to 53:47 because haptophilicity is no long present (there is no directing group on cyclohexane). The distribution changes completely in favor of the trans isomer 2A when Crabtree's catalyst is used in dichloromethane. This selectivity is both predictable and practically useful. Carbonyl groups are also known to direct the hydrogenation by the Crabtree catalyst to be highly regioselective. The directing effect that causes the stereoselectivity of hydrogenation of terpen-4-ol with Crabtree’s catalyst is shown below. History Crabtree and graduate student George Morris discovered this catalyst in the 1970s while working on iridium analogues of Wilkinson's rhodium-based catalyst at the Institut de Chimie des Substances Naturelles at Gif-sur-Yvette, near Paris. Previous hydrogenation catalysts included Wilkinson’s catalyst and a cationic rhodium(I) complex with two phosphine groups developed by Osborn and Schrock. These catalysts accomplished hydrogenation through displacement; after hydrogen addition across the metal, a solvent or a phosphine group dissociated from the rhodium metal so the olefin to be hydrogenated could gain access to the active site. This displacement occurs quickly for rhodium complexes but occurs barely at all for iridium complexes. Because of this, research at the time focused on rhodium compounds instead of compounds involving transition metals of the third row, like iridium. Wilkinson, Osborn, and Schrock also only used coordinating solvents. Crabtree noted that the ligand dissociation step does not occur in heterogeneous catalysis, and so posited that this step was limiting in homogeneous systems. They sought catalysts with “irreversibly created active sites in a noncoordinating solvent.” This led to the development of the Crabtree catalyst, and use of the solvent CH2Cl2. References Homogeneous catalysis Cyclooctadiene complexes Hydrogenation catalysts Organoiridium compounds Phosphine complexes Hexafluorophosphates