NUCLEOPHILIC AND PROTOLYTIC CATALYSIS OF PHOSPHONATE HYDROLYSIS - ISOTOPE EFFECTS AND ACTIVATION PARAMETERS

Activation parameters for the imidazole-catalyzed hydrolysis of bis(4- nitrophenyl) methylphosphonate (NMN) are DELTAH(double dagger) = 44 +/ - 2 (H2O) and 46 +/- 2 (D2O) kJ/mol and DELTAS(double dagger) = -146 /- 5 (H2O) and -145 +/- 5 (D2O) J/mol K. The activation parameters for water-catalyzed hydrolysis of NMN in H2O are DELTAH(double dagger) = 28 +/- 10 kJ/mol and DELTAS(double dagger) = -270 +/- 30 J/mol K. The solvent isotope effects for the hydrolysis of NMN at 25-degrees-C are 2.69 +/- 0.09 for imidazole catalysis and 2.91 +/- 1.24 for water cata lysis. The solvent isotope effect for the imidazole-catalyzed hydrolys is of 3,3-dimethylbutyl)methylphosphonofluoridate(soman) is similar, 2 .794-0.03 at 25-degrees-C. Nucleophilic reactions of hydroxide ion and phosphate dianion with NMN at 25-degrees-C are associated with solven t isotope effects of 0.51 +/- 0.02 and 1.54 +/-0.26, respectively, and DELTAH(double dagger) = 69 +/- 4 kJ/mol; DELTAS(double dagger) = 27 /- 12 J/mol K for hydroxide ion. For the reactions of 4-nitrophenyl 2- propyl methylphosphonate (IMN) with imidazole, water, phosphate dianio n, and hydroxide ion, the activation parameters in H2O are DELTAH(doub le dagger) = 56 +/- 2 kJ/mol, DELTAS(double dagger) = -150 +/- 4 J/mol K (imidazole); DELTAH(double dagger) = 34 +/- 2 kJ/mol, DELTAS(double dagger) = -143 +/- 5 J/mol K (hydroxide ion); DELTAH(double dagger) = 60 +/- 4 kJ/mol, DELTAS(double dagger) = -170 +/-13 J/mol K (water); and DELTAH(double dagger) = 72 +/- 3 kJ/mol, DELTAS(double dagger) = - 93 +/- 9 J/mol K (phosphate dianion); the solvent isotope effects are 1.40 +/- 0.07 (imidazole, 73-degrees-C); 1.2 +/- 0.2 (water, 25-degree s-C); 1.11 - 1.15 (phosphate dianion, 55-70-degrees-C); and 0.94 +/- 0 .02 (hydroxide ion, 25-degrees-C). Secondary beta-deuterium isotope ef fects for the reactions of hydroxide ion are 0.91 +/- 0.04 for NMN and 0.936 +/- 0.005 for IMN. All reactions occur with rate-determining ge neral base-catalyzed P-O bond formation. The discrepancy observed in t he character of the imidazole-catalyzed hydrolysis of NMN or soman and IMN reflects different electronic effects.