Electrochemical cyclization of alkynes to construct five-membered nitrogen-heterocyclic rings

Organic five-membered rings have shown significant applications in the fields of organic synthesis, natural products, organic materials and pharmaceuticals for their unique characteristics. Electrochemical construction of five-membered rings from alkynes attracted increasing attention due to the notable advantages of electrochemical transformations and facile access of alkynes. Indole skeletons were constructed successfully through electrochemical intramolecular coupling of ethynyl-involved ureas, annulation of <i>o</i>-arylalkynylanilines, cyclization of 2-ethynylanilines, selenocyclization of diselenides with 2-ethynylanilines as well as C-H indolization of 2-alkynylanilines with 3-functionalized indoles. Isoindolones were synthesized successfully by electrochemical annulation of benzamides with terminal alkynes, 5-<i>exo-dig</i> aza-cyclization of 2-alkynylbenzamides as well as reductive cascade annulation of <i>o</i>-alkynylbenzamides. Pyrroles and imidazoles were formed efficiently via electrochemical annulation of alkynes with enamides and tandem Michael addition/azidation/cyclization of alkynes, amines and azides, respectively. Imidazopyridines could be obtained by electrochemical [3 + 2] cyclization of heteroarylamines. The electrochemical oxidative [3 + 2] cycloaddition of secondary propargyl alcohols was a simple and efficient access towards 1,2,3-triazoles. In this review, electrochemical cyclizations of alkynes to construct five-membered rings are highlighted. Firstly, the property and application of five-membered rings are simply introduced. After presenting the usefulness of alkynes and the general progress of electrochemical transformations, electrochemical cyclization reactions of alkynes towards five-membered rings are classified and presented in detail. Based on different types of five-membered rings, electrochemical construction of indoles, isoindolinones, indolizines, oxazoles, imidazoles, pyrroles, imidazoles and 1,2,3-triazoles are summarized and the possible reaction mechanisms are disclosed if available.