Synthesis of Phosphinines from CoII-Catalyzed [2+2+2] Cycloaddition Reactions

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 CoI2 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.