Nickel-Catalyzed [2+2+2] Cycloaddition of Diynes and Cyanamides

The discovery of [Ni(NHC)RCN]2 species and their role as cycloaddition catalysts for the formation of pyridines

The reaction of Ni(COD)(2), IPr, and nitrile affords dimeric [Ni(IPr)RCN](2) in high yields. X-ray analysis revealed these species display simultaneous η(1)- and η(2)-nitrile binding modes. These dimers are catalytically competent in the formation of pyridines from the cycloaddition of diynes and nitriles. Kinetic analysis showed the reaction to be first order in [Ni(IPr)RCN](2), zeroth order in added IPr, zeroth order in nitrile, and zeroth order in diyne. Extensive stoichiometric competition studies were performed, and selective incorporation of the exogenous, not dimer bound, nitrile was observed. Post cycloaddition, the dimeric state was found to be largely preserved. Nitrile and ligand exchange experiments were performed and found to be inoperative in the catalytic cycle. These observations suggest a mechanism whereby the catalyst is activated by partial dimer-opening followed by binding of exogenous nitrile and subsequent oxidative heterocoupling.

Iron-catalyzed formation of 2-aminopyridines from diynes and cyanamides

Diynes and cyanamides undergo an iron-catalyzed [2 + 2 + 2] cycloaddition to form highly substituted 2-aminopyridines in an atom-efficient manner that is both high yielding and regioselective. This system was also used to cyclize two terminal alkynes and a cyanamide to afford a 2,4,6-trisubstituted pyridine product regioselectively.

Ruthenium-catalyzed [2+2+2] cycloaddition of diynes with nitriles in pure water

One ring to bind them: An efficient method for synthesizing highly functionalized pyridine derivatives from diynes and nitriles is described. The reaction system involves Cp*Ru(COD)Cl/tppts-catalyzed [2+2+2] cycloaddition in pure water. Without being accompanied by diyne dimerization or trimerization byproducts, the desired products can be obtained in moderate to high yields.