NUCLEOPHILES OF HIGH REACTIVITY IN PHOSPHORYL TRANSFER-REACTIONS - ALPHA-EFFECT COMPOUNDS AND FLUORIDE-ION

The second-order rate constants for reactions of hydrogen peroxide, acetohydroxamate, and fluoride anions with phosphorylated pyridine monoanions (H2O, 25 °C, ionic strength 1.5) are larger than those of “normal” oxygen nucleophiles of the same pKa by factors of 600-, 160-, and 30-fold, respectively. The values of {ηlg} = 0.70 and 0.86 for reactions of phosphorylated pyridines with hydrogen peroxide and fluoride anions are smaller than predicted by a plot of {ηlg} against the pAfa of normal oxygen nucleophiles; however, they fit a plot of {ηlg} against log k for the reactions of oxygen nucleophiles with a phosphorylated pyridine. Thus, the transition-state structure, as measured by {ηlg}, follows the reactivity, not the basicity of the nucleophiles. This is consistent with a thermodynamic origin for at least some of the rate enhancements for the a-effect nucleophiles and fluoride ions, such that the ratio of the phosphorus to proton basicities is larger for hydrogen peroxide and fluoride ion than for normal oxygen bases; i.e., the rate enhancements for these nucleophiles may reflect an enhanced thermodynamic affinity for the phosphoryl group that influences the stability and structure of the transition state. The observed changes in transition-state structure are not predicted for rate enhancements from hydrogen bonding in the transition state or from weak solvation of the -effect nucleophile. © 1990, American Chemical Society. All rights reserved.