Effect of the 5-substituent on the tetrazole-azide isomerization in tetrazolo[1,5-a]pyridines by ab initio calculations

The effect of substituents CH3, OH, Cl, OCH3, NO2 and COOH at the 5-positio n in tetrazolo[1,5-a]pyridines, on the tetrazoleazide equilibrium, was inve stigated at the 6-31G**/MP2 level of theory. For both, the parent and 5-CH3 compounds, the tetrazole forms are largely favored, while the other substi tuents-OH, Cl, OCH3, and NO2-stabilize the azide isomer. With the COOH grou p, the tetrazole and azide forms are predicted to be in equilibrium in near ly equal amounts. The influence of these groups on the transition state (TS ) was also studied. In general it is observed that, groups which stabilize the azide isomer, lower the E-act of the isomerization reaction, and conver sely, those that favor the tetrazole form, raise the E-act of this reaction , relative to the unsubstituted compound. The TS has a close resemblance to the tetrazole structure. (C) 2000 published by Elsevier Science Ltd.