Ab initio studies of three-membered ring formation through intramolecular nucleophilic substitution

Three-membered ring (3MR) forming processes of X--CH2-CH2-F and -CH2-C(-Y)-CH2-F (X=CH2, O, or S and Y=0 or S) through a gas phase neighboring group mechanism (S(N)i) are studied theoretically using the ab initio molecular orbital method with the 6-31 + G* basis set. When electron correlation effects are considered, the activation (Delta G(not equal)) and reaction energies (Delta G(0)) are lowered by ca. 10 kcal mol-l, indicating the importance of the electron correlation effect in these reactions. The contribution of entropy of activation (-T Delta S-not equal) at 298 K to Delta G(not equal) is very small, and the reactions are enthalpy controlled. The Delta G(not equal); and Delta G(0) values for these ring closure processes largely depend on the stabilities of the reactants and the heteroatom acting as a nucleophilic center. The Bell-Evans-Polanyi principle applies well to all these reaction series. (C) 1997 John Wiley & Sons, Inc.