Dynamic Cross‐Exchange in Halophosphonium Species: Direct Observation of Stereochemical Inversion in the Course of an S_N2 Process

Abstract

The complex fluxional interconversions between otherwise very similar phosphonium bromides and chlorides R 3 PX + X À (R = Alk, Ar,X= Cl or Br) were studied by NMR techniques.T heir energy barriers are typically ca. 11 kcal mol À1 ,b ut rise rapidly as bulky groups are attached to phosphorus,r evealing the importance of steric factors.I n contrast, electronic effects,a sm easured by Hammett analysis, are modest (1 1.46) but still clearly indicate negative charge flowt owards phosphorus in the transition state.M ost significantly,d etailed analysis of the exchange pathways unequivocally,a nd for the first time in any such process,s hows that nucleophilic attack of the nucleophilic anion on the tetrahedral centre results in inversion of configuration.