Solvation and metal ion effects on structure and reactivity of phosphoryl compounds. Part 2. Alkali metal ion catalysis in the demethylation of methyl di(3-nitrophenyl)phosphinate by iodide ion

Abstract

AGNES M. MODRO and TOMASZ A. MODRO. Can. J. Chem. 71, 469 (1993) Second-order rate constants for the demethylation of methyl di(3-nitropheny1)phosphinate by iodide in acetone-d6 at 25°C have been measured. For a series of iodide salts the rate increased in the order: NBu,I < KI 5 NaI < LiI. The addition of crown ethers or D 2 0 has a retarding effect on the reaction rate. The results are discussed in terms of the increase of the nucleofugality of the phosphinate group via the complexation of metal ions with the phosphoryl function. On a mesure les constantes de vitesse du deuxikme ordre de la reaction de dCmCthylation du di(3-nitrophkny1)-phosphinate de mkthyle par l'iodure dans l'acetone-d6, i 25°C. Pour une sCrie d'iodures, la vitesse augmente dans l'ordre NBu,I < KI 5 NaI < LiI. L'addition dlCthers couronnes ou de DzO diminue la vitesse de la rkaction. On discute des rksultats en fonction de l'augmentation du caractkre nucltofuge du groupe phosphinate via une complexation des ions metalliques par la fonction phosphoryle.[Traduit par la rkdaction]Methyl phosphotriester analogues of polynucleotides attract increasing attention (2), while nucleophilic removal of the methyl group from the POMe function (see, for example, ref.3) represents the final step in the "phosphotriester" method of oligonucleotide synthesis. Various nucleophiles can be employed in the dealkylation, and the utility of halide ions as reagents for selective removal of alkyl groups in phosphate esters was recognised 40 years ago by Zervas and Dilaris (4), as well as by Todd in his classic studies on phosphorylation (5). Although the effect of the counterion of the halide salt on the reaction rate was never the subject of an investigation, it was reported in one of Todd's papers (6) that the yield of debenzylation of benzyl diphenylphosphate by M+I-increases in the order: M' = C6H,,NH3' < Na+ < Li+. The participation of the Li' ion in the demethylation of phosphoric esters was postulated by van Lier and co-workers (7), followed by their nmr spectroscopic study of the interactions between Li' and phosphoryl substrates (8). On the other hand, Winstein (9) and others (10) demonstrated that the rate of dealkylation of alkyl arenesulfonates by halide ions in acetone or HMPTA is independent of the cation used.In a recent series of papers on the nucleophilic displacement at phosphorus and sulfur centers, Buncel and coworkers demonstrated for the first time, and quantitatively assessed, alkali metal ion catalysis in the displacement at the phosphoryl centre by oxygen nucleophiles (1 1). The catalysis, increasing in the order: K+ < ~a ' < Li+, was interpreted in terms of the association of the metal ion and the transition state. Our own studies on the effect of metal ions (K', Na', Li', M~' + ) on nmr spectroscopic parameters and the conformational preferences of phosphonic esters (1) gave evidence for strong interactions between the metal ion and the P03R, group. In this work we report our results on the 0-demethylation of methyl di(3-nitropheny1)pho...

Dramatic Effect of the Metal Cation in Dealkylation Reactions of Phosphinic Esters Promoted by Complexes of Polyether Ligands with Metal Iodides

“…The variations of rate constant obtained with these complexed cations (M + ⊂ Lig) in chlorobenzene (k (K + ⊂Lig) / k (Na + ⊂Lig) /k (Li + ⊂Lig) ) 1:10:92) are even higher than those found by other authors in analogous demethylation reactions with metal ions M + in acetone (k K + /k Na + /k Li + ) 1:1.6:9). 15 In dipolar aprotic (acetone) and protic (MeOH) media, where metal salts are soluble as such, metal ions M + in equilibrium with ion pairs M + Ycan directly interact with the substrate in the transition state. In these systems, the addition of the polyether, which selectively binds the cation 15 or destroys the ion pair, 7 inhibits electrophilic catalysis and is most likely the main reason for the decelerating effect observed.…”

“…15 In dipolar aprotic (acetone) and protic (MeOH) media, where metal salts are soluble as such, metal ions M + in equilibrium with ion pairs M + Ycan directly interact with the substrate in the transition state. In these systems, the addition of the polyether, which selectively binds the cation 15 or destroys the ion pair, 7 inhibits electrophilic catalysis and is most likely the main reason for the decelerating effect observed.…”