pH-Product and pH-rate profiles for the hydrolysis of methyl ethylene phosphate. Rate-limiting pseudorotation

Abstract

The hydrolysis of methyl ethylene phosphate in dilute acid solution has previously been shown to occur both with ring opening and with cleavage of the exocyclic methyl group, whereas the methyl ester of propylphostonic acid reacts under these conditions only with ring opening. These facts had been correlated with the theory that hydrolysis of the exocyclic methoxyl group takes place only after pseudorotation about the central phosphorus atom. The complete pH-product profile for the hydrolysis of methyl ethylene phosphate is shown in Figure 3 and explained in the text. The drop in the percentage of exocyclic cleavage in strongly acid solution may be correlated with rate-limiting pseudorotation; the rise in the percentage of exocyclic cleavage in strong alkali may be correlated with the ionization of both the hydroxyl groups in the pentacovalent intermediate so as to produce two Ogroups; according to theory, these should occupy equatorial positions, and so the ionization could force a pseudorotation that promotes exocyclic cleavage. The kinetic equations for the over-all process exhibit unusual features that are discussed. Methyl ethylene phosphate, 1, undergoes hydrolysis 101 23456 7***times as rapidly as does trimethyl phosphate. In dilute acid solution, the products are both those of ring opening and of exocyclic cleavage.1-4 to the region where/is proportional to (OH-).

Kinetics and mechanism of the hydrolysis of a (5,6)-spirophosphorane. Thermodynamics of the hydrolysis of cyclic five-membered and six-membered phosphonium ions

“…Thus, the former, as an intermediate in the hydrolysis of 1, exhibits a trend of giving only the bis-ester 9 in neutral solutions, with increasing amounts of the cyclic ester 11 as the acidity is increased (Table 1). This is exactly the same pattern as observed in the hydrolysis of methyl ethylene phosphate (30). In the latter case the explanation was advanced that the initially formed pentavalent intermediate can only undergo endocyclic cleavage, while a pseudorotation is required for exocyclic cleavage.…”

Kinetics of the Hydrolysis of Phosphate Esters

“…The esters of secondary and tertiary alcohols exhibit a higher reactivity during spontaneous hydrolysis than those of primary alcohols owing to the ease of the cleavage of the C-0 bonds via an S^l mechanism 86 , which is analogous to the behaviour of the corresponding phosphate mono-and di-esters. An unusually high reactivity has been observed for five-membered cyclic esters, which hydrolyse in neutral, acid, and alkaline media 10 6 -10 8 faster than their six-or seven -me mbered cyclic analogues and cyclic esters 86 * 97 .…”

A chemiosmotic molecular mechanism for proton‐translocating adenosine triphosphatases

“…At first it was thought that one or more covalent intermediates were involved in the ATPase reaction [ l-3,6-1 11, but this possibility became more and more unlikely as an intensive search failed to reveal any evidence for any intermediates [3,5, Nucleophilic substitution reactions at phosphate phosphorus centres are generally recognised as proceeding by way of pentavalent transitional intermediates, having trigonal bipyramidal geometry [24,25] . Classically, the attacking and leaving groups enter into and exit from the tetrahedral phosphorus centre via the long and weak apical bonds of the intermediary pentavalent trigonal bipyramid.…”