Acid-catalysed isomerisation of phenyl epoxide to aldehyde

Question

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?

Answer 1

You 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).

References:

1. Vinícius V. Costa, Kelly A. da Silva Rocha, Ivan V. Kozhevnikov, Elena V. Gusevskaya, "Isomerization of styrene oxide to phenylacetaldehyde over supported phosphotungstic heteropoly acid," Applied Catalysis A: General 2010, 383(1–2), 217-220 (https://doi.org/10.1016/j.apcata.2010.06.005).
2. Kishor V. Wagh, Bhalchandra M. Bhanage, "Synthesis of 2-phenylnaphthalenes from styrene oxides using a recyclable Brønsted acidic $\ce{[HNMP]+HSO4−}$ ionic liquid," Green Chemistry 2015, 17(8), 4446-4451 (DOI: 10.1039/C5GC01170A).