Nucleophilic substitutions at the pyridine ring. Conformational preference of the products and kinetics of the reactions of 2-chloro-3-nitro- and 2-chloro-5-nitro-pyridines with arenethiolates

The reactions of 2-chloro-3-nitropyridine 1 and 2-chloro-5-nitropyridine 2 with arenethiolates 3a-i result in arylthio-dechlorination to give 2-arylthio-3-nitropyridines 4a-i and 2-arylthio-5-nitropyridines 5a-i. A H-1 NMR study and AM1 calculations of 5a-i have shown that these compounds assume a skew conformation except the 2-(4-nitrophenylthio) derivative 5f which has a twist one. An addition-elimination mechanism is suggested based on second order kinetics and the dependence of rates on the nature of the nucleophile. It is found that 2 (para-like) reacts faster than 1 (ortho-like). Good correlations are obtained from plots of log k(2) against Hammett cr constants, pK(a) and log K (thermodynamic affinity of arenethiolates towards carbon).