AM1 studies on the intramolecular cyclization of ?-haloenolate anions

The intramolecular cyclizations of ω-haloenolate anions, −CαH2C(O)(CH2)n−3CωH2X with X = F, Cl and Br and n = 3–7, were investigated by the AM1 method. In most cases, cycloketone formation proceeds more favourably than cycloether formation, as predicted by the HSAB principle. The reactivity increases in the order X = F < Br < Cl for both processes, and for cycloether formation it is in the order n = 3 < 4 < 7 < 6 < 5 for all X but the relative order for n = 5 and 6 reverses in favour of n = 6 for cycloketone formation with X = Cl and Br. The softness of the acid centre, Cω, decreases for a harder X( = F) whereas it increases for a softer X( = Br). Thus the reactivity order with respect to X suggests that the softness of the base centres O and Cα belongs to the borderline class. The transition state (TS) structures indicate that the BEP principle is obeyed in all cases, and a less reactive process has a more product-like TS.