Dean J. Tantillo
, University of California (Davis)
Following up on our work from last year in which a combination of physical organic experiments and quantum chemical calculations was used to construct detailed mechanistic models for the Ni(0)-N-heterocyclic carbene-catalyzed vinylcyclopropane-cyclopentene rearrangement, the Ni(0)-N-heterocyclic carbene-promoted [1,3] sigmatropic rearrangements of bicyclo[3.2.0]hept-2-enes to bicyclo[2.2.1]hept-2-enes, and Rh-catalyzed [1,3] sigmatropic shifts of hydrogen, several additional sigmatropic shifts were examined. For each reaction, it was determined whether direct transition state complexation or, instead, transition metal intervention was the source of rate acceleration and stereoselection.
First, [3,3] sigmatropic shifts of heterosubstituted dienes with covalently attached Au substituents were examined in detail using density functional theory calculations. On the basis of the results of our calculations, it was proposed that Au has only a minimal effect on the barriers for these reactions when attached directly to the diene, regardless of position of attachment.
Second, Au-catalyzed [3,3] sigmatropic shifts (Cope rearrangements) were examined in collaboration with the Gagne group at the University of North Carolina. On the basis of the results of our density functional theory calculations, a model for the rate acceleration by Au was constructed. This model involves a combination of transition state complexation and transition metal intervention. Substituent effects and the regiochemistry of Au-complexation were examined in detail.
Work also continued on Pd(0)-promoted [3,5] sigmatropic shifts, which were examined in additional detail using density functional theory calculations. Experimental tests of these predictions are being pursued.