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

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• 作者：Michael C. Pirrung ; Hao Liu ; and Andrew T. Morehead ; Jr.
• 刊名：Journal of the American Chemical Society
• 出版年：2002
• 出版时间：February 13, 2002
• 年：2002
• 卷：124
• 期：6
• 页码：1014 - 1023
• 全文大小：128K
• 年卷期：v.124,no.6(February 13, 2002)
• ISSN：1520-5126

文摘

Rhodium carboxylate-mediated reactions of diazoketones involving cyclopropanation, C-Hinsertion, 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 asnitriles, ethers, and ketones, inhibit these reactions by a mixed kinetic inhibition mechanism, meaning thatthey can bind both to the free catalyst and to the catalyst-substrate complex. Substrate inhibition can alsobe exhibited by diazocompounds bearing these groupings in addition to the diazo group. The analysis ofinhibition 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, regardlessof the linking unit or the environment of the reacting alkene, suggests that the rate-determining step is thegeneration of the rhodium carbenoid. A very useful rhodium carboxylate catalyst for asymmetric synthesis,Rh₂(DOSP)₄, shows slightly slower kinetic parameters than the achiral catalysts, implying that enantioselectivity of this catalyst is based on slowing reactions from one of the enantiotopic faces of the reactant,rather than any type of ligand-accelerated catalysis. A series of rhodium catalysts derived from acids withpK_as spanning 4 orders of magnitude give very similar kinetic constants.