MODELS FOR NUCLEASE CATALYSIS - MECHANISMS FOR GENERAL ACID CATALYSIS OF THE RAPID INTRAMOLECULAR DISPLACEMENT OF METHOXIDE FROM A PHOSPHATE DIESTER

The dianion of the dialkyl phosphate diester 3 is hydrolysed in water at 50-degrees-C with a half-life of less than 2 min. The reaction involves highly efficient intramolecular nucleophilic catalysis (effective molarity greater-than-or-equal-to 10(10) mol dm-3) by the phenol OH over the whole pH range. The initial products are the cyclic phosphate diester and methanol: no phosphate migration is observed. General acid catalysis is observed for the reactions of all ionic species, and has been characterised in detail for the mono- and di-anion. An important electrostatic effect, observed for general acids with a suitably positioned second NH+ group, stabilises the transition state for the loss of methoxide from the dianion by up to 12.3 kJ mol-1, even in water. The data allow an estimate of 10(4)-10(5) for the factor by which protonation to give the triester activates a phosphate diester towards attack by a neighbouring OH group. Mechanisms involving pentacovalent addition intermediates are proposed for the reactions of the neutral ester acid and the monoanion. The phosphorane dianion is considered to be an intermediate in the cyclisation of the substrate dianion, but with a lifetime too short for diffusional equilibration.