La3+-catalyzed methanolysis of phosphate diesters.: Remarkable rate acceleration of the methanolysis of diphenyl phosphate, methyl p-nitrophenyl phosphate, and bis(p-nitrophenyl) phosphate

The interaction of La3+ 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)(s)pH. P-31 NMR shows that phosphates 3 and 4 are each associated with La3+ as a rapidly interconverting mixture of complexes having 2:1, 1:1 (or 2:2), and 1:2 stoichiometry. At [La3+] > 2 x 10(-4) M, the kinetically dominant species in the (s)(s)pH regions of interest are phosphate-bound La3+ dimers having 2-5 associated methoxides. Potentiometric titration reveals that 4 mM La3+ alone in MeOH exhibits two apparent (s)(s)pK(a)s at 7.86 and 10.44 consuming 1 and 1.5 CH3O- per metal, the suggested structures being La3+ 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 La3+ and a profile with apparent (s)(s)pK(a)s at 7.8 and 11.1 consuming 1 and 1.5 CH3O- per metal corresponding to phosphate-bridged La3+ dimers with first two and then five associated methoxides. Methanolysis of 3-5 is strongly accelerated by La3+. At (s)(s)pH 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 [La3+] and (s)(s)pH indicate that both La3+ monomers and dimers with associated methoxides are reactive species. The rate constants for La3+ monomer and dimer catalysis of the methanolysis of 5 are dependent on (s)(s)pH, 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 on [CH3O-] wherein the La3+(CH3O-)(x) + 5 reversible arrow La3+ - (CH3O-)(x):5 and (La3+)(2)(CH3O-)(y) + 5 reversible arrow 5:(La3+)(2)(CH3O-)(y) equilibria are driven to the 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.