title: Dinuclear Zn(II) complex promotes cleavage and isomerization of 2-hydroxypropyl alkyl phosphates by a common cyclic phosphate intermediate.
abstract: 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.
authors.affiliation: Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6.
authors.forename: Wing Yin
authors.initials: WY
authors.lastname: Tsang
authors.forename: David R
authors.initials: DR
authors.lastname: Edwards
authors.forename: Stephanie A
authors.initials: SA
authors.lastname: Melnychuk
authors.forename: C Tony
authors.initials: CT
authors.lastname: Liu
authors.forename: Chaomin
authors.initials: C
authors.lastname: Liu
authors.forename: Alexei A
authors.initials: AA
authors.lastname: Neverov
authors.forename: Nicholas H
authors.initials: NH
authors.lastname: Williams
authors.forename: R Stan
authors.initials: RS
authors.lastname: Brown
mesh_terms: D055598:Chemical Phenomena; D007536:Isomerism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D010755:Organophosphates / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry*
chemical_list: D010755:Organophosphates; D015032:Zinc