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

Abstract

AbstractReaction 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 LiN3. 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 Ph2P or (iPr2N)2P 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.