The [2+2] Cycloaddition‐Retroelectrocyclization (CA‐RE) Click Reaction: Facile Access to Molecular and Polymeric Push‐Pull Chromophores

Abstract

The [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction between electron-rich alkynes and electron-deficient alkenes is an efficient procedure to create nonplanar donor-acceptor (D-A) chromophores in both molecular and polymeric platforms. They feature attractive properties including intramolecular charge-transfer (ICT) bands, nonlinear optical properties, and redox activities for use in next-generation electronic and optoelectronic devices. This Review summarizes the development of the CA-RE reaction, starting from the initial reports with organometallic compounds to the extension to purely organic systems. The structural requirements for rapid, high-yielding transformations with true click chemistry character are illustrated by examples that include the broad alkyne and alkene substitution modes. The CA-RE click reaction has been successfully applied to polymer synthesis, with the resulting polymeric push-pull chromophores finding many interesting applications.

Helicenes Grafted with 1,1,4,4‐Tetracyanobutadiene Moieties: π‐Helical Push–Pull Systems with Strong Electronic Circular Dichroism and Two‐Photon Absorption

“…1,1,4,4‐Tetracyanobutadienes (TCBDs) are known to be strong electron‐accepting groups that may give rise to donor–acceptor (push–pull) molecular structures when conjugated to donor fragments, and provide remarkably strong one‐photon absorption in the visible and red regions, along with nonlinear optical responses. , TCBDs possess electron‐accepting properties that are able to significantly tune the absorption features of molecules and, more generally, have a strong impact on the physical characteristics of the moieties they are linked to . Their synthesis implies a sequence of [2+2] cycloaddition–retroelectrocyclization (CA‐RE) between tetracyanoethylene (TCNE) and electron‐rich alkynes activated by donating groups (see Figure S1.0) .…”

Strain‐Promoted Reactivity of Alkyne‐Containing Cycloparaphenylenes

“…[17] This strategy enables rapid access to nonplanar donor-acceptor chromophores based on molecular and polymeric scaffolds,w hich feature arange of attractive properties,such as intramolecular charge-transfer bands,n onlinear optical properties,a nd pronounced redox activity. [18] Despite the rapidly increasing popularity of angle-strained alkynes as tools for synthetic chemistry,t he number of suitable scaffolds available is surprisingly limited. Herein, we merge the concept of alkyne-based strain-promoted reactions with [n]cycloparaphenylenes ([n]CPPs)-inherently warped conjugated macrocycles that have garnered widespread attention because of their relationship to carbon nanotubes and their unique sizedependent properties.…”

Reactivity of Polyynes: Complex Molecules from Simple Carbon Rods

“…[91,92] The first step of such reaction is [2+2] cycloaddition between alkyne and alkene and in the second step corresponding cyclobutene undergoes ring opening retroelectrocyclic reaction yielding non-cyclic product. [91,92] The first step of such reaction is [2+2] cycloaddition between alkyne and alkene and in the second step corresponding cyclobutene undergoes ring opening retroelectrocyclic reaction yielding non-cyclic product.…”