Conspectus The field of supramolecular chemistry has its foundation in molecular recognition and selective binding of guest molecules, often with remarkably strong binding affinities. The field ...evolved to leverage these favorable interactions between the host and its guest to catalyze simple, often biomimetic transformations. Drawing inspiration from these early studies, self-assembled supramolecular hosts continue to capture a significant amount of interest toward their development as catalysts for increasingly complex transformations. Nature often relies on microenvironments, derived from complex tertiary structures and a well-defined active site, to promote reactions with remarkable rate acceleration, substrate specificity, and product selectivity. Similarly, supramolecular chemists have become increasingly intrigued by the prospect that self-assembly of molecular components might generate defined and spatially segregated microenvironments that can catalyze complex transformations. Among the growing palette of supramolecular catalysts, an anionic, water-soluble, tetrahedral metal–ligand coordination host has found a range of applications in catalysis and beyond. Early work focused on characterizing and understanding this host and its various host–guest phenomena, which paved the path for exploiting these features to selectively promote desirable chemistries, including cyclizations, rearrangements, and bimolecular reactions. Although this early work matured into achievements of catalysis with dramatic rate accelerations as well as enantioenrichment, the afforded products were typically identical to those produced by background reactions that occurred outside of the host microenvironment. This Account describes our recent developments in the application of these anionic tetrahedral hosts as catalysts for organic and organometallic transformation. Inspiration from natural systems and unmet synthetic challenges led to supramolecular catalysis displaying unique divergences in reactivity to give products that are inaccessible from bulk solution. Additionally, these tetrahedral assemblies have been shown to catalyze a diverse range of transformations with notable rate acceleration over the uncatalyzed background reaction. The pursuit of complexity beyond supramolecular catalysis has since led to the integration of these tetrahedral catalysts in tandem with natural enzymes, as well as their application to dual catalysis to realize challenging synthetic reactions. Variation in the structure, including size and charge, of these tetrahedral catalysts has enabled recent studies that provide insights into connections between specific structural features of these hosts and their reactivities. These mechanistic studies reveal that the solvent exclusion properties, hydrophobic effects, confinement effects and electrostatic effects play important roles in the observed catalysis. Moreover, these features may be leveraged for the design of supramolecular catalysis beyond those described in this Account. Finally, the supramolecular chemistry detailed in this Account has presented the opportunity to emulate some of the mechanisms nature engages to achieve catalysis; however, this relationship need not be entirely unidirectional, as the examples describe herein can stand as simplified model systems for unravelling more complex biological processes.
Herein is reported the first asymmetric utilization of aryldiazonium cations as a source of electrophilic nitrogen. This is achieved through a chiral anion phase‐transfer pyrroloindolinization ...reaction that forms C3‐diazenated pyrroloindolines from simple tryptamines and aryldiazonium tetrafluoroborates. The title compounds are obtained in up to 99 % yield and 96 % ee. The air‐ and water‐tolerant reaction allows electronic and steric diversity of the aryldiazonium electrophile and the tryptamine core.
Live and let diazene: Chiral anion phase transfer of aryldiazonium cations has been utilized to prepare C3‐diazenated pyrroloindolines. The air‐ and water‐tolerant reaction allows electronic and steric diversity in the aryldiazonium electrophile and the tryptamine core, with the products being obtained in up to 99 % yield and 96 % ee (MTBE=methyl tert‐butyl ether).
The limited selectivity of existing CO2 reduction catalysts and rising levels of CO2 in the atmosphere necessitate the identification of specific structure–reactivity relationships to inform catalyst ...development. Herein, we develop a predictive framework to tune the selectivity of CO2 reduction on Cu by examining a series of polymeric and molecular modifiers. We find that protic species enhance selectivity for H2, hydrophilic species enhance formic acid formation, and cationic hydrophobic species enhance CO selectivity. ReaxFF reactive molecular dynamics simulations indicate that the hydrophilic/hydrophobic modifiers influence the formation of surface hydrides, which yield formic acid or H2. These observations offer insights into how these modifiers influence catalytic behavior at the non-precious Cu surface and may aid in the future implementation of organic structures in CO2 reduction devices.
Reductive elimination of carbon-carbon bonds occurs in numerous metal-catalysed reactions. This process is well documented for a variety of transition metal complexes. However, carbon-carbon bond ...reductive elimination from a limited number of Au(III) complexes has been shown to be a slow and prohibitive process that generally requires elevated temperatures. Herein we show that oxidation of a series of mono- and bimetallic Au(I) aryl complexes at low temperature generates observable Au(III) and Au(II) intermediates. We also show that aryl-aryl bond reductive elimination from these oxidized species is not only among the fastest observed for any transition metal, but is also mechanistically distinct from previously studied alkyl-alkyl and aryl-alkyl reductive eliminations from Au(III).
A highly enantioselective carboalkoxylation of alkynes catalyzed by cationic (DTBM-MeO-Biphep)gold(I) complexes is reported. Various optically active β-alkoxyindanone derivatives were obtained in ...good yields with high enantioselectivities. Furthermore, this methodology was extended to the enantioselective synthesis of 3-methoxycyclopentenones. The reaction is proposed to proceed through an enantioselective cyclization of intermediates containing vinylgold(I) and prochiral oxocarbenium moieties.
Nearly one hundred years ago, the fermentative production of acetone by Clostridium acetobutylicum provided a crucial alternative source of this solvent for manufacture of the explosive cordite. ...Today there is a resurgence of interest in solventogenic Clostridium species to produce n-butanol and ethanol for use as renewable alternative transportation fuels. Acetone, a product of acetone-n-butanol-ethanol (ABE) fermentation, harbours a nucleophilic α-carbon, which is amenable to C-C bond formation with the electrophilic alcohols produced in ABE fermentation. This functionality can be used to form higher-molecular-mass hydrocarbons similar to those found in current jet and diesel fuels. Here we describe the integration of biological and chemocatalytic routes to convert ABE fermentation products efficiently into ketones by a palladium-catalysed alkylation. Tuning of the reaction conditions permits the production of either petrol or jet and diesel precursors. Glyceryl tributyrate was used for the in situ selective extraction of both acetone and alcohols to enable the simple integration of ABE fermentation and chemical catalysis, while reducing the energy demand of the overall process. This process provides a means to selectively produce petrol, jet and diesel blend stocks from lignocellulosic and cane sugars at yields near their theoretical maxima.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A combination of the advantages of homogeneous and heterogeneous catalysis could enable the development of sustainable catalysts with novel reactivity and selectivity. Although heterogeneous ...catalysts are often recycled more easily than their homogeneous counterparts, they can be difficult to apply in traditional organic reactions and modification of their properties towards a desired reactivity is, at best, complex. In contrast, tuning the properties of homogeneous catalysts by, for example, modifying the ligands that coordinate a metal centre is better understood. Here, using olefin cyclopropanation reactions catalysed by dendrimer-encapsulated Au nanoclusters as examples, we demonstrate that changing the dendrimer properties allows the catalytic reactivity to be tuned in a similar fashion to ligand modification in a homogeneous catalyst. Furthermore, we show that these heterogeneous catalysts employed in a fixed-bed flow reactor allow fine control over the residence time of the reactants and thus enables the control over product distribution in a way that is not easily available for homogeneous catalysts.
Selective reactions on structures of high complexity can move beyond the mind’s eye and proof-of-principle. Enhanced understanding of noncovalent interactions and their interdependence, revealed ...through analysis of multiple parameters, should accelerate the discovery of efficient reactions in highly complex molecular environments.
Herein we report the mechanism of oxidative addition of CF3I to Au(I), and remarkably fast Caryl–CF3 bond reductive elimination from Au(III) cations. CF3I undergoes a fast, formal oxidative ...addition to R3PAuR′ (R = Cy, R′ = 3,5-F2-C6H4, 4-F-C6H4, C6H5, 4-Me-C6H4, 4-MeO-C6H4, Me; R = Ph, R′ = 4-F-C6H4, 4-Me-C6H4). When R′ = aryl, complexes of the type R3PAu(aryl)(CF3)I can be isolated and characterized. Mechanistic studies suggest that near-ultraviolet light (λmax = 313 nm) photoinitiates a radical chain reaction by exciting CF3I. Complexes supported by PPh3 undergo reversible phosphine dissociation at 110 °C to generate a three-coordinate intermediate that undergoes slow reductive elimination. These processes are quantitative and heavily favor Caryl–I reductive elimination over Caryl–CF3 reductive elimination. Silver-mediated halide abstraction from all complexes of the type R3PAu(aryl)(CF3)I results in quantitative formation of Ar–CF3 in less than 1 min at temperatures as low as −10 °C.
The development of an enantioselective palladium-catalyzed 1,1-fluoroarylation of unactivated aminoalkenes is described. The reaction uses arylboronic acids as the arene source and Selectfluor as the ...fluorine source to generate benzylic fluorides in good yields with excellent enantioselectivities. This transformation, likely proceeding through an oxidative Heck mechanism, affords 1,1-difunctionalized alkene products.
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