Mechanisms of Thiol-Ene Reactions

The TEC reaction can be accomplished under a variety of experimental conditions, including acid/base catalysis (Li et al., 2008), nucleophilic catalysis (Chan et al., 2009), radical mediation (most commonly induced photochemically) (Dandoni, 2008), and via a solvent-promoted process (Tolstyka et al., 2008). However, the reaction is most commonly performed under radical mediation where it is apphcable to many substrates, or under nucleophilic mediation for activated enes where the process proceeds via an anionic chain process. 

The radical mediation of the thiol-ene reaction is brought about photochemically or via thermal decomposition of a radical initiator and the reaction is known to proceed by a radical mechanism to give a thioether, as shown in Fig. 12.31(A). Under nucleophile-initiated conditions, such as employing dimethylphenylphosphine (Mc2PPh, DMPP) as an initiator, the TEC reaction takes place extremely rapidly as well as quantitatively. The anionic chain mechanism proposed for the amine/phosphine mediated nucleophilic thiol-ene reaction with an acrylate is shown in Fig. 12.31(B). 

Under radical-mediated conditions, a thiol also adds to a yne. The radical thiol-yne reaction can be considered as a sister reaction to the radical thiol-ene reaction because it possesses characteristics virtually identical to tiiose of the radical thiol-ene reactions. As with the thiol-ene reaction, the thiol-yne reaction, in general, proceeds rapidly under a variety of experimental conditions in an air atmosphere at ambient temperature and humidity, selectively yielding the mono or bisaddition products with little to no clean up required. In the case of the double addition products formed imder radical conditions, the reaction of two equivalents of thiol with a terminal alkyne is itself a two-step process (Fig. 12.32). The rst, slower, step 1 involves the addition of thiol to the CsC bond to yield an intermediate vinylic radical that subsequently imdergoes chain transfer to 

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