H-Bonded Counterion-Directed Catalysis: Enantioselective Gold(I)-Catalyzed Addition to 2-Alkynyl Enones as a Case Study

H-bonded counterion-directed catalysis (HCDC) is a strategy wherein a chiral anion that is hydrogen-bonded to the achiral ligand of a metal complex is responsible for enantioinduction. In this article we present the application of H-bonded counterion-directed catalysis to the Au(I)-catalyzed enantioselective tandem cycloisomerization-addition reaction of 2-alkynyl enones. Following the addition of C-, N- or O-centered nucleophiles, bicyclic furans were obtained in moderate to excellent yield and enantioselectivity (28 examples, 59-96 % yield, 62 : 38 to 95 : 5 er). The optimal catalytic system, comprising a phosphinosquaramide Au(I) chloride complex and a BINOL-derived phosphoramidate Ag(I) salt, was selected in a combinatorial fashion from a larger library with the help of high-throughput screening. An enantioselectivity switch of ca. 120 Δee% was observed upon addition of the achiral Au(I) component to the Ag(I) salt.

Helical Chiral N-Heterocyclic Carbene Ligands in Enantioselective Gold Catalysis

The first chiral helicene-NHC gold(I) complexes efficient in enantioselective catalysis were prepared. The L-shaped chiral ligand is composed of an imidazo[1,5-a]pyridin-3-ylidene (IPy) scaffold laterally substituted by a configurationally stable [5]-helicenoid unit. The chiral information was introduced in a key post-functionalization step of a NHC-gold(I) complex bearing a symmetrical anionic fluoreno[5]helicene substituent, leading to a racemic mixture of complexes featuring three correlated elements of chirality, namely central, axial and helical chirality. After HPLC enantiomeric resolution, X-ray crystallography and theoretical calculations enabled structural and stereochemical characterization of these configurationally stable NHC-gold(I) complexes. The high potential in asymmetric catalysis is demonstrated in the benchmark cycloisomerization of N-tethered 1,6-enynes with up to 95 : 5 er.

Carbohydrate‐functionalized triazolylidene iridium complexes: hydrogenation catalysis in water with asymmetric induction

Two sets of carbohydrate‐NHC hybrid iridium complexes were synthesised in order to combine properties of carbohydrates and triazolylidene (trz) ligands in organometallic catalysis. One set features a direct trz linkage to the anomeric carbohydrate carbon, while the second set is comprised of an ethyl linker between the two functional units. Deprotection of the carbohydrate afforded hybrid complexes that efficiently catalyse the direct hydrogenation of ketones in water. The catalytic activity of the hybrid complexes was influenced by the pH of the aqueous medium and surpassed the activity of carbohydrate‐free or acetyl‐protected analogues (>90 % vs 13 % yield). While no enantiomeric induction was observed for the ethyl‐linked hybrids, a moderate enantiomeric excess (ee) was induced by the directly linked systems. Moreover, these carbohydrate‐trz hybrid complexes displayed mixed inhibitory activity towards a glycosidase from H. orenii that contain a glucose binding site.

Au⋅⋅⋅H-C Hydrogen Bonds as Design Principle in Gold(I) Catalysis

Secondary ligand-metal interactions are decisive in many catalytic transformations. While arene-gold interactions have repeatedly been reported as critical structural feature in many high-performance gold catalysts, we herein report that these interactions can also be replaced by Au⋅⋅⋅H-C hydrogen bonds without suffering any reduction in catalytic performance. Systematic experimental and computational studies on a series of ylide-substituted phosphines featuring either a PPh3 (Ph YPhos) or PCy3 (Cy YPhos) moiety showed that the arene-gold interaction in the aryl-substituted compounds is efficiently compensated by the formation of Au⋅⋅⋅H-C hydrogen bonds. The strongest interaction is found with the C-H moiety next to the onium center, which due to the polarization results in remarkably strong interactions with the shortest Au⋅⋅⋅H-C hydrogen bonds reported to date. Calorimetric studies on the formation of the gold complexes further confirmed that the Ph YPhos and Cy YPhos ligands form similarly stable complexes. Consequently, both ligands showed the same catalytic performance in the hydroamination, hydrophenoxylation and hydrocarboxylation of alkynes, thus demonstrating that Au⋅⋅⋅H-C hydrogen bonds are equally suited for the generation of highly effective gold catalysts than gold-arene interactions. The generality of this observation was confirmed by a comparative study between a biaryl phosphine ligand and its cyclohexyl-substituted derivative, which again showed identical catalytic performance. These observations clearly support Au⋅⋅⋅H-C hydrogen bonds as fundamental secondary interactions in gold catalysts, thus further increasing the number of design elements that can be used for future catalyst construction.

New-Generation Ligand Design for the Gold-Catalyzed Asymmetric Activation of Alkynes

Gold(I) catalysts are ideal for the activation of alkynes under very mild conditions. However, unlike allenes or alkenes, the triple bond of alkynes cannot be prochiral. In addition, the linear coordination displayed by gold(I) complexes places the chiral ligand far away from the substrate resulting in an inefficient transfer of chiral information. This poses a significant challenge for the achievement of high enantiocontrol in gold(I)-catalyzed reactions of alkynes. Although considerable progress on enantioselective gold(I)-catalyzed transformations has recently been achieved, the asymmetric activation of non-prochiral alkyne-containing small molecules still represents a great challenge. Herein we summarize recent advances in intra- and intermolecular enantioselective gold(I)-catalyzed reactions involving alkynes, discussing new chiral ligand designs that lie at the basis of these developments. We also focus on the mode of action of these catalysts, their possible limitations towards a next-generation of more efficient ligand designs. Finally, square planar chiral gold(III) complexes, which offer an alternative to chiral gold(I) complexes, are also discussed.

Merging Molecular Recognition and Gold(I) Catalysis with Triphoscalix[6]arene Ligands

We report the synthesis and characterization of novel triphosphine calix[6]arene ligands. These supramolecular wheels, with recognition features governed by the hydrogen-bonding domain, were employed to synthesize multitasking trinuclear gold(I) complexes as a new platform for the synthesis of interwoven (pseudo)rotaxane species. In parallel, the multivalent, metal-bonded upper rim displayed catalytic features promoting highly selective gold-catalyzed cycloisomerization reactions of 1,6-enynes.

Enantioselective Alkoxycyclization of 1,6-Enynes with Gold(I)-Cavitands: Total Synthesis of Mafaicheenamine C

Chiral gold(I)-cavitand complexes have been developed for the enantioselective alkoxycyclization of 1,6-enynes. This enantioselective cyclization has been applied for the first total synthesis of carbazole alkaloid (+)-mafaicheenamine C and its enantiomer, establishing its configuration as R. The cavity effect was also evaluated in the cycloisomerization of dienynes. A combination of experiments and theoretical studies demonstrates that the cavity of the gold(I) complexes forces the enynes to adopt constrained conformations, which results in the high observed regio- and stereoselectivities.

Programmed Synthesis of Hepta-Differentiated β-Cyclodextrin: 1 out of 117655 Arrangements

Cyclodextrin poly-functionalization has fueled progress in their use in multiple applications such as enzyme mimicry, but also in the polymer sciences, luminescence, as sensors or for biomedical applications. However, regioselective access to a given pattern of functions on β-cyclodextrin is still very limited. We uncover a new orienting group, the thioacetate, that expands the toolbox available for cyclodextrin poly-hetero-functionalization using diisobutylaluminum hydride (DIBAL-H) promoted debenzylation. The usefulness of this group is illustrated in the first synthesis of a precisely hepta-hetero-functionalized β-cyclodextrin. By way of comparison, a random hepta-functionalization would give 117655 different molecules. This synthesis is not simply the vain quest for the Holy Grail of CD hetero-functionalization, but it illustrates the versatility of the DIBAL-H oriented hetero-functionalization strategy, opening the way to a multitude of useful functionalization patterns for new practical applications.

Gold Catalysis in (Supra)Molecular Cages to Control Reactivity and Selectivity

Gold catalysis has experienced a tremendous development over the past decades, and is nowadays widely used in organic synthesis to perform chemical transformations of π-bond-containing molecules. Catalyst development has been based mostly on ligand development and counter-ion strategies. More recently, the encapsulation of gold catalysts in (supra)molecular cages was explored as a new way to control selectivity and reactivity of gold catalysts. In this review, we describe the cages that have been employed as hosts for gold complexes, along with their impact on the catalytic performance. Covalent and supramolecular approaches to encapsulate single metal complexes will be described and the impact on the catalytic performance will be discussed. Also, recent strategies to pre-organize multiple metal centers will be discussed.

Confinement Effects in Catalysis Using Well-Defined Materials and Cages

This review focuses on the effects that confinement of molecular and heterogeneous catalysts with well-defined structure has on the selectivity and activity of these systems. A general introduction about catalysis and how the working principles of enzymes can be used as a source of inspiration for the preparation of catalysts with enhanced performance is provided. Subsequently, relevant studies demonstrate the importance of second coordination sphere effects in a broad sense (in homogeneous and heterogeneous catalysis). Firstly, we discuss examples involving zeolites, MOFs and COFs as heterogeneous catalysts with well-defined structures where confinement influences catalytic performance. Then, specific cases of homogeneous catalysts where non-covalent interactions determine the selectivity and activity are treated in detail. This includes examples based on cyclodextrins, calix[n]arenes, cucurbit[n]urils, and self-assembled container molecules. Throughout the review, the impact of confined spaces is emphasized and put into context, in order to get a better understanding of the effects of confinement on catalyst performance. In addition, this analysis intends to showcase the similarities between homogeneous and heterogeneous catalysts, which may aid the development of novel strategies.

D3h -Symmetric Porphyrin-Based Rigid Macrocyclic Ligands for Multicofacial Multinuclear Complexes in a One-Nanometer-Sized Cavity

The one-step synthesis of D3h -symmetric cyclic porphyrin trimers 1 composed of three 2,2'-[4,4'-bis(methoxycarbonyl)]bipyridyl moieties and three porphyrinatozinc moieties was achieved from a nickel-mediated reductive coupling of meso-5,15-bis(6-chloro-4-methoxycarbonylpyrid-2-yl)porphyrinatozinc. Although cyclic trimers 1 were obtained as a mixture that included other cyclic and acyclic porphyrin oligomers, an extremely specific separation was observed only for cyclic trimers 1 when using columns of silica gel modified with pyrenylethyl, cyanopropyl, and other groups. Structural analysis of cyclic trimers 1 was carried out by means of NMR spectroscopy and X-ray crystallography. Treatment of an η(3) -allylpalladium complex with a cyclic trimer gave a tris(palladium) complex containing three η(3) -allylpalladium groups inside the space, which indicated that the bipyridyl moieties inside the ring could work as bidentate metalloligands.

Gold(I) catalysis at extreme concentrations inside self-assembled nanospheres

Homogeneous transition-metal catalysis is a crucial technology for the sustainable preparation of valuable chemicals. The catalyst concentration is usually kept as low as possible, typically at mM or μM levels, and the effect of high catalyst concentration is hardly exploited because of solubility issues and the inherent unfavorable catalyst/substrate ratio. Herein, a self-assembly strategy is reported which leads to local catalyst concentrations ranging from 0.05 M to 1.1 M, inside well-defined nanospheres, whilst the overall catalyst concentration in solution remains at the conventional mM levels. We disclose that only at this high concentration, the gold(I) chloride is reactive and shows high selectivity in intramolecular CO and CC bond-forming cyclization reactions.

Confining phosphanes derived from cyclodextrins for efficient regio- and enantioselective hydroformylation

Two confining phosphane ligands derived from either α- or β-cyclodextrin produce singly P(III) -ligated metal complexes with unusual coordination spheres. High-pressure NMR studies have revealed that rhodium hydride complexes of the same type are also formed under hydroformylation conditions. This unique feature strongly favors the formation of the branched aldehyde at the expense of the linear one with high enantioselectivity in the rhodium-catalyzed hydroformylation of styrene.