The PTC reaction shown in the diagram is a base catalyzed cyclization and rearrangement that performs better when using solid NaOH than when using aqueous base.
The data for the use of aqueous base are shown in the table. The reaction times are longer, the conversions lower and the yields lower.
Before discussing the potential mechanism, it is worthwhile stating an observation that I have been using as a guideline for decades which is that under PTC conditions, when hydroxide has a choice of acting as a base or as a nucleophile, it acts primarily as a base and when there are no “acidic” protons to be deprotonated, hydroxide will act as a nucleophile.
The question here is how hydroxide may be catalyzing the reaction. One possibility may start with hydroxide nucleophilically adding to the electron-deficient carbon of the quinone or addition to the alkene. However, the methylene group that is allylic and alpha to the ring, likely has a pKa low enough to be deprotonated by PTC-NaOH, especially if the NaOH is solid which means that there is little hydration to deactivate the basicity. There are many examples of PTC being able to deprotonate substrates with pKa’s even in the 30’s (my Ph.D. Thesis from 1983 shows many examples).
Therefore, we speculate that the reaction starts by PTC-NaOH deprotonation to form an allylic carbanion that attacks the carbonyl carbon of the quinone. This results in cyclization and the formation of an alkoxide on the ring that is protonated by the molecule of water formed during the deprotonation. This forms an alcohol on the ring that undergoes tautomerization and aromatization that leads to the hydroquinone derivative.
The phase-transfer catalyst is essential to promote the solid-liquid transfer and activation of the hydroxide in the non-hydrated toluene phase. The presence of additional water hinders the transfer of hydroxide from the aqueous phase in which it is “very comfortable.” Small amounts of hydration that may accompany the hydroxide into the toluene phase or at the interface, reduce its basicity.
The overall process results in high yield and easy workup.
