= Solution
Hydroxide initially deprotonates a little <thiophenol>, generating the reactive <thiolate> $\mathrm{PhS^-}$. 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. \b[The product is 3-\[2-(phenylsulfanyl)ethyl\]cyclohex-2-en-1-one], drawn below. Its side chain is $\mathrm{-CH_2CH_2SPh}$; 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, \b[$\mathrm{C_8H_{10}O+C_6H_6S=C_{14}H_{16}OS}$]; <carbonyl> 1,2 addition would instead produce a different functional group and is not favored for this soft <sulfur> <nucleophile>.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-1-thiol-additions.png]
{title=Regioselective sulfur addition to an extended enone and an activated allene}
Back to article page
