Thiol-Ene Radical Click Reaction

The thiol-ene radical reactions can sometime lead to the formation of unwanted by-products through radical recombination reactions. The addition of a thiyl radical to olefins is reversible, until the free radical product formed abstracts a hydrogen radical from another thiol giving a thioether product and propagating the radical reaction [41]. 

The formation of the carbon sulfur bond follows an anti-Markovnikov regios-electivity, which ensures the formation of the most stable carbon radical [42]. There are several reports establishing a general trend for the reaction of thiols with alkenes. Comprehensive reports in this regard were published by Hoyle et al. [43], where they compared the reaction of three families of thiols, namely alkyl-3-mercaptopropionates, alkyl thioglycolates, and alkyl thiols, with various alkenes. The reactivity order provided by them is as follows norbornene vinyl ethers propenyl alkenes allyltriazines allyl isocyanurates acrylates N-substituted maleimides acrylonitrile methacrylates styrene conjugated dienes. 

Among the various families of thiol that have been studied, propionates and glycolates are more reactive than alkyl thiols. This difference in reactivity is proposed to be resulting from the weakening of S-H bond through H-bonding with the carbonyl of the ester function and from polar effects [44]. 

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]. 

It has been found that the reactions of thiols with internal alkynes are slower than those with terminal alkynes. These reactions are generally very sensitive to steric crowding. Sulfanyl and related radicals are electron-deficient in nature and 

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