisomer that is expected from an E2 reaction. If you are rusty on Newman Projections, you should go back and review the first two sections in Chapter 6 in this book. Then come back to here, and try to use Newman projections to determine the stereochemical outcome of the following reactions. 

S,3Sj-2-Bromo-3-phenylbutane undergoes an E2 reaction when treated with a strong base to produce (E)-2-phenyl-2-butene. Use Newman projections to explain the stereochemical outcome of this reaction. 

Because the E2 reaction is a single-step process, with precise stereochemical requirements, we can do more to control the outcome. As far as the substrate is concerned, branching at a- and/ or p-positions slows substitution more than it does elimination. Because in substitution, there is a requirement for a five-coordinate carbon at the transition state, the steric demand of this process is much higher than for elimination. Also, branching at the a- or p-positions stabilizes an alkene-like transition state. As we have previously noted, stereochemistry is critically important, especially to E2 reactions. Elimination will be inhibited if the tram-anti arrangement of the atoms or groups to be eliminated is not available or is disfavored. Substitution will be unaffected. If we compare the reactions of 10.11a and 10.11b using ethoxide (which can be either a... 

---