η 2 ‐Coordination of a Phosphaalkyne to an Amino Olefin Nickel Complex and Regioselective Catalyzed Cyclooligomerization to Dewar 1,3,5‐Triphosphabenzene

We report herein an efficient synthesis of a Dewar‐triphosphabenzene derived from [2+2+2] cycloaddition of three phosphaalkyne molecules promoted by an amino olefin Ni 0 complex as the catalyst. Amino diolefin Ni I complex 1 serves as a precursor complex that reacts with triphenylmethylphosphaethyne under reducing conditions at low temperature to give Ni 0 complex 3 with a coordinated phosphaalkyne in a η 2 ‐mode. The structure of 3 was determined by X‐ray diffraction methods and suggests a bonding situation for the phosphaalkyne that is between a side‐on coordinated C≡P molecule and a metallaphosphacyclopropene structure. The reaction of 3 with a 10‐fold excess of phosphaethylene under reducing conditions at room temperature resulted in the formation of 2,4,6‐tris(triphenylmethyl)‐Dewar‐1,3,5‐triphosphabenzene ( 4 ) in good yield.