A PM3 study of the molecular mechanism for the cycloaddition between cyclopentadiene and protonated pyridine-imine derivatives

The molecular mechanism of the cycloaddition reaction between cyclopentadie ne (CP) and three pyridine-imine derivatives has been studied by means of P M3 semiempirical method. The role of the protonation, endo/exo and pi -faci al stereoselectivity are analyzed and discussed. In neutral conditions the cycloaddition between CP and the pyridine-imine takes place along a concert ed mechanism. Protonation on both nitrogen atoms brings out that the reacti on pathway takes with a very low barrier along a stepwise mechanism. PM3 re sults are capable to find the key factors governing this chemical reaction and to explain the experimental outcome. (C) 2001 Elsevier Science B.V. All rights reserved.