In light of your answer to Problem 11-64, which alkene, E or...

In light of your answer to Problem $11-64,$ which alkene, $E$ or $Z,$ would you expect from an $\mathrm{E} 2$ reaction on the tosylate of $(2 R, 3 \mathrm{R})-3$ -phenyl2-butanol? Which alkene would result from E2 reaction on the $(2 S, 3 R)$ and $(2 S, 3 S)$ tosylates? Explain.

Key Concepts

E2 Reaction Mechanism

The E2 reaction is a one-step bimolecular elimination process in which a base abstracts a ??proton while the leaving group departs simultaneously, resulting in the formation of a double bond. The rate of the process depends on both the substrate and the base, and the reaction is stereospecific because the geometry in the transition state influences which alkene is formed.

Anti-Periplanar Geometry

A critical requirement for the E2 mechanism is the anti-periplanar alignment of the hydrogen to be abstracted and the leaving group. This orientation allows for optimal orbital overlap during the removal of the proton and the departure of the leaving group, which in turn governs the stereochemical outcome of the alkene product.

Alkene Stereochemistry (E/Z Isomerism)

The stereochemistry of an alkene is analyzed using the E/Z system, which classifies products based on the relative positions of the highest priority substituents on either side of the double bond. The specific E or Z configuration that results from an E2 reaction is influenced by the geometric constraints imposed by the starting material’s conformation.

Influence of Substrate Stereochemistry

The configuration (R or S) of the stereocenters in the starting material dictates the available conformations and the spatial relationship between the leaving group and the adjacent hydrogen atoms. This stereochemical arrangement determines which proton is in an anti-periplanar position to the leaving group, thereby influencing whether an E or Z alkene is preferentially formed during the E2 elimination.

Tosylate as a Leaving Group

Tosylate groups (OTs) are excellent leaving groups due to their ability to stabilize the negative charge once they depart from the molecule. Their effectiveness in this role promotes the elimination process, facilitating the concurrent loss of the tosylate and the proton abstraction characteristic of the E2 mechanism.

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