Dinuclear Zn(II) Complex Promotes Cleavage and Isomerization of 2-Hydroxypropyl Alkyl Phosphates by a Common Cyclic Phosphate Intermediate
| Autor: | Nicholas H. Williams, Wing Yin Tsang, Stephanie A. Melnychuk, Chaomin Liu, David R. Edwards, Alexei A. Neverov, R. Stan Brown, C. Tony Liu |
|---|---|
| Rok vydání: | 2009 |
| Předmět: |
chemistry.chemical_classification
Magnetic Resonance Spectroscopy Chemical Phenomena Chemistry Kinetics Inorganic chemistry General Chemistry Nuclear magnetic resonance spectroscopy Phosphate Biochemistry Medicinal chemistry Organophosphates Catalysis Zinc chemistry.chemical_compound Colloid and Surface Chemistry Isomerism Proton NMR Methanol Isomerization Alkyl |
| Zdroj: | Journal of the American Chemical Society. 131:4159-4166 |
| ISSN: | 1520-5126 0002-7863 |
| Popis: | The kinetics and cleavage products of 2-hydroxypropyl p-nitrophenyl phosphate were determined in methanol containing the di-Zn(II) complex of bis-1,3-N1,N1'-(1,5,9-triazacyclododecyl)propane (4). Time-dependent 1H NMR spectra of the reaction mixture at sspH 9.8 +/- 0.1 show that the catalytic reaction proceeds via a cyclic phosphate (4-methylethylene phosphate, 2) that is subsequently cleaved into a kinetic mixture of two isomeric products, 2-hydroxypropyl methyl phosphate (3) and 1-hydroxypropan-2-yl methyl phosphate (3a), in a 29/71 ratio. In the presence of 4, the kinetic mixture of 3/3a is transformed into a thermodynamic mixture of 72/28 3/3a. The time-dependent 1H NMR spectra of 4 and a 22/78 mixture of 3/3a in CD3OH show that the formation of the thermodynamic mixture occurs on the same time scale as replacement of the P-OCH3 group of the 3/3a starting materials with OCD3. Detailed kinetic studies indicate that the dominant process for loss of the OCH3 group and equilibration of 3/3a is via a 4-catalyzed process where each of the isomers cyclizes to methylethylene phosphate (2), which subsequently reforms the 3/3a thermodynamic mixture. The kcatmax for 4-catalyzed cyclization of 3 and three other 2-hydroxypropyl O-alkyl phosphates (alkyl = CF3CH2- (6a), CH2FCH2- (6b), and CH3CH2- (6c)) has been determined, and the Brønsted plot comprising the log kcatmax vs leaving group sspKa that includes several previously studied 2-hydroxypropyl aryl phosphates is linear, following the expression log kcatmax = (-0.85 +/- 0.02) sspKa + (12.8 +/- 0.4). The betalg value of -0.85 suggests that the catalyzed cleavage of the P-OAr/OR bond has progressed to about 45% in the transition state. The combined results are analyzed in terms of two possible processes involving either a concerted reaction leading to the cyclic phosphate 2 from which the thermodynamic mixture of 3/3a is formed or a stepwise one involving a transient phosphorane whose predominant fate is to eliminate methoxide and proceed to 2 rather than partitioning between 3, 3a, and 2. |
| Databáze: | OpenAIRE |
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