Direct kinetic measurements for the fast interconversion process between trigonal boronic acid and tetragonal boronate ion at low temperatures
摘要:
The rate constant, k(1), for the following addition reaction between 4-isopropyltropolonate (ipt(-)) and meta-nitrophenylboronic acid (m-NO2PhB(OH)(2)) in acetonitrile was measured at various temperatures by using a low-temperature stopped-flow spectrophotometer:m-NO2PhB(OH)(2) + ipt(-) reversible arrow m-NO2PhB(OH)(2)(ipt)(-);k(1) = 7.9 x 10(4)M(-1) s(-1) at 25 degrees C, Delta H-1* = 34.7 +/- 1.0 kJ mol(-1), Delta S-1* = -34.8 +/- 3.8 J mol(-1) K-1, which indicates that the intercoriversion between the trigonal boronic acid and the tetrahedral boronate ion is much faster than the chelate formations for the complexation of boronic acid with bidentate ligands, but much slower than the diffusion-controlled reactions. (C) 2005 Elsevier B.V. All rights reserved.