La~(3+)-Catalyzed Methanolysis of Phosphate Diesters. Remarkable Rate Acceleration of the Methanolysis of Diphenyl Phosphate, Methyl p-Nitrophenyl Phosphate, and Bis(p-nitrophenyl) Phosphate

摘要

The interaction of La~(3+) with diphenyl phosphate, methyl p-nitrophenyl phosphate, and bis(p-nitrophenyl)phosphate (4,5, and 3) and the ensuing catalysis of methanolysis has been studied in methanol under completely homogeneous conditions of known s~spH. ~(31)P NMR shows that phosphates 3 and 4 are each associated with La~(3+) as a rapidly interconverting mixture of complexes having 2:1, 1:1 (or 2:2), and 1:2 stoichiometry. At [La~(3+)] . 2 X 10~(-4) M, the kinetically dominant species in the s~spH regions of interest are phosphate-bound La~(3+) dimers having 2-5 associated methoxides. Potentiometric titration reveals that 4 mM La~(+) alone in MeOH exhibits two apparent s~spKas at 7.86 and 10.44 consuming 1 and 1.5 CH3O~- per metal, the suggested structures being La~(3+) dimers with first two and then five associated methoxides. In the presence of 0.5 equiv of 4 the potentiometric titration reveals strong complexation of 4 to La~(3+) and a profile with apparent s~spKas at 7.8 and 11.1 consuming 1 and 1.5 CH3O~- per metal corresponding to phosphate-bridged La~(3+) dimers with first two and then five associated methoxides. Methanolysis of 3-5 is storngly accelerated by La~(3+). At s~spH 8.9 or 11.1, respectively, as little as 5 X 10~(-4) M La(Otf)_3 accelerates the methanolysis of 3 or 4 by 10~(10)-fold relative to the background reaction. Detailed kinetic studies of the methanolysis of 5 at varying [Ls~(3+)] and s~spH indicate that both la~(3+) monomers and dimers with associated methoxides are reactive species. The rate constnats for La~(3+) monomer and dimer catalysis of the methanolysis of 5 are dependent on s~spH, the slopes of the log plots being 0.35 and 0.5, respectively. A scheme is proposed to account for the non-first-order dependence of [CH3O~-] wherein the La~(3+)(CH3O~-)_x + 5 <=> La~(3+)(CH_3O~-)_x:5 and (La~(3+)_2(CH_3O~-)_y + 5 <=> 5: (La~(3+))_2(CH_3o~-)_y equilibria are driven tot he left with increasing numbers of associated methoxides and that attack on these complexes, whether by external or metal-associated methoxide, is slower due to the reduced net positive charge on the complex.