A theoretical MNDO and AM1 SCF-MO study of dihydrogen elimination reactions

Abstract

Semiempirical SCF-MO calculations for the four-centre elimination of molecular hydrogen from substituted cyclohexadienes and cyclopentenes reveal the MNDO and AM1 enthalpic barriers to be too high by ca. 50 and 20 kcal mol^–1 respectively compared with experiment, whereas the calculated entropies of activation are in good agreement with experiment. The alternative stepwise mechanism involving intermediate formation of a hydrogen atom and an alkenyl radical by C–H bond homolysis is significantly lower in energy than the concerted path at the MNDO level, but very similar at the AM1 level. Bis-α(oxy-anion) substituents are calculated to reduce the concerted barrier to hydrogen elimination by 22–26 kcal mol^–1 compared to the neutral diol. Kinetic deuterium isotope effects calculated for the concerted reaction of cyclohexa-1,4-diene or cyclopentene agree very well with experimentally reported values. The importance of tunnelling contributions is discussed. The corresponding isotope effects for the two-centre stepwise reaction are calculated to be significantly greater.