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 chemistry [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 synthetic chemistry. Supramolecular catalysis can enable aqueous reaction conditions, 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 supramolecular 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 noncovalent interactions, with the added requirement that the host possess an interior 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. rights All Incorporated. Sons, & Wiley John 2017. © 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–catalyst interactions through molecular recognition processes that function independent of the reactive functional groups. Finally, it is possible to install catalytic moieties within the cavity of a molecular host, which can then bind substrate 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 provide 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 products typically requires energy- or solvent-intensive purification steps. Supra-

Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © 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

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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 synthetically 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) interactions 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 center 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 functionalization [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. rights All Incorporated. Sons, & Wiley John 2017. © 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 terpyridine 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 acid-containing substrates such as ibuprofen (Scheme 7.3). The binding mode positions a single C H bond near the active metal center, and the substrate is regio- and stereo- selectively!! hydroxylated. Lower selectivity is observed when the reaction is per- formed using a dimanganese terpyridine catalyst lacking the molecular recognition element [59]. Bach and coworkers recently reported the design of a ruthenium–porphyrin catalyst bearing a chiral molecular recognition element. A chiral lactam

Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright moiety is linked to the porphyrin through a rigid alkyne linker and is responsible for binding lactam substrates (Scheme 7.4). The catalyst is capable of

Scheme 7.3

Scheme 7.4

enantioselective C H oxidation of prochiral spirocyclic lactam substrates with high enantioselectivity,!! but modest yields [60]. Recently an iridium–bypyridine complex with an attached urea group was disclosed by Kanai and coworkers for the catalytic C–H borylation of arenes. The pendant urea moiety complexes the carbonyl group of substrate benzamides, and the rigid ligand framework positions the iridium center closest to the C H bond meta to the amide substituent (Scheme 7.5). As a result, the borylation!! reaction

Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright is meta-selective, while analogous complexes lacking the substrate-binding group give a mixture of isomers [61].

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Scheme 7.5

Photochemical reactions are extremely valuable synthetic transformations, but controlling the selectivity of such reactions can be challenging. This is due to the extremely short lifetimes and high intrinsic reactivity of excited-state reaction intermediates, which provide little opportunity for directing the reaction outcome. Chiral, supramolecular triplet sensitizers have been developed by Bach and coworkers to perform enantioselective photochemical reactions. The catalyst design links a triplet sensitizer) to a chiral lactam-binding group derived from the Kemp triacid, and it both ensures close contact between the sensitizing group and the substrate while controlling the stereochemical outcome of the reaction (Scheme 7.6). This family of catalysts enables the synthesis of enantioen- riched products via photochemical cyclization and [2 + 2] cycloaddition [62–65].

7.2.2 Scaffolding Ligands

As an alternative to catalyst directing groups that operate through noncovalent bonds, it is also possible to use reversible covalent bonding to colocate substrate and a metal catalyst. Scaffolding ligands, which contain both a catalyst binding unit and a site for reversible covalent substrate binding, are used for this pur- pose [66,67]. The reversibility of the substrate binding allows the scaffolding lig- and to be used in catalytic quantities. The initial application of this approach to the rhodium-catalyzed hydroformylation reaction was independently reported by the groups of Breit and Tan [68,69]. The scaffolding ligands employed in this system contain a phosphine group for metal binding and a site for reversible, covalent bonding of substrate (Scheme 7.7).

Scheme 7.7

The reversibly bound directing group is able to effectively impose regiocontrol over the hydroformylation of homoallyllic alcohols, which is followed by oxidation to provide lactone products. In the absence of the scaffolding ligand, a mixture of products favoring the linear aldehyde, which cyclizes to form a six- membered lactone after oxidation (Scheme 7.8). The scaffolding ligand is able to override the intrinsic selectivity of the reaction, selectively producing the branched product (which forms a ﬁve-membered lactone after oxidation). This approach could also be applied to hydroformylation of alkene substrates bearing

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. 146 7 Supramolecular Catalysis as a Tool for Green Chemistry

Scheme 7.8

7.2.3 Selectivity through Conﬁnement and Binding Effects

The chemical environment and confined space inside of self-assembled hosts can impart selectivity to reactions mediated by supramolecular catalysts. When catalysis occurs within a confined space, it is possible to impart product selectivity that is difficult to achieve with conventional catalysts. A second type of selectivity is the control over which substrates are allowed to react by limiting the size and shape of molecules that penetrate the host interior. Both of these types of selectivity are also hallmarks of enzymatic catalysis. Seminal work published by the van Leeuwen and Reek groups has explored the effect of supramolecular encapsulation on the selectivity of rhodium hydroformylation catalysts, an important reaction in which selectivity is difficult to control [72]. A monodentate tripyridylphosphine ligand is capable of binding a zinc porphyrin panel through each pyridine, creating a well-defined ligand-templated assembly that encapsulates a phosphine-bound rhodium center (Scheme 7.9). Compared to the rhodium complex without the associated porphrins, the ligand-templated catalyst is more active and more selective for the branched isomer. The increased selectivity produced by the encapsulated catalyst is due to the steric restrictions imposed by the assembly interior. A related ligand-templated assembly was created from the tris(zinc (II) porphyrin)phosphite ligand, which self-assembles in the presence of three bridging diamines to form a sandwich structure with a rhodium center in the interior cavity. This supramolecular catalyst is an active hydroformylation catalyst and is highly selective for the linear hydroformylation product [73]. Rebek and coworkers have designed a family of open-ended resorcinarene- derived hosts in which a diversity of functional groups are positioned over the host rim, protruding into the cavity. The host is functionalized with a carboxylic acid group, which is attached to the host rim and dangles into the binding pocket (Scheme 7.10). The intramolecular epoxide ring opening of a 1,5-epox- yalcohol is catalyzed by the host to form a hydroxymethyltetrahydrofuran product [74]. The host-catalyzed reaction is substantially accelerated when compared to the reaction catalyzed by a carboxylic acid that is electronically similar but

lacks any substrate-recognizing cavity. This difference underscores the enhanced Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright reactivity that results from enforcing the close proximity of substrate and a

Scheme 7.9

Scheme 7.10

catalytic functional group. Additionally, the 26-catalyzed reaction produces a mixture of regioisomers, the result of intramolecular nucleophillic attack at both epoxide positions, while the host-catalyzed reaction yields a single regioisomer. An important self-assembling catalyst system for epoxide formation was published by the Hupp and Nguyen groups. The supramolecular box self-assembles from rigid porphyrin-based components and forms a large, cavity-containing structure bearing interior manganese porphyrins [75,76]. While alkenes such as stillbene can be oxidized to the corresponding epoxide by the encapsulated cata-

Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright lyst, larger derivatives do not interact with the catalyst as easily and are undergo epoxidation less efﬁciently (Scheme 7.11). The ability to discriminate between

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substrates on the basis of size alone is due to the steric constraints imposed by encapsulation. A recent from deBruin and coworkers report detailed the cyclopropanation

behavior of a cobalt–porphyrin catalyst encapsulated within a M8L6 cubic assembly [77]. The constricted cage pores of the host modulate how easily substrates reach the encapsulated catalyst, with smaller substrates having easier access. In a competition experiment, 8:2 selectivity for the smaller substrate was exhibited (Scheme 7.12). In contrast, no selectivity is observed when the same experiment is conducted with the unencapsulated catalyst.

Scheme 7.12

A micellar system disclosed by Scarso and coworkers exhibits high levels of substrate selectivity in the palladium-catalyzed hydrogenation of α,β-unsaturated aldehydes [78]. In this example, the catalytic species is a surfactant-encapsulated Pd nanoparticle. Lipophilic substrates bearing long alkyl chains react faster than

C4 and C5 substrates by a factor of 300 (Scheme 7.13). The opposite trend in reaction rates is observed when the reaction is conducted in organic solvent. This is due to their increased ability to associate with the micellar phase due to the hydrophobic effect, allowing easier access to the catalytic nanoparticle surface. Similar effects are seen in the Diels–Alder and Heck reactions, catalyzed by micelle-encapsulated Cr(III)–salen and Pd(II) catalysts, respectively [79,80]. Interestingly, larger substrates react faster in these systems, in contrast to the

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. 150 7 Supramolecular Catalysis as a Tool for Green Chemistry

Scheme 7.13

7.3 Reactions in Water

Solvents account for a large fraction of waste generated in chemicals reactions, and switching to environmentally innocuous solvents is one of the twelve princi- ples of green chemistry [1,2,81,82]. Water is perhaps the most obvious green solvent because it is nontoxic, nonﬂammable, inexpensive, and requires no synthesis. Despite these advantages, water is seldom used as a solvent for organic reactions because many substrates and reagents are either insoluble in or incompatible with water [83]. A related issue is that water can react with some reaction intermediates, producing undesired side products. Many supramolecular hosts are water soluble while possessing a hydrophobic interior cavity, and the host interior presents a chemical environment to encapsulated guests that is dissimilar to water. Thus, water can be used as the bulk solvent while the reaction itself takes place within the inner phase of the host, where reaction conditions are more favorable. Enzyme mimicry under biologically relevant reaction conditions (e.g., water as the solvent, physiological temperature, and pH) has been a long-standing goal of supramolecular chemistry, and many reviews have been published that summarize these efforts [23,25,27,29–33,35,38,54,84–87]. Likewise, conducting organic reactions in water using self-assembled micellar nanoreactors is a research area that has received considerable interest, and several reviews have been published [20,37,88,89]. Because these excellent reviews are quite comprehensive, this section will discuss selected examples that illustrate how supramolecular catalysis can improve reactions in water.

7.3.1 Water-Soluble Nanoreactors

A substantial fraction of self-asssembled molecular hosts are soluble in water and possess hydrophobic interiors. When the reactants and/or catalyst of an organic reaction are encapsulated within a hydrophobic cavity, the molecular

host can act as a nanometer-sized reaction ﬂask, bringing together reactants that Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright

would otherwise be insoluble [31]. While many examples of supramolecular catalysis in water now exist, the most practical and synthetically useful strategy that has emerged is the use of micellar hosts that spontaneously self-assemble in water. Advantages of these systems over other water-soluble hosts include their reliable self-assembly under a wide range of conditions, the commercial availa- bility and low cost of many micelle-forming surfactants, and wide range of hydrophobic molecules that are encapsulated [20,37,88,89]. Micelles lack defined structure compared to other supramolecular structures, which is responsible for their broad encapsulation behavior. A corresponding shortcoming of these systems is that they do not produce confinement effects found in other host- catalyzed systems, such as shape-based substrate gating or the enforcement of specific substrate orientations. Kobayashi and coworkers have developed a number of useful reactions in water using Lewis acid–surfactant-combined catalysts (LASCs). Crucial to these reactions was the counterintuitive discovery that rare earth metal triflates are water-compatible Lewis acid catalysts for the Mukaiyama aldol reaction, and that water is in fact required for catalyst activitiy [90–92]. This led to the discover of the prototypical LASC, scandium tris(dodecyl sul-

fate) (Sc(DS)3), in which the Lewis acidic scandium atom possesses ligands with surfactant properties. While the LASC is soluble in water, it creates a hydrophobic environment for reactants that slows the rate of silyl enolate hydrolysis, a major decomposition pathway in water. The Mukaiyama aldol reaction between silyl

enolates and aldehydes proceeds rapidly and in high yield using Sc(DS)3 as a catalyst in pure water as the solvent (Scheme 7.14) [93,94]. In addition to the afore- mentioned advantages of using water as the reaction solvent, this system allows the use of aqueous formaldehyde instead of gaseous or polymeric forms of the valuable C1 electrophile, which is not possible under anhydrous conditions [91].

Scheme 7.14

Kobayashi and coworkers have disclosed several additional reactions that are amenable to LASC catalysis in water (Scheme 7.15). The three-component Man- nich reaction of amines, aldehydes, and silyl ketene acetals is catalyzed by Sc(DS) and in higher yield by a related Cu(II) LASC, copper bis(dodecyl sulfate) Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright 3 (Cu(DS)2) [94,95]. A similar three-component Abramov-type reaction of amines,

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. 152 7 Supramolecular Catalysis as a Tool for Green Chemistry

Scheme 7.15

aldehydes, and phosphite ester nucleophiles is catalyzed by Sc(DS)3 [96]. Conju- gate additions to electron-deﬁcient oleﬁns with beta-ketoester and indole

nucleophiles in water using Sc(DS)3 as a catalyst were reported [97,98]. It is noteworthy that these reactions are all operationally simple and conducted at ambient temperature. Asymmetric reactions are also possible using LASC catalysis (Scheme 7.16).

The ring opening of meso-epoxides with aromatic amines, catalyzed by Sc(DS)3 Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright Scheme 7.16

and a chiral a bipyridine ligand, proceeded with high enantioselectivity [99]. Asymmetric catalysis of the Mukaiyama aldol reaction was also achieved using

Cu(DS)2, a chiral bisoxazoline ligand, and a Brønsted acid additive [93,100]. In this case, however, the enantioselectivity was modest. Lipshutz and coworkers have made important contributions by advancing a series of designer surfactants that serve as nanoreactors for green, practical, and synthetically useful reactions in water (Scheme 7.17) [88,89]. Central to the suc- cess of this research effort has been the design of surfactants that self-assemble to form nanoreactors with optimal properties for mediating organic reactions in water. The size and morphology of particles formed by these surfactants were found to be particularly important, with 50–100 nm diameter particles being most effective. The structures of surfactants TPGS-750-M and Nok were both optimized with this property in mind, and accordingly are the most effective for performing reactions in water. It should also be noted that these surfactants are environmentally innocuous, being derivatives of nontoxic compounds vitamin E

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These designer surfactants, particularly TPGS-750-M and Nok, have been explored extensively for performing organic reactions in water as the bulk reaction solvent. All of the most common transition-metal-catalyzed reactions can be per- formed efﬁciently under micellar conditions, including oleﬁn metathesis, Sonoga- shira coupling, Suzuki–Miyaura coupling, Heck coupling, Stille coupling, Miyaura borylation, and Buchwald–Hartwig amination (Scheme 7.18) [101–113].

Other indispensable organic transformations, such as amide formation, nucleophilic aromatic substitution, and nitroarene reduction, can also be per- formed in water under micellar conditions (Scheme 7.19) [114–116]. Excellent yields are obtained for each of these micellar reactions at room temperature, while many of the corresponding reaction run in organic solvent under conventional conditions require elevated temperatures. This is due to the high local concentration found within the micellar nanoreactors, which accelerates reaction rates.

Scheme 7.19

Beyond the advantages of switching the reaction solvent to water, Lipshutz and coworkers have demonstrated that using micellar nanoreactors leads to dra- matic reductions in waste when compared to traditional methods. The E factors (the ratio of waste to product produced by a chemical product) [82] of some representative reactions run under micellar and conventional conditions were compared, showing that micellar reaction E factors were typically reduced by an order of magnitude relative to conventional reactions. Finally, the aqueous reaction mixture left over after product extraction contains surfactant and catalyst, and can be recycled several times without detrimental effect on yield, further reducing the amount of generated waste [117]. Very recently, scientists at Novartis published an analysis of the environmental and economic beneﬁts of using TPGS-750-M in water instead of conventional organic solvents for the kilogram-scale production of an Active Pharmaceutical Ingredient (API) [118]. Although the exact nature of the API and intermediates had to be obscured due to the commercial sensitivity of the project, the authors reported a 50% reduction in the quantity of organic solvents used and a 50% reduction in the quantity

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7.3.2 Dehydration Reactions

Performing dehydration reactions in aqueous solvent is typically a challenging task, due to either thermodynamic (such as forming an amide or ester through condensation) or kinetic factors (such as when a carbocation can undergo elimi- nation or nucleophilic attack by water). Counter to intuition, however, it is possible to bias reactions toward dehydration products in pure water if the reaction takes place within the hydrophobic interior of a supramolecular nanoreactor. In 2002, Kobayashi and coworkers reported that a Brønsted acid-functionalized surfactant acted as a catalyst for esteriﬁcation of carboxylic acids and alcohols in water as the sole solvent [119]. Run under conventional conditions, removal of water formed as a coproduct of this reaction is typically necessary to bias the equilibrium toward the desired ester and achieve high yields. Esteriﬁca- tion is successful in water using a catalytic quantity of dodecylbenzenesulfonic acid (DBSA), a micelle-forming Brønsted acid (Scheme 7.20). This reaction is limited to reaction partners that are quite hydrophobic, which is necessary for them to partition into the micelle interior. The inability for water to penetrate into the micelle core alters the thermodynamics of the system, producing high yields of ester. A similar approach was successful with other dehydration reactions, such as ether formation, thioether formation, and thioacetal formation. Since this report, several micelle-mediated dehydration reactions have been reported.

Scheme 7.20

A tetrahedral, self-assembled metal–organic cage (Ga4L6, where L is a bisbi- dentate organic ligand) developed catalyzes the monoterpene-like Prins cyclization of citronellal, as reported by the Raymond and Bergman groups in 2012 [120,121]. This cyclization proceeds through the intermediacy of a carboca-

tion, which can be deprotonated to form an alkene product, or trapped with Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright water to form the corresponding diol (Scheme 7.21). When the reaction is

Scheme 7.21

conducted in buffered acidic water, the diol is the major product, while the alkene product predominates when the reaction is catalyzed by encapsulation

within the Ga4L6 assembly. This effect is also seen in the gold-catalyzed eneyne cycloisomerization, which similarly proceeds through a cationic intermediate

(Scheme 7.22). In this case, the gold catalyst, PMe3AuBr, produces a product resulting from water incorporation. When a gold catalyst encapsulated within + the Ga4L6 assembly (PMe3Au Ga4L6, where denotes encapsulation) is used instead, a formally dehydrated product is also produced. In both of these cases, rate of water addition is substantially decreased within the hydrophobic interior of the supramolecular assembly, an effect that is responsible for the stabilization of various water-sensitive species within the same host [122–126].

Scheme 7.22

Fujita and coworkers have reported the catalysis of the Knoevenagel condensa-

tion by inclusion within a self-assembled metal–ligand cage (Pd6L4) bearing a + ﬁ

Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright 12 charge. Low catalyst loading (1 mol%) of the cage is suf cient to catalyze the condensation of 2-naphthaldehyde with Meldrum’s acid in high yield in

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neutral water as the solvent (Scheme 7.23). The Pd2+ centers located near the cage openings stabilize the anionic reaction intermediates, increasing the reaction rate. Finally, the reaction product is too large to ﬁt within the host cavity, facilitating catalytic turnover [127].

Scheme 7.23

Lipshutz and coworkers recently reported the gold(III)-catalyzed dehydrative cyclization of propargyl diols and propargyl amino acids in water under micellar conditions. When the equivalent reaction is conducted in organic solvent, acti- vated molecular sieves are added to remove water, which is generated as a stoichiometric by-product [128,129]. Using TPGS-750-M as a micellar host for this reaction, the cyclization reaction proceeds smoothly, producing furan and pyr- role products in high yields (Scheme 7.24). No dehydrating agents are required to drive the reaction forward, despite the presence of a vast excess of water [130].

Scheme 7.24

7.4 Catalyst/Reagent Protection

Catalyst and reagent stability is an important issue in many synthetic reactions, and the prevention of off-pathway decomposition is critical for achieving low catalyst loadings and good atom economy. Supramolecular encapsulation of a

reactive catalyst or reagent within a host cavity can protect it from detrimental Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright interactions, providing longer catalyst lifetimes. This can produce higher yields

and allow lower catalyst loading, which is particularly important when consider- ing the low earth abundance of many precious metals used in catalysis.

7.4.1 Catalyst Protection

Manganese porphyrins are useful catalysts for the oxidation of unactivated C H bonds, but they rapidly decompose, limiting their synthetic utility. The decom-!! position occurs through a bimolecular mechanism, forming an oxo-bridged Mn dimer (Mn-O-Mn). The supramolecular metal-ligand square published by Hupp and coworkers binds a single Mn porphyrin molecule through pyridine-Zn association (Scheme 7.11). The lifetime of the encapsulated Mn porphyrin is increased by 18-fold, and the turnover numbers are increased 10–100-fold as well. The stabilization is due to the suppression of the bimolecular decomposition by supramolecular protection [75]. Similar stabilization of Co–porphyrin catalysts are provided by encapsulation in work reported by de Bruin and coworkers (Scheme 7.12). The Bergman and Raymond groups reported the isomerization of allyl alcohols (Scheme 7.25) is catalyzed by a ruthenium(II) catalyst encapsulated within a

self-assembled metal–ligand cage Ga4L6 (Scheme 7.21) [131]. This reaction exhibits host-mediated size selectivity, substrate inhibition, and the reaction proceeds in water. The catalyst lifetime is prolonged by encapsulation, and compared to the performance of the unencapsulated catalyst in organic solvent, encapsulation leads to higher turnover numbers.

Scheme 7.25

7.4.2 Protection of Water-Sensitive Reagents

An intriguing and surprising ﬁnding from the Lipshutz group is that their micellar systems allow for the generation and reaction of several moisture-sensitive

Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright organometallic reagents, even when water is the reaction solvent [102]. Negishi- like couplings between alkyl and aryl halides can be accomplished using zinc

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dust and a palladium catalyst under micellar conditions in water (Scheme 7.26). The water-sensitive organozinc reagent is formed in situ at the metal surface and then partitions into the anhydrous micelle interior more rapidly that it can react with water. At this point, transmetallation to a less sensitive organopalladium species occurs.

Scheme 7.26

This concept was extended to perform cuprate conjugate addition reactions. The reaction of alkyl halides with zinc dust forms organozinc species, which undergoes transmetalation to form an organocopper species [132]. The organ-

copper reagent undergoes conjugate addition to an enone, catalyzed by AuCl3 as a Lewis acid (Scheme 7.27). Remarkably, this reaction proceeds smoothly despite the fact that the reaction must proceed through two water-sensitive intermediates in water as the bulk solvent. An additional feature is that this chemistry proceeds at room temperature instead of the cryogenic temperatures often necessary for organocopper chemistry.

Scheme 7.27

7.5 Tandem Reactions

Tandem reactions, in which multiple reaction events occur sequentially in a single reaction vessel, offer an appealing alternative to iterative chemical synthesis [133]. The isolation and puriﬁcation of intermediate products is energy consuming, produces large quantities of chemical waste, and costly, so tandem reactions are particularly desirable from a green chemistry standpoint. The exe- cution of tandem reactions is difﬁcult because the conditions required for multiple reactions are often incompatible. It is possible to use supramolecular

Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright encapsulation as a tool to circumvent this problem by partitioning an incompatible reaction event into a host interior, where it will no longer interfere

with other reactions. This approach is inspired by enzymatic catalysis, in which incompatible reactions occur in active sites that are isolated from other reaction processes in solution. Even in vitro, extremely impressive tandem processes are possible using enzymatic catalysis. For instance, the one-pot synthesis of the pol- yketide natural product enterocin from simple precursors benzoic acid and malonyl-CoA was accomplished. The tandem process forms 10 C Cbonds, 5C O bonds, and 7 stereocenters, is catalyzed by 12 puriﬁed enzymes,!! and proceeds!! in 25% overall yield [134].

7.5.1 Synthetic Tandem Reactions

An impressive three-reaction tandem process enabled by supramolecular encapsulation was recently disclosed by the Nitschke group [135]. In this process, furan ﬁrst reacts with singlet oxygen (photogenerated by methylene blue) to form an endoperoxide, which is converted to fumaraldehydic acid in a step catalyzed by encapsulation within a self-assembled metal–organic host. Finally, the proline-catalyzed aldol reaction between nitromethane and fumaraldehydic acid yields the ﬁnal lactone product in 30% yield (Scheme 7.28). All reactants and catalysts are present at the beginning of the reaction, which proceeds without any of the three reaction cycles interfering with each other. Not only does encapsulation within the cage catalyze the endoperoxide rearrangement, but it also suppresses nonproductive reaction pathways that occur when the cage is absent. It is also noteworthy that the cage itself self-assembles in the reaction mixture, and that the process occurs in water.

Scheme 7.28

As described in Section 3.1, a large number of transition-metal-catalyzed reactions can be conducted in water within the hydrophobic core of a self- assembled micelle. Except for the catalysts and reagents, the conditions required

are nearly identical for each reaction. The generality of these reaction conditions Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright has enabled the Lipshutz group to design several multireaction tandem processes

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in water, which proceed in a single reaction ﬂask and require no puriﬁcation of intermediates [111,113,136]. For instance, the diamination of 1-iodo-4-bromobenzene was accomplished by an initial installation of a carbamate group, followed by a subsequent coupling with a second carbamate (Scheme 7.29). Both amination events are facilitated by the same catalyst, and the second reaction event is conducted by simply adding the second carbamate and increasing the reaction temperature.

Scheme 7.29

Micellar conditions allow for the formation of C C bonds using the Suzuki or Sonogashira coupling of 1-iodo-4-bromobenzene!! can be followed by a Pd- catalyzed amination step, furnishing the difunctionalized product (Scheme 7.30). In this case, it is worth noting that the reactions proceed with good yields, despite requiring two different Pd catalysts for the two coupling steps.

Scheme 7.30

7.5.2 Chemoenzymatic Tandem Reactions

The importance of enzymes in organic synthesis is growing [137–140], and given that enzymes are particularly suited for tandem processes, the use of supramolecular encapsulation to enable chemoenzymatic tandem reactions under bio- logical conditions is desirable. Bergman and Raymond have reported two tandem chemoenzymatic systems involving a self-assembled metal–ligand host [141]. In ﬁ

Copyright © 2017. John Wiley & Sons, Incorporated. All rights reserved. rights All Incorporated. Sons, & Wiley John 2017. © Copyright the rst example, the initial allenic ester or amide is hydrolyzed by an esterase or lipase, followed by allene hydroalkoxylation catalyzed by an encapsulated gold

complex (Scheme 7.31). This one-pot process proceeds in water affords product tetrahydrofurans in high yield. Supramolecular encapsulation prevents unwanted interactions between the gold catalyst and enzyme; in the absence of the host, both catalytic reactions are negatively impacted.

Scheme 7.31

A second chemoenzymatic tandem process involves an allyl alcohol isomerization catalyzed by an encapsulated Ru(II) catalyst followed by NADPH-dependent enzymatic reduction of the resulting aldehyde (Scheme 7.32). Overall, the process represents a formal alkene reduction. Cofactor regeneration was accomplished by a second enzyme, allowing sodium formate to be the terminal reductant instead of the expensive NADPH. In tandem processes involving synthetic reactions, incorporating additional reaction cycles is extremely challenging, if not impossible. In this example, however, the second cofactor recycling enzyme was added without any change in reaction conditions. The design of more complex chemoenzymatic processes will no doubt be aided by supramolecular substrate gating and other forms of reaction control [23,86] to mini-

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7.6 Conclusion

Supramolecular catalysis is still in an early stage of development, and much work has been dedicated to establishing proof-of-concept rather than developing practical processes. However, the examples in this chapter demonstrate how supramolecular binding can be applied as a tool to achieve reaction outcomes that are desirable from a green chemistry standpoint. Indeed, the past decade has seen the productive application of supramolecular binding and encapsulation toward synthetically useful and green reactions, particularly in the area of micellar catalysis, directed C–H functionalization, and hydroformylation. As supramolecular chemistry concepts continue to be adopted by the wider synthetic community, further practical and green applications can be expected. Finally, although using supramolecular protection to enable tandem reactions is still a research area in the earliest stages of development, it holds great potential for reducing the time, waste, and energy involved in chemical synthesis. Particularly if one considers the integration of synthetic and enzymatic chemistry into chemoenzymatic tandem processes, it is now possible to imagine a truly ideal reaction process in which a desired product is made from simple reactants in a single operation at ambient temperature using water as the solvent. Again, it will be the adoption of these concepts by the wider synthetic community that will lead to practical and impactful new processes.

References

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131 Brown, C.J., Miller, G.M., Johnson, M.W., 136 Bhattacharjya, A., Klumphu, P., and Bergman, R.G., and Raymond, K.N. Lipshutz, B.H. (2015) Organic Letters, 17, (2011) Journal of the American Chemical 1122. Society, 133, 11964. 137 Pàmies, O. and Bäckvall, J.-E. (2003) 132 Lipshutz, B.H., Huang, S., Leong, Chemical Reviews, 103, 3247. W.W.Y., Zhong, G., and Isley, N.A. 138 Leresche, J. and Meyer, H. (2006) Organic (2012) Journal of the American Chemical Process Research and Development, 10, Society, 134, 19985. 572. 133 Fogg, D.E. and dos Santos, E.N. (2004) 139 Meyer, H. (2011) Organic Process Coordination Chemistry Reviews, 248, Research and Development, 15, 2365. 180–188. 134 Cheng, Q., Xiang, L., Izumikawa, M., 140 Clouthier, C.M. and Pelletier, J.N. Meluzzi, D., and Moore, B.S. (2007) (2012) Chemical Society Reviews, 41, Nature Chemical Biology, 3, 557. 1585. 135 Salles, A.G., Zarra, S., Turner, R.M., and 141 Wang, Z.J., Clary, K.N., Bergman, R.G., Nitschke, J.R. (2013) Journal of the Raymond, K.N., and Toste, F.D. (2013)