Thiol-ene click reaction

Scheme 15 General scheme for the synthesis of thioether-linked diubiquitins through thiol-ene click reaction...

Figure 14 The first examples of click reactions explored for dendrimer synthesis (a) CuAAC click reaction (b) Diels-Alder click reaction and (c) thiol-ene click reaction...

The thiol-ene (click) reaction for the s mthesis of plant oil derived polymers has been reviewed (73). The thiol-ene (click) reaction is a chain reaction with basic steps shown in Eq. 4.2. 

With the thiol-ene (click) reaction unsaturated moieties in a polymer can be modified. This is of interest, because some monomers are not readily pol5mcierizable to give the desired polymer. In fact, postpolymerization modification of a polymer backbone is an important strategy in pol5mcier science. 

The thiol-ene click reaction has also been applied extensively in recent studies to unsaturated oils and fatty acids. This free-radical process operates on terminal mono-substituted and internal di-substituted unsaturations, the latter coupling being shown schematically in Scheme 2.8. 

This chapter does not intend to be exhaustive, but aims instead at illustrating the impressive versatility and potential of the thiol-ene click reactions associated with fatty acids and their derivatives, as well as to highlight examples demonstrating the broad utility of this particular combination in monomer and polymer synthesis. 

Renewable diisocyanates can be synthesised for the preparation of PU using the thiol-ene click reaction. This approach was first described by the research team of Cramail [27]. They reacted the prepared diisocyanates with commercially available and fatty acid-based diols to obtain partially and fully bio-based PU. These materials were obtained with values of weight average molecular weight of 6-38 kDa and displayed fair thermal stability with no significant weight loss below 235 °C. Scheme 6.6 shows the approach adopted in this study for the synthesis of the novel diisocyanate. 

Scheme 6.6 Synthesis of a renewable diisocyanate via the thiol-ene click reaction.

Application of the thiol-ene click reaction as a direct polymerisation tool consists of the synthesis of monomers in the form of a,(0-dienes that are photo-polymerised with dithiol componnds (Scheme 6.10a). That is, the coupling, via thiol-ene reactions, of an AA (with ene-ene end groups) and a BB (with thiol-thiol end groups) structure or, alternatively, in the synthesis of AB monomers (with ene and thiol functions as end-groups. Scheme 6.10b). Studies reporting on the latter approach are scarce. 

PA can also be obtained via the thiol-ene click reaction from a,ro-diene monomers bearing amide groups in the main backbone or as side moieties via the Ugi four-component reaction [36, 37]. In one such study [38], the authors used 10-undecenoic acid as carboxylic acid, 10-undecen-l-al as an aldehyde, and different primary amines and isonitriles to synthesise a,C0-diene monomers, and then applied them to polymerisation reactions. The main focus of this specific work was the ADMET mechanism, though thiol-ene photopolymerisation was also implemented to prepare linear aliphatic PA with Mn values of 3-9 kDa and Tg values of 1-10 °C. 

The research team of Cramail also carried out a pol) hioether synthesis but used a different approach. They undertook the synthesis of AB-type monomers and then polymerised them via the thiol-ene click reaction [41, 42]. The work consisted of the synthesis of 10-undecene-l-thiol from 10-undecenoic acid. Photochemical or thermal initiations were tested for self-polymerisation of this AB monomer, and also by varying the reaction time. Polymers with Mn = 15-40 kDa were obtained and oxidised further, leading to materials with an increased T [21]. 

Yang, M. Mao, J. Nie, W. Dong, Z. Wang, D. Zhao, Z. Ji, X., Facile Synthesis and Responsive Behavior of PDMS-fc-PEG Dihlock Copolymer Brushes via Photoinitiated Thiol-ene Click Reaction. J. Polym. Sci., Part A Polym. Chem. 2012,50,2075-2083. 

TFMO-1 with a 2-D hexagonal mesoporous structure has been designed via consecutive surface functionalization of SBA-15 with 3-mercaptopropyl triethoxysilane followed by a thiol-ene click reaction with 2,4,6-triallyloxy-l,3,5-triazine in the presence of azobisisobutylronitrile (AIBN) initiator. 

The BET surface areas for the pure SBA-15 and TFMO-1 were 610 and 405 mVg, respectively. A considerable decrease in the BET surface area upon covalent grafting of the organic triazine groups through the thiol-ene click reaction at the surface of the thiol-functionalized SBA-15 material suggests that the organic groups have been anchored at the surface of the mesopores. 

Lowe, A.B., 2010. Thiol-ene click reactions and recent applications in polymer and materials synthesis. Polym. Chem. 1, 17—36. 

Scheme 11.15 Schematic illustration of the two synthesis routes for boronic acid modified Fe304 MNPs via thiol-ene click reaction. (Reproduced from ref. 74 with permission. Copyright 2014, Royal Society of Chemistry.)...

Chan, J.W., Yu, B., Hoyle, C.E., and Lowe, A.B. (2009b) The nucleophilic, phosphine-catalyzed thiol-ene click reaction and convergent star synthesis with RAFT-prepared homopolymers. Polymer, 50,3158. 

Figure 8.14 Divergent synthesis of star-shaped thioether oligomers by using successive thiol-ene click reaction and esterification reaction. (Reprinted from Tetrahedron Letters, 50, 28, C. Rim, D.Y. Son, Facile and efficient synthesis of star-shaped oligomers from a triazine core, 4161-4163, 2009, with permission from Elsevier.)...

RAFT Alkene functional HEMA scaffolds and ethylene glycol methacrylate Glucothiose Thiol-ene Click reaction Con-A Micelles [102]... 

HEMA followed by reaction with 4-pentenoic anhydride resulting in polymers bearing alkene side chains (Scheme 13). The thiol-ene Click reaction was reported to be complete in less than 2 h. The produced glycomicelles showed bioactivity with Con-A [102]. 

Scheme 37 Synthesis of cyclic PSTY, P BA, PDMA, and PNIPAM through the combination of RAFT polymerization and thiol-ene click reactions...

Tucker-Schwartz, A.K., Farrell, R.A., and Garrell, R.L. (2011) Thiol-ene click reaction as a general route to functional... 

Ordered mesoporous silica containing triazine moieties (Figure 3), based on the SBA-15 structure, has also been reported as an efficient and a reusable catalyst for the synthesis of chromenes 20 xmder solvent-free reaction conditions (Scheme 12) [75]. The catalyst was prepared from 3-mercaptopropyl-grafted SBA-15 by thiol-ene click reaction with 2,4,6-triallyloxy-l,3,5-triazine. The characterization data of the solid demonstrated the total disappearance of C=C bonds in triazine moiety. This feature strongly suggested the formation of bridges between the triazine and the mesopore walls. 

Lluch, Ronda JC, Galia M, LUgadas G, Cddiz V. 2010. Rapid Approach to Biobased Telechehcs through Two One-Pot Thiol-Ene Click Reactions. Biomacromolecules 11(6) 1646-1653. 

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