A Lewis Base Nucleofugality Parameter, N_B, 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_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-RX (X = P, N; R' = aryl, alkyl) has allowed the formulation of related substituent parameters (n_PB, n_AB), providing a means of calculating N_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_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.