TRANSVERSE COMPRESSION AND THE SECONDARY H D ISOTOPE EFFECTS IN INTRAMOLECULAR S(N)2 METHYL-TRANSFER REACTIONS/

Using ab initio molecular orbital theory mainly at the 3-21+G level, i ntramolecular S(N)2 methyl transfer between two oxygens confined withi n a rigid template is found to proceed exclusively by a high energy re tention mechanism when the oxygens are separated by three or four bond s, and by a high energy inversion mechanism when the oxygens are separ ated by six bonds. Both mechanisms exist when the oxygens are separate d by five bonds. The CH3/CD3, kinetic isotope effects are normal (1.21 -1.34) in the retention processes and inverse (0.66-0.81) in the inver sion reactions. In the case of inversion, compression of C-H bonds of the transition state by structural effects in the plane perpendicular to the O-C-O plane increases the inverse isotope effect. The retention barriers are high because retention is inherently unfavorable, even w hen pericyclic stabilization of the transition state is possible. The inversion barriers are high because a rigid template cannot accommodat e a linear O-CH3-O structure, and the O-C-O bending vibration is stiff (the Eschenmoser effect). Using a navel design strategy, a nonrigid t emplate has been found in which the barrier and the CH3/CD3 kinetic is otope effect are the same as in an intermolecular reaction.