The Claisen rearrangement can be used to attach allyl groups to benzene via initial alkylation of a phenol.
(a) Predict the identity of A and B
(b) provide an arrow-pushing mechanism for each step. [The phenol alkylation is simply a Williamson ether synthesis reaction.]

검증된 단계별 안내

Step 1: Begin with the phenol and treat it with sodium hydride (NaH), a strong base. This deprotonates the hydroxyl group, forming a phenoxide ion.

Step 2: The phenoxide ion then reacts with allyl bromide in a Williamson ether synthesis. The oxygen of the phenoxide ion attacks the carbon attached to the bromine, displacing the bromide ion and forming an allyl phenyl ether. This is compound A.

Step 3: Upon heating, the allyl phenyl ether undergoes a Claisen rearrangement. This involves a [3,3]-sigmatropic rearrangement where the allyl group migrates to the ortho position relative to the oxygen, forming an ortho-allyl phenol. This is compound B.

Step 4: The ortho-allyl phenol can tautomerize between its keto and enol forms. The keto form is a cyclohexenone, while the enol form is an ortho-allyl phenol.

Step 5: Provide an arrow-pushing mechanism for each step: (a) Deprotonation of phenol by NaH, (b) Nucleophilic attack of phenoxide on allyl bromide, (c) [3,3]-sigmatropic rearrangement in Claisen rearrangement, and (d) Keto-enol tautomerism.

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Williamson Ether Synthesis

The Williamson ether synthesis is a reaction that forms an ether from an alkoxide ion and an alkyl halide. In this process, a phenol is deprotonated by a strong base like NaH to form a phenoxide ion, which then reacts with an allyl bromide to form an ether. This step is crucial for forming the intermediate that undergoes the Claisen rearrangement.

Claisen Rearrangement

The Claisen rearrangement is a pericyclic reaction where an allyl vinyl ether is converted into a γ,δ-unsaturated carbonyl compound upon heating. This rearrangement involves a [3,3]-sigmatropic shift, where the allyl group migrates to the ortho position of the aromatic ring, forming a ketone. It is a key step in attaching allyl groups to benzene derivatives.

Keto-Enol Tautomerism

Keto-enol tautomerism is an equilibrium between a ketone and its corresponding enol form. In this process, the ketone form, which is typically more stable, can convert to an enol form through the migration of a hydrogen atom and the shift of a double bond. This tautomerism is important for understanding the final structure of the product in the Claisen rearrangement.

The Claisen rearrangement can be used to attach allyl groups to benzene via initial alkylation of a phenol.(a) Predict the identity of A and B (b) provide an arrow-pushing mechanism for each step. [The phenol alkylation is simply a Williamson ether synthesis reaction.]