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With triethylamine present, proton transfer from thiophenol produces a more nucleophilic thiolate population. sulfur attacks the anomeric epoxide carbon C1 from the face opposite its epoxide C–O bond. Displacement by backside attack opens the epoxide, breaks C1–O, and leaves the former epoxide oxygen attached to C2. protonation of that alkoxide by thiophenol or triethylammonium gives G: SPh at C1 on the front face and OH at C2 on the back face, a trans relationship. This is inversion at C1 with retention of the C2 C–O bond; there is no need to invert every stereocentre. Rapid protonation also suppresses competing silyl transfer by the alkoxide.

Without added base, neutral thiophenol is less nucleophilic. acid-assisted cleavage at the anomeric epoxide bond can instead be accompanied by donation from the ring oxygen, giving an oxocarbenium ion with charge shared with the ring oxygen. thiophenol capture from the face that puts SPh back on the original epoxide face, followed by deprotonation, gives H with SPh and C2–OH both on the back face, the net-retention pathway. An ion-pair or highly asynchronous opening can provide this outcome; the drawing does not establish a long-lived free carbocation.

The distinction is configuration, not a mandatory fixed chair. For the closely matching trisilylated substrate, the primary study obtained both anomers with thiophenol alone, and established an unusual inverted chair for the alpha thioglycoside by NMR and crystallography. Thus H should be explained as an accessible product, not asserted to be the sole product or necessarily to have axial SPh in its preferred conformation.^[ref]

Base again generates $\mathrm{Ph}{\mathrm{S}}^{-}$ catalytically. In $\mathrm{P}{\mathrm{h}}_2\mathrm{P}(\mathrm{O})-\mathrm{CH}=\mathrm{C}=\mathrm{C}{\mathrm{H}}_2$, attack at the central allene carbon moves the adjacent C=C pi electrons onto the phosphorus-bearing carbon. This makes the stabilized alpha-phosphoryl carbanionprotonation at that carbon gives $\mathrm{P}{\mathrm{h}}_2\mathrm{P}(\mathrm{O})-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}(\mathrm{SPh})=\mathrm{C}{\mathrm{H}}_2$, the second product drawn above. The remaining double bond is terminal, so no E/Z pair is possible. The new sulfur bond is at the central carbon of the original allene, not at the terminal methylene.

This regiochemistry places the developing negative charge beside the electron-withdrawing phosphine-oxide group, instead of at a less stabilized vinylic position. It is an ionic addition/proton-transfer cycle, rather than radical thiol–ene chemistry. proton donation by another thiophenol molecule regenerates thiolate. One thiophenol is incorporated without loss of atoms, giving ${\mathrm{C}}_{21}{\mathrm{H}}_{19}\mathrm{OPS}$.

Hydroxide initially deprotonates a little thiophenol, generating the reactive thiolate $\mathrm{Ph}{\mathrm{S}}^{-}$. Number the extended conjugated system $\mathrm{O}=C1-C2=C3-C4=C5$, where C1–C2–C3 is the ring enone and C4=C5 is the attached vinyl group. sulfur attacks the terminal delta carbon C5. The pi bonds relay toward oxygen, generating an extended enolate.

proton transfer at C4, either directly from thiophenol or through solvent-mediated enolization, gives the product retaining the conjugated ring enone. The product is 3-[2-(phenylsulfanyl)ethyl]cyclohex-2-en-1-one, drawn below. Its side chain is $-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{SPh}$; the ring C2=C3 double bond remains. This is net sulfur/hydrogen addition to the terminal part of the extended system, proceeding through 1,6-Conjugate addition and protonation of the extended enolate.

The terminal site is less crowded than the ring beta carbon, and the isolated product retains favorable carbonyl–alkene conjugation. Reversible proton transfers and conjugate addition under thiolate catalysis can favor that product over an unconjugated enone-derived alternative. Transfer of a proton from another PhSH molecule regenerates PhS${}^{-}$, explaining why a catalytic quantity of hydroxide suffices. The net balance is one thiophenol added, ${\mathrm{C}}_8{\mathrm{H}}_{10}\mathrm{O}+{\mathrm{C}}_6{\mathrm{H}}_6\mathrm{S}={\mathrm{C}}_{14}{\mathrm{H}}_{16}\mathrm{OS}$; carbonyl 1,2 addition would instead produce a different functional group and is not favored for this soft sulfur nucleophile.

Thiophenol is $\mathrm{PhSH}$. Its conjugate base is thiophenolate, $\mathrm{Ph}{\mathrm{S}}^{-}$, a sulfur-centered nucleophile.