Thiol-yne reaction

Thiol-ene and Thiol-yne Reactions for the Transformation of Oleochemicals into Monomers and Polymers... 

Thiol-yne Reaction as a Tool for the Synthesis of Monomers and Polymers Derived from Vegetable Oils... 

Similar to the thiol-ene reaction, the thiol-yne reaction is also classified as click. Therefore, it has attracted the attention of many researchers interested in applying more environmentally friendly approaches to their work. 

The mechanism of the thiol-yne reaction resembles that of its thiol-ene sister. However, a second possible cycle involving further addition of a thiol into the remaining doublebond reaction is also possible in the thiol-yne version (Scheme 6.17) [49]. As a matter of fact, this second cycle is, by definition, a thiol-ene radical coupling. 

Despite being sister reactions , works reporting on use of thiol-yne reactions for the synthesis of monomers and polymers from fatty-acid derivatives are much scarcer than those using the thiol-ene approach. 

Cadiz and co-workers [50] demonstrated use of the thiol-yne reaction for the synthesis of polyols for application in partially renewable PU (Scheme 6.18). The starting materials were 10-undecynoic acid and 9-octadecynoic acid, which were prepared by dehydrobromination of the corresponding derivatives of 10-undecenoic acid and 9-octadecenoic acid. 

A very similar strategy was adopted recently by Pham and co-workers [52] when propargylic fatty ester (PFE) and propargylic fatty diester (PFD) derived from propargyl alcohol and a derivative of oleic acid were converted into lipidic polyols (OHl) and (OFf2), respectively, via thiol-ene and thiol-yne reactions with mercaptoethanol (Scheme 6.19). 

Scheme 6.19 Application of thiol-ene and thiol-yne reactions of propargylic fatty ester and propargylic fatty diester with mercaptoethanol [52]...

Since the thiocarbonylthio end groups of the polymer produced by RAFT polymrization can be easily mod-i ed to a variety of species including cleavage to a thiol (typically with a primary or secondary amine) that affords subsequent TEC reaction, it will be convenient to use the RAFT method to make PNIPAm of the desired DP. Yu et al. (2009) used 1-cyano-l-methylethyl dithiobenzoate (CPDB) as the RAFT CTA to make PNIPAm precursors to well-de ned ene and yne terminal polymers, as shown in Scheme P12.15.1. The latter are used to carry out thiol-ene and thiol-yne reactions. 

Scheme P12.15.1 RAFT polymerization of NIPAm and subsequent end group modi cations via tandem nucleophilic thiol-ene/radical thiol-ene and nucleophilic thiol-ene/radical thiol-yne reactions. (From Yu et at., 2009. With permission from John Wiley Sons, Inc.,)...

In a typical procedure for photochemical, radical thiol-ene and thiol-yne reactions (Yu et al., 2009), a mixture of PNIPAm-S-ALMA or PNIPAm-S-PROPA (0.2 g), 100% excess of target thiol (based on the molarity of ene or yne ), benzil dimethyl ketal (Irganure 651) photoinitiator (15.0 mg) in an appropriate volume of THF is exposed to UV light (A = 350 nm) for 2 h. The sample is dialyzed against methanol. It should be noted that the thiol-ene and thiol-yne reactions yield mono and bis end-functionalized PNIPAm homopolymers, respectively (see Scheme P12.15.1). 

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... 

Figure 2.10 (a) The UV-promoted radical addition of thiols to alkynes - the thiol-yne reaction (b) kinetics of the thiol-yne reaetion monitored via FT-IR spectroscopy (Chan et ai, 2009a). DMPA 2,2-dimethoxy-2-phenylacetophenone. (Reprinted with permission from J.W. Chan, C.E. Hoyle and A.B. Lowe, Sequential phosphine-eatalyzed, nucleophilic thiol-ene/radical-mediated thiol-yne reactions and the facile orthogonal synthesis of polyfunctional materials, Journal of the American Chemical Society, 131, 16, 5751-5753, 2009. 2009 American Chemical Society.)... 

Chan, J.W., Hoyle, C.E., and Lowe, A.B. (2009a) Sequential phosphine-catalyzed, nucleophiUc thiol-ene/radical-mediated thiol-yne reactions and the facile orthogonal synthesis of polyfimctional materials. Journal of the American Chemical Society, 131,5751. 

As with all condensation reactions, thiol-ene polymerizations are sensitive to the stoichiometry of the thiol- and ene-containing monomer substrates. Typically, the best results are obtained when there is a molar equivalent concentration of thiol and double bonds present. Recently, there has been a considerable effort to extend the photoinduced radical addition of thiols to alkynes (the so-called thiol-yne reaction). 

This concept was refined by the research group of Li, who employed the thiol-yne reaction instead of the olefin cross-metathesis reaction as key step [60]. In this case, the carboxylic acid component served as anchor, whereby terminal alkynes were introduced by the remaining components (5-hexyn-l-al and propargyl isocyanoacetamide). Interestingly, thiol-yne addition of 3-mercaptopropionic acid to the pendant alkynes enabled not only the incorporation of further carboxylic acids, but also resulted in additional branching. Therefore, the second generation dendrimer, synthesized in three steps, exhibited 16 peripheral triple bonds. Moreover, this concept offers the opportunity to introduce structural diversity into the dendrimer architecture because the use of only one alkyne-functionalized compound in the Passerini-3CR still results in branching due to the thiol-yne reaction. Here, a structural sequence of employed phenylacetaldehyde and 2-nitrobenzaldehyde was demonstrated. 

Various click chemistries, building the foundation for most post-polymerization modifications, have also been applied for fabrication of reactive porous polymer monoliths [118-125]. Click reactions include alkyne-azide, thiol-ene, and thiol-yne reactions, and these reactions can be conducted under mild conditions, unlike many other polymer graft reactions. Svec and coworkers used the thiol-ene reaction to functionalize porous polymer monoliths [121]. Monoliths were thiolated with cysteamine, followed by cleavage of the disulfide bonds using tri(2-carboxylethyl) phosphine to expose the desired thiol groups. Then, lauryl methacrylate monomers were clicked onto the monoliths using either heat or UV initiation. Thiol-yne and... 

In short, the thiol-yne radical click reaction is the formation of a 1,2-dithioether through double addition of thiols on to an alkyne. The reaction has largely been used to generate multifunctional polymer structures. Repetitive thiol-yne reactions are used to form multifunctional molecules, which are further used to make dendrimers [42] or hyperbranched polymers [46]. 

Akin to thiol-ene reaction, thiol-yne was used by Stenzel and coworkers for the synthesis of dendrimers, where thiol-yne reaction was fast (10 min UV irradiation) and proceeded by reacting 2 equiv. of thiol with alkyne therefore, a limited number of steps were needed to produce a high number of terminal functional... 

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