Iron-catalyzed formation of six-membered rings from alkyne

Iron compounds, exhibiting high catalytic behavior and excellent compatibility, have been considered as low-toxic, cheap, efficient and green catalysts for many important organic transformations. Organic six-membered rings play a crucial role in the fields of natural products, agricultural chemicals, pharmaceuticals and organic materials due to their significant properties. The construction of six-membered rings from alkynes through iron-catalyzed annulation attracted increasing attention to the remarkable advantages of iron catalysts. Benzenes, pyridines and pyrans could be formed facilely through iron-catalyzed [2+2+2] annulation of alkynes. Pyrimido cycles were afforded successfully via iron-catalyzed [3+2+1] annulation of alkynes, while iron-catalyzed [4+2] cyclization of alkynes could be applied to access quinolones, chromenones, isoquinolone, etc. Iron-catalyzed intramolecular cyclization of alkynes formed isoquinolinium, isocoumarin, phenanthrene, 1,2-dihydroquinolines, and spiro[5.5]trienones. In this review, the generation of six-membered rings from alkynes by iron-catalyzed cyclization was highlighted. Firstly, the property, superiority and application of iron catalysts were briefly introduced. After presenting the usefulness of six-membered rings and the unique reactivities of alkynes, iron-catalyzed synthesis of six-membered rings from alkynes were classified and demonstrated in detail. Based on different kinds of reactions, iron-catalyzed [2+2+2], [3+2+1], [4+2] and intramolecular cyclizations of alkynes were summarized and the plausible reaction mechanism would be also showed if available. The recent progress on iron-catalyzed formation of six-membered rings from alkyne was highlighted. And six-membered rings like benzenes, pyridines, pyrans, quinolones, spiro[5.5]trienones, etc. were facilely obtained through iron-catalyzed annulation of alkynes.