REACTION OF 2-BROMO-2-ALKENOIC CARBONYL-COMPOUNDS WITH AMIDINES .2. THEORETICAL (PM3) STUDIES OF THE MECHANISM OF HETEROCYCLIZATION INTO DIHYDROIMIDAZOLE AND PYRIMIDIN-4-(3H)-ONE

Different pathways have been considered to describe heterocyclization between monosubstituted or unsubstituted amidines and 2-bromo-2-alkeno ic carbonyl compounds, giving N-substituted 2-dihydroimidazole heteroc ycles. Semiempirical calculations (PM3) have been carried out on the r eactants and intermediates. The transition states have been analysed. From both thermodynamic and kinetic arguments, it appears that mechani sms involving a 6-exo-trig ring closure leading to pyrimidin-4-(3H)-on e, or a 5-endo-trig ring closure process in the case of N-substituted amidines, are more probable than the one expected in which the interme diate is an aziridine.