Supramolecular Catalysis of Unimolecular Rearrangements: Substrate Scope and Mechanistic Insights

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• 作者：Dorothea Fiedler ; Herman van Halbeek ; Robert G. Bergman ; Kenneth N. Raymond
• 刊名：Journal of the American Chemical Society
• 出版年：2006
• 出版时间：August 9, 2006
• 年：2006
• 卷：128
• 期：31
• 页码：10240 - 10252
• 全文大小：430K
• 年卷期：v.128,no.31(August 9, 2006)
• ISSN：1520-5126

文摘

A cavity-containing metal-ligand assembly is employed as a catalytic host for the 3-aza Coperearrangement of allyl enammonium cations. Upon binding, the rates of rearrangement are accelerated forall substrates studied, up to 850-fold. Activation parameters were measured for three enammonium cationsin order to understand the origins of acceleration. Those parameters reveal that the supramolecular structureis able to reduce both the entropic and enthalpic barriers for rearrangement and is highly sensitive to smallstructural changes of the substrate. The space-restrictive cavity preferentially binds closely packed,preorganized substrate conformations, which resemble the conformations of the transition states. Thishypothesis is also supported by quantitative NOE studies of two encapsulated substrates, which place thetwo reacting carbon atoms in close proximity. The capsule can act as a true catalyst, since release andhydrolysis facilitate catalytic turnover. The question of product hydrolysis was addressed through detailedkinetic studies. We conclude that the iminium product must dissociate from the cavity interior and theassembly exterior before hydroxide-mediated hydrolysis, and propose the intermediacy of a tight ion pairof the polyanionic host with the exiting product.