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Mechanistic Studies of Catalysis and Molecular Recognition by Synthetic Supramolecular Enzyme Mimics

Abstract

Chapter 1 – A summary and overview of the application of self-assembled synthetic microenvironment catalysts is presented. The various clusters for which catalytic applications have been reported are introduced together, grouped by mechanism of self-assembly (i.e. hydrogen bond network or metal ligand coordination). The examples of catalysis are then discussed in the context of the nature of charge buildup in the transition state, with neutral reactions presented first, followed by reactions that develop anionic charge in the transition state, and finally reactions that develop cationic charge in the transition state.

Chapter 2 – The tetrahedral [Ga4L6]12- supramolecular assembly developed in the Raymond group is shown to catalyze a bimolecular aza-Prins cyclization featuring an unexpected transannular 1,5-hydride transfer. This reaction pathway is promoted by the constrictive binding of the interior microenvironment of the cluster, and is kinetically inaccessible in bulk solution. A thorough investigation of the mechanism of this process is presented, including isotope labeling studies and kinetic analysis, indicating that the rate limiting step of the process is encapsulation of a transiently formed iminium ion and supports the proposed 1,5-hydride transfer pathway. This work represents a striking example of the enzyme-like ability of synthetic microenvironment catalysts to selectively modulate kinetic barriers in order to promote otherwise inaccessible selectivity.

Chapter 3 – Catalysis of alkyl-alkyl reductive elimination from high valent transition metal complexes [such as gold(III) and platinum(IV)] by the [Ga4L6]12- Raymond cluster is described. Kinetic experiments delineate an enzyme-like Michaelis-Menten mechanism, with rate accelerations (kcat/kuncat) up to 1.9 × 107. Indirect evidence for the intermediacy of a coordinatively unsaturated encapsulated species is garnered from the observation of several persistent donor-arrested inclusion complexes, including a crystallographically characterized gold(III) cation. The catalysis of reductive elimination is further incorporated into a dual-catalytic cross coupling for which the presence of the supramolecular cluster is necessary in order to achieve efficient turnover, and the full catalytic cycle of this process is elucidated through a series of stoichiometric experiments.

Chapter 4 – A novel supramolecular assembly of M4L4 stoichiometry is reported for which the addition of a guest effects an increase from S4- to T-symmetry. A mechanistic investigation of this guest-induced host isomerization revealed that the guest binding occurs by a mechanism similar to the conformational selection model for ligand binding in proteins. A comprehensive study of this simple system provides insight into analogous behavior in biophysics and enzymology, as well as important information to support future efforts in the design of more efficient self-assembled microenvironment catalysts.

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