Alkali-metal ion catalysis and inhibition in nucleophilic displacement reactions at carbon, phosphorus and sulfur centres. IX. p-Nitrophenyl diphenyl phosphate

We report on the catalytic effects by alkali-metal ions in the ethanolysis of p-nitrophenyl diphenyl phosphate, in continuation of our studies on alkali-metal ion catalysis and inhibition in nucleophilic displacement reactions at carbon, phosphorus and sulfur centres. The following selectivity order of catalytic reactivity was observed for nucleophilic displacement at the phosphorus center with p-nitrophenoxide as leaving group: Li⁺ > Na⁺ > K⁺ > Cs⁺ A minor reaction pathway with phenoxide leaving was also found. The k_obs data have been dissected into reaction pathways by free ions (k_EtO) and by ion pairs (k_MOEt) with the latter being dominant, in a 4-membered transition-state. Further analysis is given in terms of initial-state and transition-state stabilization by the alkali-metal ions in terms of the Eisenman model (electrostatic interaction vs. desolvation). Results of ab-initio MO calculations are presented based on interaction between M⁺ and a model bipyramidal phosphorane intermediate and compared with the sulfurane analogue.