1092940-55-0Relevant articles and documents
Dissociative solvolytic cleavage of methyl (ortho-Carboxymethyl)Aryl phosphate diesters mediated by Yb3+ in methanol gives a 10 12-fold rate acceleration attributable to leaving group assistance
Edwards, David R.,Neverov, Alexei A.,Brown, R. Stan
supporting information; experimental part, p. 368 - 377 (2009/06/28)
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 ≤ spHs ≤ 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/Yb3+ complex and catalytic rate constant (A cat) that varies between about 2 × 10-3 and 50 × 10-3 s-1 overthe range of substrates. Detailed analysis indicates that at very low c oncentration 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 spHs 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 Jaffe plot of the kcat vs substituent ?-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 p phosphate and p 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 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(Kb ind) = (0.96 ± 0.04)s spKa- (1.73 ± 0.4), (r2 = 0.99). For the catalyzed reacti on 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.
