text: For a reaction to be considered a click reaction, it must satisfy certain characteristics:
  * modularity
  * insensitivity to solvent parameters
  * high chemical yields
  * insensitivity towards oxygen and water
  * regiospecificity and  stereospecificity
  * a large thermodynamic driving force (>20 kcal/mol) to favor a reaction with a single reaction product. A distinct exothermic reaction makes a reactant "spring-loaded".
  The process would preferably:
  * have simple reaction conditions
  * use readily available starting materials and reagents
  * use no solvent or use a solvent that is benign or easily removed (preferably water)
  * provide simple product isolation by non-chromatographic methods (crystallisation or distillation)
  * have high atom economy.
  Many of the click chemistry criteria are subjective, and even if measurable and objective criteria could be agreed upon, it is unlikely that any reaction will be perfect for every situation and application. However, several reactions have been identified that fit the concept better than others:
  * [3+2] cycloadditions, such as the Huisgen 1,3-dipolar cycloaddition, in particular the Cu(I)-catalyzed stepwise variant, are often referred to simply as Click reactions 
  * Thiol-ene reaction
  * Diels-Alder reaction and inverse electron demand Diels-Alder reaction
  * [4+1] cycloadditions between isonitriles (isocyanides) and tetrazines
  * nucleophilic substitution especially to small strained rings like epoxy and aziridines
  * carbonyl-chemistry-like formation of ureas  but not reactions of the aldol type due to low thermodynamic driving force.
  * addition reactions to carbon-carbon double bonds like dihydroxylation or the alkynes in the thiol-yne reaction.
  * Sulfur (VI) Fluoride exchange