Through theoretical and experimental studies, chemists have learned how a catalytic asymmetric reaction called the combined C–H activation/Cope rearrangement works (J. Am. Chem. Soc., DOI: 10.1021/ja111408v). The reaction (shown) is generally selective for one stereochemical outcome, but the new work suggests it’s possible to obtain several products at will. The rhodium-mediated reaction is “a powerful transformation that has been used effectively in complex molecule assembly,” says organic chemist Joseph M. Fox of the University of Delaware. However, the mechanism for the process wasn’t clear before now. Emory University’s Jørn H. Hansen and Huw M. L. Davies, director of the NSF Center for Stereoselective C–H Functionalization, and colleagues determined that the transformation proceeds through a hydride transfer and carbon-carbon bond formation. Energy differences between the reaction’s possible transition states are small, which indicates that different chiral products should be obtainable in a controlled manner with modified reagents, something Davies’ lab has accomplished but not yet reported. This study “places the reaction on sound mechanistic footing and provides chemists with a predictive computational tool for understanding chemoselectivity and regioselectivity,” Fox says.