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

The Yb 3+ -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 ≤ s s pH ≤ 3.34 at 25 °C. All substrates show saturation binding of the metal ion that is analyzed to provide a conditional binding constant ( K b ) for a 1:1 substrate/Yb 3+ complex and catalytic rate constant ( k cat ) 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 Yb 3+, 3 equiv of substrate are bound, and with increasing [Yb 3+ ], the binding changes to a 1:1 complex which decomposes by a pathway independent of s s pH over the range investigated. Control studies show that substrates without the o -methoxycarbonyl group still bind to the Yb 3+ with approximately the same strength as do the o -methoxycarbonyl containing substrates but have no observable reaction when bound. A Jaffé plot of the k cat vs substituent σ-values indicates that, during the catalyzed reactions of 2a − h, the phenoxy-O and C(O)OCH 3 groups accommodate negative and positive charge respectively, the ρ phosphate and ρ C(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 Yb 3+ complex of the phenoxide. A study of the binding of the parent phenols to Yb 3+ indicates that log( K bind ) = (0.84 ± 0.06) s s p K a + (3.4 ± 0.9), r 2 = 0.9664 for phenols containing the o -methoxycarbonyl group; for those lacking that substituent log( K bind ) = (0.96 ± 0.04) s s p K a − (1.73 ± 0.4), ( r 2 = 0.99). For the catalyzed reaction the β lg = −0.48, while the β eq = −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 Yb 3+ 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 Yb 3+ provides leaving group assistance on the order of 10 12 -fold, stabilizing the transition state for cleavage by some 16 kcal/mol.

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