Kinetic Study on Pyridinolysis of Aryl Benzenesulfonates: Factors Governing Regioselectivity and Reaction Mechanism

A kinetic study is reported for reactions of 2,4-dinitrophenyl X-substituted-benzenesulfonates (1a-1f) and Y-substituted-phenyl benzenesulfonates (2a-2f) with Z-substituted-pyridines. The reactions proceed through S‒O and C‒O bond scissions competitively. The Yukawa-Tsuno plot for the reactions of 1a-1f with 4-oxypyridine (S‒O bond fission) exhibits an excellent linearity. The Brønsted-type plot for the reactions of 2,4-dinitrophenyl benzenesulfonate (1d) with pyridines (S‒O bond fission) is also linear with βnuc = 0.62. The Brønsted-type plot for the reactions of 2a-2f with 4-oxypyridines (S‒O bond fission) is linear with βlg= ‒1.17. Thus, the reactions have been concluded to proceed through a concerted mechanism, in which leaving-group expulsion is significantly more advanced than bond formation between the electrophilic center and nucleophile at the transition state. The Hammett plot for the reactions of 1a-1f with 4-oxypyridine (C‒O bond fission) exhibits scattered points with ρX= 0.98. The Brønsted-type plot for the reactions of 1d with Z-substituted-pyridines (C‒O bond fission) results in an excellent linear correlation with βnuc = 0.38. Thus, the reactions (C‒O bond fission) have been concluded to proceed through a stepwise mechanism with a Meisenheimer complex, in which expulsion of the leaving group occurs after the rate-determining step. Factors governing regioselectivity and reaction mechanism are discussed.

A Lewis Base Nucleofugality Parameter, NFB, and Its Application in an Analysis of MIDA-Boronate Hydrolysis Kinetics

The kinetics of quinuclidine displacement of BH₃ from a wide range of Lewis base borane adducts have been measured. Parameterization of these rates has enabled the development of a nucleofugality scale (N^F_B), shown to quantify and predict the leaving group ability of a range of other Lewis bases. Additivity observed across a number of series R'_3-nR_nX (X = P, N; R' = aryl, alkyl) has allowed the formulation of related substituent parameters (n^f_PB, n^f_AB), providing a means of calculating N^F_B values for a range of Lewis bases that extends far beyond those experimentally derived. The utility of the nucleofugality parameter is explored by the correlation of the substituent parameter n^f_PB with the hydrolyses rates of a series of alkyl and aryl MIDA boronates under neutral conditions. This has allowed the identification of MIDA boronates with heteroatoms proximal to the reacting center, showing unusual kinetic lability or stability to hydrolysis.