A semiempirical study on the ring-opening process for the cyclopropanone, 2,2-dimethylcyclopropanone,trans-2,3-di-tert-butylcyclopropanone, and spiro(bicyclo[2.2.1]heptane-2.1?-cyclopropan)-2?-one systems in solution

Abstract

The molecular mechanisms for the ring openings of cyclopropanone, 2,2-dimethylcyclopropanone, trans-2,3-di-tert-butylcyclopropanone, and spiro(bicyclo[2.2.1]heptane-2.1′-cyclopropan)-2′-one systems were studied at the PM3 semiempirical level in the gas phase and including solvent effects. The behavior of the solvent polarity was considered by using the SCRF polarizable continuum method. Six solvents were selected: hexane, ether, tetrahydrofuran, pyridine, acetone, and acetonitrile. An extensive exploration of the potential energy surface using analytical gradient techniques allows the characterization of stationary points associated to the stereomutation conversion. Along a disrotatory ring-opening mechanism, cyclopropanone, 2,2-dimethylcyclopropanone and trans-2,3-di-tert-butylcyclopropanone are intraconverted via an oxyallyl intermediate. The epimeric forms of the spiro(bicyclo[2.2.1]heptane-2.1′-cyclopropan)-2′-one intraconvert along two competitive pathways correspond to two-step processes by a disrotatory ring-opening mechanism. Two oxyallyl intermediates and four transition structures were obtained and the corresponding transition vectors are associated to the carbon–carbon bond-breaking process and the dihedral angle measuring the conrotatory movement of the plane defined by the three carbon atoms of the cyclopropanone ring. The oxyallyl intermediates and the transition structures for the four model systems present similar structures and energies and they are located on a rather flat region. The analysis of the theoretical results shows that the solvent reaction field decreases the energy barriers for the ring-opening processes and a stabilization of the oxyallyl intermediates takes place. The calculated relative barrier heights are in good agreement with the experimental data available, and the trends in the kinetics can be explained primarily by steric interactions. Nevertheless, for the spiro(bicyclo[2.2.1]heptane-2.1′-cyclopropan)-2′-one system, it is necessary to include a specific interaction of a discrete molecule of the nucleophilic solvent on the quantum mechanical representation to explain the experimental behavior. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 729–738, 1997