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

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 Jaffe plot of the kcat vs substituent σ-values indicates that, duri...