Supramolecular catalysis

Supramolecular catalysis is not a well-defined field but it generally refers to an application of supramolecular chemistry, especially molecular recognition and guest binding, toward catalysis. This field was originally inspired by enzymatic system which, unlike classical organic chemistry reactions, utilizes non-covalent interactions such as hydrogen bonding, cation-pi interaction, and hydrophobic forces to dramatically accelerate rate of reaction and/or allow highly selective reactions to occur. Because enzymes are structurally complex and difficult to modify, supramolecular catalysts offer a simpler model for studying factors involved in catalytic efficiency of the enzyme. Another goal that motivates this field is the development of efficient and practical catalysts that may or may not have an enzyme equivalent in nature.

A closely related field of study is asymmetric catalysis which requires molecular recognition to differentiate two chiral starting material or chiral transition states and thus it could be categorized as an area of supramolecular catalysis, but supramolecular catalysis however does not necessarily have to involve asymmetric reaction. As there is already written about small molecule asymmetric catalysts, this article focuses primarily on large catalytic host molecules. Non-discrete and structurally poorly defined system such as micelle and dendrimers are not included.

The term supramolecular chemistry is defined by Jean-Marie Lehn as "the chemistry of intermolecular bond, covering structures and functions of the entities formed by association of two or more chemical species" in his Nobel lecture in 1987, but the concept of supramolecular catalysis was started way earlier in 1946 by Linus Pauling when he founded the theory of enzymatic catalysis in which rate acceleration is the result of non-covalent stabilization of the transition state by the enzymes. Nevertheless, it was not until a few decades later that an artificial enzyme was developed. The first simple enzyme mimics were based on crown ether and cryptand. In 1976, less than ten years after the discovery of crown ether, Cram et al. developed a functionalized binapthyl crown ether that catalyze transacylation. The catalyst makes use the crown ether motif's ability to capture cation to bind to the ammonium ion part of the substrate and subsequently employs the nearby thiol motif to cleave the ester.

From the early 1970s, cyclodextrins have been extensively studied for its encapsulation properties and used as binding sites in supramolecular catalyst. Cyclodextrins have rigid ring structure, hydrophilic surface, and hydrophobic cavity on the inside; therefore, they are capable of binding organic molecules in aqueous solution. In 1978, with the background knowledge that the hydrolysis of m-tert-butylphenyl acetate is accelerated in the presence of 2-benzimidazoleacetic acid and alpha-cyclodextrin, Brewslow et al. developed a catalyst based on a beta-cyclodextrin carrying two imidazole groups. This cyclodextrin catalytic system mimics ribonuclease A by its use of a neutral imidazole and an imidazolium cation to selective cleave cyclic phosphate substrates. The rate of the reaction is catalyzed 120 times faster, and unlike a hydrolysis by simple base NaOH that gives a 1:1 mixture of the products, this catalysts yield a 99:1 selectivity for one compound.

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