Synthesis of Phosphinines from Co<sup>II</sup>-Catalyzed [2+2+2] Cycloaddition Reactions
Tim Gläsel, Haijun Jiao, Marko Hapke
Abstract
The Co^II-catalyzed [2+2+2] cycloaddition of functionalized diynes and phosphaalkynes resulting in the formation of an array of structurally interesting phosphinines is reported. A combination of 1,2-bis(diphenylphosphino)benzene (dppbenz) and CoI_2 turned out to be the most suitable and active catalyst system under microwave reaction conditions. Chemoselective cyclizations of phosphaalkynes in the presence of nitriles have been observed, and a number of carbonyl functionalities are tolerated as well. The reaction can successfully be conducted with a 1:1 ratio of phosphaalkyne/diyne substrate. Model reactions with benzonitrile and diyne demonstrated that Co^II catalysts in the presence of bisphosphines prefer carbocyclization, while reduction to lower oxidation states promotes heterocyclization to the corresponding pyridine. A reaction mechanism comprising a Co^II oxidation state for the cyclotrimerization process is proposed and rationalized based on control experiments and theoretical calculations.