The reactivity of triesters is discussed in the general context of phosphate transfer, as usually studied for the reactions of mono- and diesters. Systematic work has typically concentrated on the Linear Free Energy Relationships measuring the dependence of reactivity on the nucleophile and the leaving group, but new results indicate that it can depend equally strongly on the two non-leaving (sometimes known as spectator) groups. This conclusion is supported by first results from theoretical calculations: which also predict that a two-step mechanism can be favored over a concerted S(N)2(P) mechanism even for reactions involving leaving groups as good as p-nitrophenolate. This article is part of a Special Issue entitled: Chemistry and mechanism of phosphatases, diesterases and triesterases.

The spontaneous hydrolysis of a series of five triaryl and two dialkyl aryl phosphate triesters, previously studied experimentally, is examined theoretically using two different hybrid density functional methods, B3LYP and M06; two basic sets, 6-31+G(d) and 6-311++G(d,p); and the Gaussian 09 program. The B3LYP/6-31+G(d) methodology combined excellent accuracy with minor computational cost. The calculations show excellent quantitative agreement with experiment, which is best in the presence of three discrete water molecules. The results support a two-step mechanism involving a pentacovalent addition intermediate, with a lifetime of tenths of a millisecond. The rate-determining formation of this intermediate involves general base catalysis, defined by concerted proton transfers in a six-membered cyclic activated complex (TS1), which involves two hydrogen-bonded water molecules supporting a well-developed H(2)O···P bond (mean % evolution 77.83 ± 0.97). The third water molecule is hydrogen-bonded to P═O and subsequently involved in product formation via TS2. The effects on reactivity of all the groups attached to phosphorus in TS1 are examined in detail: the two non-leaving groups in particular are found to play an important role, accounting for the substantial difference in reactivity between triaryl and dialkyl aryl phosphate triesters.

Al 3+ -Mg 2+ mixed oxides were prepared by coprecipitation and characterized with scanning electron microscopy (SEM), energy dispersive Xray fluorescence (EDXRF), temperature programmed desorption of CO 2 (CO 2 -TPD), and N 2 adsorption/desorption isotherms (BET and BJH methods). By increasing the MgO concentration up to 31.8% (w/w), X-ray diffraction (XRD) measurements suggested an incipient magnesium aluminate spinel (MgAl 2 O 4 ) phase. However, the spinel crystalline structure was obtained only after calcination at 950 °C. These materials were tested as catalysts in the propanolysis reaction of methyl paraoxon. This reaction in the presence of the more efficient incipient MgAl 2 O 4 spinel is of the order of 2.5 × 10 5 -fold faster than the spontaneous propanolysis reaction and results in the formation of a product that is structurally related to a family of flame retardants. The different products of propanolysis and hydrolysis were identified by electrospray ionization mass spectrometry (ESI(+)-MS), ESI(+)-MS/MS) and liquid chromatography mass spectrometry (LC-MS/MS).

Notwithstanding its half-life of 70 years at 25 °C, the spontaneous hydrolysis of the anion of di-2-pyridyl phosphate (DPP) is thousands of times faster (ca. 3000 at 100 °C, over 10000-fold at 25 °C) than expected for a diester with leaving groups of pK(a) 9.09. The kinetic parameters do not permit a conclusive choice between five possible mechanisms considered, but the combination of kinetics and calculational evidence supports a single-step, concerted, S(N)2(P) mechanism involving the attack of solvent water on phosphorus assisted by intramolecular catalysis by a (weakly basic) pyridine nitrogen acting as a general base. Catalysis is relatively efficient for this mechanism, with an estimated effective molarity (EM) of the general base of >15 M, consistent with the absence of catalysis by typical buffers. Further new results confirm that varying the nonleaving group has minimal effect on the rate of spontaneous diester hydrolysis, in striking contrast to the major effect on the corresponding reaction of triesters: though protonation of one nitrogen of DPP(-) increases the rate of hydrolysis by 6 orders of magnitude, in line with expectation.