Single Transition State for Sulfuryl Group (-SO3-) Transfer between Pyridine Nucleophiles

Article abstract of DOI:10.1021/ja00300a042

The second-order rate constants for the nucleophilic attack of pyridines on isoquinoline-N-sulfonate (25 deg C, 0.1 M ionic strength) obey an excellent linear relation, log k_Xpy^isq = 0.23 pK_Xpy - 1.91 (r=0.995), with the pK of the attacking - equation - pyridine over eight pK units.The complete absence of curvature in the relationship indicates a single transition state for the reaction consistent with a concerted, symmetrical mechanism.The attack of pyridine on substituted pyridine-N-sulfonates (25 deg C, ionic strength at 0.1 M) obeys the Bronsted equation log K_py^Xpy = -0.9 pK_Xpy + 4.22 (r=0.998).The Β_EQ for the equilibrium transfer of the -SO3- group from a constant pyridine leaving group to a variant pyridine nucleophile ( +1.13) is close to that predicted from a previous study.The electronic structure of the transition state possesses considerable sulfur trioxide character as deduced from the changes in effective charge on the entering and leaving pyridine nitrogen atoms.