The Metal-Initiated Cyclooligomerization of Phospha-Alkynes and its Consequences [1]

1,3-Diphosphacyclobutadienes 4 are almost certainly intermediates in the conversion by hexachloroethane of phosphaalkyne dimer complexes 3 to the tetraphosphacubanes 5. We now describe trapping reactions of 4, generated in the same way, with phosphaalkyne 1 (→9), ynamines 10 (→13), and electron-poor alkynes 11 (→14). The cyclooligomerization of 1 initiated by t-Bu-N=VCl3DME (15) leads to the azatetraphosphaquadricyclanes 20 while the reaction with the stronger Lewis acid t-Bu-N=VCl3 (21) furnishes the 1,3,5-triphosphabenzenes 23 in high selectivity.