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Supramolecular Catalysts Green Chemistry 139 7 Supramolecular Catalysis as a Tool for Green Chemistry Courtney J. Hastings 7.1 Introduction Catalysis is central to advancing green chemistry in the area of synthetic chemis- try [1,2]. Beyond replacing stoichiometric reagents, catalysts have the potential to streamline multistep synthesis by enabling new bond-forming processes to shorten synthetic sequences and achieve better step economy [3,4]. Supra- molecular catalysis and the application of supramolecular concepts to catalytic reactions is emerging as a valuable tool for improving catalytic reactions for syn- thetic chemistry. Supramolecular catalysis can enable aqueous reaction condi- tions, improve reactions selectivity, improve catalyst lifetime, and enable tandem reactions, all of which can have positive impacts on the cost, waste, and energy associated with a reaction. The field of supramolecular chemistry concerns the design of molecular enti- ties that are defined by reversible, noncovalent interactions. While each supra- molecular interaction is quite weak individually, the effect of many such interactions working in concert can produce strongly associated and structurally well-defined molecular species [5–7]. Such additive effects are responsible for the spectacular structural complexity found in biomacromolecules such as pro- teins. Efforts to characterize these interactions have provided chemists with a “toolbox” of reliable methods to program the association between two or more molecules to form a single complexed species. Thus, supramolecular chemistry represents a complementary approach toward molecular construction, and one that offers certain advantages over covalent chemistry [5–8]. Like supramolecular interactions, host–guest binding relies on manifold non- covalent interactions, with the added requirement that the host possess an inte- rior cavity that is complementary in size and shape to the guest molecule [9–11]. Quite frequently, the “inner phase” of a synthetic host presents a dramatically different chemical environment to a bound guest than what it would experience in the surrounding bulk solvent. In fact, the environment within a synthetic host Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. All Wiley & Sons, Incorporated. © 2017. John Copyright is frequently unlike anything that a molecule would experience in any solvent, Handbook of Green Chemistry Volume 12: Tools for Green Chemistry, First Edition. Edited by Evan S. Beach and Soumen Kundu. 2017 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2017 by Wiley-VCH Verlag GmbH & Co. KGaA. Handbook of Green Chemistry, Green Products, Tools for Green Chemistry, edited by Evan S. Beach, and Soumen Kundu, John Wiley & Sons, Incorporated, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/dal/detail.action?docID=4883029. Created from dal on 2017-09-29 07:02:31. 140 7 Supramolecular Catalysis as a Tool for Green Chemistry particularly with respect to confinement effects. Many hosts themselves are con- structed through supramolecular interactions, self-assembling from relatively simple subunits into highly complex and symmetric structures [12–16]. The design of synthetic self-assembled host molecules requires control over the geometry of the individual components and how the components interact with each other. This control can be achieved by choosing the subunits to interact with each other through known and predictable noncovalent interactions. Supramolecular catalysis relies upon noncovalent interactions to provide the primary associative interaction between catalyst and substrate, a factor that is responsible for the spectacular selectivity and reactivity of enzymes. Supra- molecular interactions can be involved in catalysis in a number of ways. Supra- molecular encapsulation of one or more substrate molecules within a host (which itself is often self-assembled through supramolecular chemistry) can pro- mote or modulate reactivity. Supramolecular binding can enforce substrate–cat- alyst interactions through molecular recognition processes that function independent of the reactive functional groups. Finally, it is possible to install cat- alytic moieties within the cavity of a molecular host, which can then bind sub- strate molecules. Since the field of supramolecular catalysis and related research areas have been the subject of many excellent reviews, the aim of this chapter is not to pro- vide a comprehensive review of supramolecular catalysis [17–39]. Rather, the goal is to summarize the types of reaction improvements that can be made, and to provide representative examples where supramolecular catalysis was used a tool for obtaining a favorable reaction outcome. Special emphasis is placed on examples that involve widely used and synthetically useful transformations, such as cross-coupling, hydroformylation, and C H functionalization reactions. Finally, conceptually related work on encapsulation-mediated!! reaction control using metal–organic frameworks [40–44], the inner phase of polymers [45–48], and dendrimers [49–51], and other such species are beyond the scope of this chapter, and will be omitted. 7.2 Control of Selectivity through Supramolecular Interactions Supramolecular binding and encapsulation can exert large effects on reaction selectivity, influencing which products are formed (regioselectivity, stereoselec- tivity) and which substrates are allowed to react (substrate gating). This aspect of supramolecular catalysis parallels the high levels of selectivity achieved by enzymes, which are also due in large part due to the cumulative influence of many noncovalent interactions between enzyme and substrate. Imposition of selectivity on synthetic reactions is an important goal, since separation of prod- ucts typically requires energy- or solvent-intensive purification steps. Supra- Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. All Wiley & Sons, Incorporated. © 2017. John Copyright molecular control of selectivity is particularly attractive in reactions where many sites in a substrate molecule are equally reactive (e.g., C–H functionalization) or Handbook of Green Chemistry, Green Products, Tools for Green Chemistry, edited by Evan S. Beach, and Soumen Kundu, John Wiley & Sons, Incorporated, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/dal/detail.action?docID=4883029. Created from dal on 2017-09-29 07:02:31. 7.2 Control of Selectivity through Supramolecular Interactions 141 where selectivity is difficult to achieve using traditional catalyst engineering approaches (e.g., photochemistry and hydroformylation). Thus, representative reactions in which supramolecular interactions improve selectivity to syntheti- cally useful levels will be the focus of this section. 7.2.1 Catalysis with Supramolecular Directing Groups Reactions in which attractive substrate–reagent (or substrate–catalyst) interac- tions exist often proceed with greater selectivity or altered selectivity compared to cases where a directing group is absent, and as such substrate-directed reactions are valuable in synthesis (Scheme 7.1) [52,53]. Typical directing groups influence selectivity by binding directly to the group that is reacting with the substrate. In the case of transition metal catalysis, this means that the metal cen- ter is both the reactive center and the site of molecular recognition. This strategy limits the possible substrate directing groups to those that will bind to, but not inhibit the catalytic metal. A more flexible strategy is for the molecular recognition element to be separate from the reactive center (Scheme 7.1). In addition to expanding the toolbox of noncovalent interactions that may be used for molecular recognition, this approach also enables remote functionaliza- tion [54–57], while traditional directing groups tend to favor activation of proxi- mal positions. Scheme 7.1 This approach was pioneered by Breslow and coworkers, who developed a cyclodextrin-modified Mn–porphyrin catalyst for aliphatic C H hydroxylation. This catalyst selectively hydroxylates an unactivated position !!of a steroid deriva- Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. All Wiley & Sons, Incorporated. © 2017. John Copyright tive (Scheme 7.2) [58]. The steroid substrate androstanediol was derivatized with Handbook of Green Chemistry, Green Products, Tools for Green Chemistry, edited by Evan S. Beach, and Soumen Kundu, John Wiley & Sons, Incorporated, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/dal/detail.action?docID=4883029. Created from dal on 2017-09-29 07:02:31. 142 7 Supramolecular Catalysis as a Tool for Green Chemistry Scheme 7.2 two ester groups bearing both water-solublizing moieties and a tert-butylphenyl for binding to the cyclodextrin. When this substrate is subjected to the catalyst in the presence of iodosobenzene as the terminal oxidant in water, the steroid is regio- and stereospecifically hydroxylated. It is noteworthy that the methylene position where hydroxylation occurs is not the most intrinsically reactive site on the substrate, and that supramolecular binding is responsible for the observed selectivity. Crabtree and coworkers reported a dimanganese terpyridine catalyst bearing two molecular recognition sites for binding carboxylic acid substrates. The ter- pyridine ligands are functionalized with a phenylene group and then the Kemp triacid, which provides a U-turn geometrical element. This orients a carboxylic acid directing group that is capable of binding carboxylic
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