Transfer of the Diethoxyphosphoryl Group between Imidazole and Aryloxy Anion Nucleophiles

Article abstract of DOI:10.1021/jo00245a015

Rate constans have been measured for reaction of imidazole with aryl diethyl phosphate (k₁) and of aryloxy anions with N-(diethoxyphosphoryl)imidazolium ion (k_-1) in aqueous solution at 25 deg C; they obey the following linear Broensted equations: log k₁ = -1.02pK_ArOH + 1.83 (n = 6, r = 0.989); log k_-1 = 0.85pK_ArOH - 0.48 (n = 10, r = 0.957).The value of β_eq (1.87) obtained from β_lg and β_nuc supports a previously determined value (1.83) for the transfer of the neutral phosphoryl group <(HO)2PO> from phenolate ion nucleophiles.The pK_aof (diethoxyphosphoryl)imidazolium ion is 6.04.The equilibrium constant for reaction of 4-nitrophenyl diethyl phosphate with imidazole is 5.9 x 10^-6; in the case of the aryl ester from phenol with pK_ArOH = 4.34 the equilibrium constant is calculated to be unity.The Broensted β_eq data are used to calibrate effective charges derived from previously measured β_lg values for attack of nucleophiles at phosphorus bearing phenolate ion leaving groups.