| Description |
One of the greatest challenges in synthetic chemistry is the development of reactions that can efficiently afford target compounds without creating byproducts. One such class of reactions are [2+2+2] cycloadditions. Here, three unsaturated coupling partners are combined to create six-membered rings, often with high regioselectivity, high yield, and no byproducts (i.e., secondary products). To further increase the attractiveness of these reactions, iron, a cheap and abundant catalyst, can be used. However, for the [2+2+2] synthesis of pyridines, a very important class of compounds, iron has traditionally been a very poor catalyst. By tethering unreactive nitriles to the more reactive alkynes, the first general iron-catalyzed [2+2+2] method to synthesize pyridines has been developed. This reactivity was further explored with the cycloaddition of diynes and cyanamides. Cyanamides demonstrated remarkable chemo- and regioselectivity. The three-component cycloaddition of terminal alkynes and a cyanamide could be performed with high yield and complete chemoselectivity. By combining aforementioned strategies of nitrile incorporation, the entirely novel cycloaddition of alkynenitriles and cyanamides has been developed. In this case, iron demonstrated reactivity that few [2+2+2] cycloaddition catalyst exhibited by incorporating multiple nitrogen atoms into the resulting 6-membered ring. Finally, isolation of presumed inorganic and organometallic intermediates provides a preliminary understanding of the mechanistic sequences involved in these iron-catalyzed [2+2+2] cycloadditions. |
