Specific and Reversible Alkynyl Transfer Reactions of an N‐Heterocyclic Phosphane

Abstract

Abstract Reaction of P ‐chloro‐1,3,2‐diazaphospholenes with alkynyllithium or Grignard reagents produced N‐heterocyclic alkynylphosphanes 3a – c . Terminal alkynylphosphane 3c reacted in the presence of a catalytic amount of CuCl with formation of bis‐phosphanylalkyne 4 , ethyne and chlorodiazaphospholene 2b , which was converted into the appropriate azide 6 upon addition of LiN 3 . Detailed studies allowed us to explain the formal alkynyl/Cl metathesis and alkynyl scrambling as arising from a combination of Cu‐catalysed cross‐coupling steps and the reverse processes, which involved Cu‐induced P–C bond breaking. The superposition of individual steps allows the formulation of an equilibrium reaction: 2 3c [lrarr2] 4 + HCCH. Shift of the equilibrium by enforced release of acetylene into the gas phase enabled quantitative conversion of 3c into 4 . Traces of P–C bond‐activation products were also detected upon action of CuCl on 3b with an internal triple bond, but alkynylphosphanes with Ph 2 P or ( i Pr 2 N) 2 P moieties were unreactive. Alkynylphosphanes 3a – c and 4 were characterised by spectroscopic data and single‐crystal X‐ray diffraction studies. The presence of notably elongated P–C bonds points to a bond‐weakening effect, which is responsible for explaining the unusual P–C bond reactivity.