Nucleophilic Addition Click Reactions of Thiols

Thiols and thiolate anions are very good nucleophiles. Various click reactions have been developed based on the nucleophilic attack of thiols on to the electrophilic substrates such as epoxides, isocyanates, halides, and Michael acceptors. The reactions are generally initiated by bases, which are added in catalytic amounts or are produced in catalytic amounts by photolatent bases that act as photoinitiators for these reactions [48]. The rates of these reactions are dependent on the substrates 

Fringuelli and coworkers have explored the use of InCl3 (Lewis acid), TsOH (Bronsted acid), n-Bu3P (Lewis base), K2C03 (Bronsted base), and so on, as catalysts in solvent-less conditions for thiol-epoxide click reactions [51]. 

Isocyanates are very reactive compounds, which react readily with alcohols, amines, water, and thiols or thiolate ions. Reaction of isocyanates with thiols 

Polyurethanes are extremely versatile polymeric materials due to their elasticity, responsive nature toward impact, stretchability, and other possible physical manipulations. They have been utilized in various fields to make optical devices, adhesives, coatings, and also in many biomedical applications. Thiol-isocyanate click reactions have been used to generate polythiourethanes, a sulfur analog of polyurethane, in a very efficient way and in high yields [50]. 

Chan et al. studied bulk reaction of hexanethiol (5 mmol) with hexyl acrylate (5 mmol) in the presence of 0.43 M hexylamine (p/C = 10.56), -dipropylamine (p/C3 = ll), and NEtj (pfC = 10.75) under ambient condition for 500 s. After 500 s, reaction with hexylamine showed approximately 95% conversion, n-dipropylamine showed approximately 60% conversion, and NEtj showed less than 1% conversion. These amines have almost the same p/T, but there is a huge difference in their kinetic profiles. Apart from these amines, they also studied various weak nucleophiles, which have varying basicity, such as pyridine (p/C3 = 5.14), aniline (pA3 = 9.34), and l,8-bis(dimethylamino)naphthalene (a proton sponge with pA = 12.1), and found that they yield less than 1% conversion. Based on these observations, the catalysis was attributed to the kinetic 

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