Electron-rich alkynes undergo an efficient, atom-economic cycloaddition–retroelectrocyclization (CA-RE) click reaction with electron-deficient olefins, such as tetracyanoethene (TCNE), to produce donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes. This transformation is used to combine organometallic acetylides and electron-deficient olefins, as well as in purely organic transformations.

Tsuyoshi Michinobu, Tokyo Institute of Technology, Japan, and François Diederich, Swiss Federal Institute of Technology (ETH) Zurich, Switzerland, have reviewed the broad range of alkyne and alkene substitution modes that are tolerated by the CA–RE click reaction. Many of the non-planar molecular push-pull products have properties suited to device fabrication, including for waveguide and photovoltaics applications.

The CA–RE click reaction has been applied to polymer chemistry in the side-chain postfunctionalization of polymers and the formation of polymer main chains. The method gives easy access to a diverse range of soft functional materials and chromophoric polymers with a wide array of structural, optoelectronic, and redox properties. Therefore, the researchers expect a further increase in the preparative use of the CA-RE reaction and the investigation of the resulting molecular and polymeric products by many research groups. Also the scope of the target materials is expected to widen.

• The [2+2] Cycloaddition-Retroelectrocyclization (CA-RE) Click Reaction: Facile Access to Molecular and Polymer Push-Pull Chromophores,
  Tsuyoshi Michinobu, François Diederich,
  Angew. Chem. Int. Ed. 2018.
  https://doi.org/10.1002/anie.201711605