How does $\ce{H3O+}$ cause the formation of an aldehyde from the following epoxide?
I thought that the epoxide would undergo opening to form a 1,2-diol, as illustrated below. Am I wrong?
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Sign up to join this communityYou are correct if this epoxide is a alkyl peroxide. However, this is a 2-phenyloxirane (styrene oxide or phenylethylene oxide), which can stabilize a protonated intermediate by resonance, specifically with 2-hydroxyphenyl group. Thus, it can isomerize to phenylacetaldehyde through 1,2-hydride shift (similar to pinacol-pinacolone rearrangement). The following diagram illustrate how styrene oxide converted to phenylacetaldehyde (Ref.1):
Note that styrene oxide is not stabilized by extra hydroxy group here. That's probably the reason the authors have used slica-supported phosphotungstic heteropoly acid in cyclohexane (no other nucleophile there).
The identical isomerization has been used in the synthesis of 2-phenylnaphthalenes from styrene oxides (Ref.2).
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