Kinetics and Mechanism of the Pyridinolysis of Diisopropyl Chlorophosphate in Acetonitrile

초록

The nucleophilic substitution reactions of diisopropyl chlorophosphate (1) with substituted pyridines (XC5H4N) in acetonitrile at 35.0 °C are investigated kinetically. The pyridinolysis of 1 shows biphasic concave upward free energy correlations with a break point at X = 3-Ph (3-phenyl pyridine). The Hammett ρX = -5.12 and Bronsted βX = 1.05 for strongly basic pyridines (X = 4-MeO, 4-Me, 3-Me, H, 3-Ph), and ρX = -2.28 and βX = 0.39 for weakly basic pyridines (X = 3-Ph, 3-MeO, 3-Cl, 3-Ac, 4-Ac, 3-CN, 4-CN) are obtained. A concerted SN2 mechanism with a dominant frontside nucleophilic attack with greater bond formation for strongly basic pyridines and a dominant backside nucleophilic attack for weakly basic pyridines are proposed. Relatively smaller rates compared to other chlorophosphates is rationalized by the stereoelectronic effects of the isopropyl ligands.