Promotion of phosphaalkyne cyclooligomerisation by a Sb(V) to Sb(III) redox process

A high yield of the tetraphosphaladderene, anti-tetraphosphatricyclo[4.2.0.0(2,5)] octa-3,7-diene, is obtained from reaction of the zirconocene 1,3-diphosphabicyclo[1.1.0] butane with Ph2SbCl3 in THF or CH2Cl2. Exploration of the reaction pathway using density functional theory suggests that an envelope-type adduct of Ph2SbCl and 1,3-diphosphabicyclo[1.1.0] butane plays a pivotal role in the reaction. The zwitterionic character of this intermediate species allows it to act simultaneously as both an ene and an eneophile, and a symmetry-allowed bimolecular reaction leads to the tetraphosphaladderene species via a spirocyclic intermediate.