A Versatile, Functional Group‐Tolerant, and Bench‐Stable Iron Precatalyst for Building Arene and Triazine Rings by [2+2+2] Cycloadditions

Abstract

AbstractWe report an efficient iron‐catalyzed cycloaddition procedure leading to the construction of (hetero)aromatic rings by alkyne [2+2+2] cycloisomerization. This method relies on the use of an air‐stable (N,N)Fe(II) precursor easily prepared from a commercially available ligand derived from 1,10‐phenanthroline, reduced in situ into a catalytically active non‐innocent (N,N ⋅−)2Fe(II) species. This system displays a large scope application, operates under mild conditions and at low catalytic charges (25 cycloadducts formed, up to 1.5 mol% catalyst). Moreover, this method also enables access to 29 cycloadducts by cross‐cycloisomerization between 1,6‐ or 1,7‐diynes and alkynes in near‐equimolar conditions. 1,3,5‐Triazines can also be prepared with this procedure starting from the corresponding cyanamides. Scale‐up reactions and post‐functionalization of several cycloadducts also show that this [2+2+2] cycloaddition can be used in multistep sequences.