Dissociative Solvolytic Cleavage of Methyl (ortho-Carboxymethyl)Aryl Phosphate Diesters Mediated by Yb3+ in Methanol Gives a 1012-Fold Rate Acceleration Attributable to Leaving Group Assistance

Abstract

The Yb3+-catalyzed cleavage of a series of eight methyl aryl phosphates (2a−h) where the aryl groups all contain an ortho-methoxycarbonyl group was studied in acidic methanol from 1.34 ≤ sspH ≤ 3.34 at 25 °C. All substrates show saturation binding of the metal ion that is analyzed to provide a conditional binding constant (Kb) for a 1:1 substrate/Yb3+ complex and catalytic rate constant (kcat) that varies between about 2 × 10−3 and 50 × 10−3 s−1 over the range of substrates. Detailed analysis indicates that at very low concentration of Yb3+, 3 equiv of substrate are bound, and with increasing [Yb3+], the binding changes to a 1:1 complex which decomposes by a pathway independent of sspH over the range investigated. Control studies show that substrates without the o-methoxycarbonyl group still bind to the Yb3+ with approximately the same strength as do the o-methoxycarbonyl containing substrates but have no observable reaction when bound. A Jaffé plot of the kcat vs substituent σ-values indicates that, during the catalyzed reactions of 2a−h, the phenoxy-O and C(O)OCH3 groups accommodate negative and positive charge respectively, the ρphosphate and ρC(O)OMe values being (1.84 ± 0.11) and (−0.85 ± 0.14). For all these substrates, the final reaction products are dimethyl phosphate and the Yb3+ complex of the phenoxide. A study of the binding of the parent phenols to Yb3+ indicates that log(Kbind) = (0.84 ± 0.06)sspKa + (3.4 ± 0.9), r2 = 0.9664 for phenols containing the o-methoxycarbonyl group; for those lacking that substituent log(Kbind) = (0.96 ± 0.04)sspK a − (1.73 ± 0.4), (r2 = 0.99). For the catalyzed reaction the βlg = −0.48, while the βeq = −0.95, leading to a Leffler parameter of α = 0.51. A mechanism is presented for the catalyzed reaction which is highly dissociative, having a transition state where the Yb3+ translocates during the cleavage reaction to assist the leaving group’s departure with weak nucleophilic assistance by the solvent methanol. A comparison of the catalyzed rate of reaction with a computed rate of reaction attributable to solvent alone indicates that Yb3+ provides leaving group assistance on the order of 1012-fold, stabilizing the transition state for cleavage by some 16 kcal/mol.