The electrophilicity and chemistry of phosphate esters

Cocks, Sarah

The electrophilicity and chemistry of phosphate esters

Doctoral Thesis

1986

Publisher

University of Cape Town

Department

Department of Chemistry

Faculty

Faculty of Science

Abstract

A series of phosphate esters containing the E.-nitrophenyl moiety as a leaving group were synthesised and their base catalysed hydrolysis and electron impact-induced fragmentations studied. Heterocyclic ester substituents were present in some of the substrates and in-this sense they were considered as models for a nucleotide molecule containing both the 11 energy-rich11 phosphate bond and the nitrogen heterocyclic ring. The relative reactivity of the phosphorus-E,-nitrophenoxide bond was taken as a probe for nitrogen (pyridyl or quinolyl) participation in the substitution reaction. Although the heterocyclic derivatives show that the introduction of the nitrogen centre increases the rate of base catalysed hydrolysis, the reactivity enhancement is. not significant and results probably from polar effects rather than from intra-molecular catalysis. A comparison of the mass spectra of the heterocyclic and carbocyclic analogues, reveal differences which are significant (in the pyridyl substrates) and dramatic (in the quinolyl substrate) with regard to the fragmentation involving the E.-nitrophenoxy radical expulsion and formation of the corresponding phosphorylium ion. Synthetic approaches towards dimethylaryl and dimethyl(arylalkyl) phosphates are discussed. Nucleophilic displacement at the methyl ester carbon, and in particular the oxygen+ nitrogen methyl group transfer in these substrates occurs readily, complicating the synthetic procedures and lowering the stability of products. Dimethyl-(2-pyridylmethyl) phosphate isomerises in water to the zwitterionic N-methylpyridinium derivative via bimolecular methylation. The kinetics of the reaction were studied and no evidence for intra-molecular methyl transfer was obtained. - ii - The isomerisation of dimethyl-(quinolin-8-yl) phosphate in water also provides evidence for the bimolecular methylation. The crystal and molecular structure of dimethyl-(quinolin-8-yl) phosphate was determined and has revealed that the orientation of the CH 3 groups in the crystal provide no indication for the occurrence of methyl transfer in the solid state. The dynamics of the prepared phosphate derivatives studied in solutions were also investigated by means of mass spectrometry. Fragmentation patterns and their relative contributions are presented and discussed. The effect of solvent on the rates of the reaction between trimethyl phosphate and i) pyridine and ii) 4-(dimethylamino)pyridine, was studied at 25 - 65°C using NMR techniques. The solvents used were DzO, CD30D, CD3CN and DzO/CD30D and DzO/CD3CN mixtures. Water (DzO) is the most effective medium for the methyl transfer reaction. We attribute this to the strong hydrogen bonding between water and the departing dimethyl phosphate anion in the transition state. Activation parameters obtained for the reactions in pure solvents confirmed our interpretation of the hydrogen bonding effect operating in the transition state. Fragmentation of S-arylethyl phosphates has been investigated. A novel feature is exhibited by S-arylethyl phosphorochloridates. They easily decompose thermally yielding 1-chloro-2-arylethanes and/or arylethylenes presumably via concerted C-O and P-Cl bond fission involving a phenonium ion intermediate and expulsion of a metaphosphate species.

Description

Bibliography: pages 380-395.

Keywords

Organic Chemistry

Reference:

Collections

PhD / Doctoral

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