Question

The acetolysis product(s) of the given reaction (solvolysis of the benzylic tosylate in AcOH) is(are) 

 

Hint:

Benzylic solvolysis next to an aryl group often proceeds via a {phenonium} ion; nucleophiles open the bridge {anti}, giving net inversion at that carbon. Stereocenters not involved in bond making/breaking retain configuration. When options show pairs, look for “inversion vs retention” at the reacting center to decide.

Answer 1

Step 1: Ionization and anchimeric assistance.
In \( \mathrm{AcOH} \), the benzylic tosylate undergoes solvolysis. The departure of \( \mathrm{OTs}^- \) generates a benzylic cation. This benzylic cation is then stabilized by a neighboring group, in this case, the adjacent phenyl ring, which provides anchimeric assistance. This neighboring-group participation shields one face of the cationic center, resulting in a unique intermediate. The cation formed is not planar, as the adjacent phenyl group plays a significant role in stabilizing the cation by forming a bridged phenonium intermediate. This mechanism allows the system to minimize the energetic instability of the benzylic cation.

Step 2: Regio-/stereochemical capture.
The nucleophilic attack by \( \mathrm{AcO}^- \) occurs via an anti opening of the phenonium bridge at the reacting carbon. The anti opening is a key factor here because it results in an inversion of configuration at the substitution center. This means that the attack occurs from the opposite side of the leaving group, similar to an SN2 reaction mechanism. This leads to an overall inversion at the reacting center. Importantly, the other stereocenter, which is not directly involved in the reaction (since its bonds remain unbroken), retains its original configuration. This is a classic feature of reactions involving anti opening and inversion at the center undergoing substitution.

Step 3: Products formed.
As a result of the acetolysis, two diastereomers are formed. Both of these diastereomers exhibit inversion at the reacting center, which occurs due to the attack from the two accessible conformers formed during the anti opening of the phenonium intermediate. Therefore, options (A) and (C) are the correct products because they reflect the inversion of configuration at the reacting center, consistent with the described mechanism. On the other hand, options (B) and (D) depict retention of configuration at the reacting center, which is not favored in this case due to the anchimeric assistance. The neighboring-group participation leads to an anti attack and inversion, thus making options (B) and (D) incorrect.

Final Answer: \( \boxed{\text{Anchimeric assistance by Ph } \Rightarrow \text{ anti attack and inversion at the benzylic center } \Rightarrow \text{ (A) and (C).}} \)