Thiol-yne coupling

Thiol-yne coupling is a click-chemistry reaction between a thiol (R-SH) and an alkyne (C≡C) that forms one or two new C–S covalent bonds. Because the alkyne triple bond can react twice, a single thiol-yne molecule can form a denser crosslink network than a thiol-ene molecule of equivalent size.

Background

Mechanism: a thiyl radical adds across the alkyne to form a vinyl sulfide intermediate, which can react with a second thiol to form a 1,2-bis-sulfide product. In practice the reaction yield depends on stoichiometry, radical initiator, and substrate accessibility — but two C–S bonds per alkyne is the upper bound.

In haircare, thiol-yne complements thiol-ene. ANATOMY uses Aminalyl S, a bifunctional molecule with an alkyne functional group, alongside Pro-amino X (alkene-bearing) — the pair drives both thiol-ene and thiol-yne reactions in the same application, producing a denser crosslink network than either alone.

Why thiol-yne is useful: - Higher crosslink density per molecule. Two C–S bonds vs one. - Reaction with thiols already present. Damaged hair has elevated free-thiol concentration. - Compatible with thiol-ene in the same protocol. No mutual interference.

The published bis-adduct formation rate from LC-MS analysis of treated bleached hair: 81.5%. That figure quantifies how often the alkyne carries a second thiol addition vs stops at the vinyl sulfide stage. Higher bis-adduct rate equals denser crosslinking.