Rhodium Chemzymes: Michaelis−Menten Kinetics in Dirhodium(II) Carboxylate-Catalyzed Carbenoid Reactions

Rhodium carboxylate-mediated reactions of diazoketones involving cyclopropanation, C−H insertion, and aromatic C−C double bond addition/electrocyclic ring opening obey saturation (Michaelis−Menten) kinetics. Axial ligands for rhodium, including aromatic hydrocarbons and Lewis bases such as nitriles, ethers, and ketones, inhibit these reactions by a mixed kinetic inhibition mechanism, meaning that they can bind both to the free catalyst and to the catalyst−substrate complex. Substrate inhibition can also be exhibited by diazocompounds bearing these groupings in addition to the diazo group. The analysis of inhibition shows that the active catalyst uses only one of its two coordination sites at a time for catalysis. Some ketones exhibit the interesting property that they selectively bind to the catalyst−substrate complex. The similarity of the kinetic constants from different types of reactions with similar diazoketones, regardless of the linking unit or the environment of the reacting alkene, suggests that ...