AM1 STUDIES OF E2 REACTIONS .2. REGIOSELECTIVITY, STEREOCHEMISTRY, KINETIC ISOTOPE EFFECTS, AND COMPETITION WITH SN2 REACTIONS

The regioselectivity and stereochemistry of bimolecular elimination (E2) reactions have been studied by use of the AM 1 model. Calculations are first carried out for reactions of compounds of the type H-CHR-CHS-X, where X is the leaving group while R and S are H, CH3, C2H5, or phenyl. In the first series, X is neutral (halogen) and the base anionic (methoxide ion), while in the second, X is cationic (onium ion) and the base neutral (ammonia). The results account for the different regioselectivities shown by the two types of elimination, as embodied in the Saytzeff and Hoffmann rules. Our calculated kinetic isotope effects also agree with experiment and support our formulation of the two types of elimination (in the gas phase) as E2/ElcB and E2, respectively. Calculations for reactions of methoxide ion with cyclic bromides show that anti(axial-axial) elimination is favored in all cases. In derivatives of cyclohexane, equatorial-equatorial gauche elimination is predicted to be more facile than axial-equatorial. Calculations for parallel E2 and SN2 reactions predict the E2 reactions of methoxide ion with alkyl halides to be preferred. Substitution becomes more competitive in the reactions of ammonia with onium ions. © 1990, American Chemical Society. All rights reserved.