Anion recognition by silanetriol in acetonitrile

Anion recognition ability and organocatalytic activity of a silanetriol are firstly presented by comparing with those of a series of silanol derivatives.

Enantiopure planar chiral [2.2]paracyclophanes: Synthesis and applications in asymmetric organocatalysis

This short review focuses on enantiopure planar chiral [2.2]paracyclophanes (pCps), a fascinating class of molecules that possess an unusual three-dimensional core and intriguing physicochemical properties. In the first part of the review, different synthetic strategies for preparing optically active pCps are described. Although classical resolution methods based on the synthesis and separation of diastereoisomeric products still dominate the field, recent advances involving the kinetic resolution of racemic compounds and the desymmetrization of meso derivatives open up new possibilities to access enantiopure key intermediates on synthetically useful scales. Due to their advantageous properties including high configurational and chemical stability, [2.2]paracyclophanes are increasingly employed in various research fields, ranging from stereoselective synthesis to material sciences. The applications of [2.2]paracyclophanes in asymmetric organocatalysis are described in the second part of the review. While historically enantiopure pCps have been mainly employed by organic chemists as chiral ligands in transition-metal catalysis, these compounds can also be used as efficient catalysts in metal-free reactions and may inspire the development of new transformations in the near future.

Catalytic asymmetric C-Si bond activation via torsional strain-promoted Rh-catalyzed aryl-Narasaka acylation

Atropisomers are important organic frameworks in bioactive natural products, drugs as well as chiral catalysts. Meanwhile, silanols display unique properties compared to their alcohol analogs, however, the catalytic synthesis of atropisomers bearing silanol groups is challenging. Here, we show a rhodium-catalyzed torsional strain-promoted asymmetric ring-opening reaction for the synthesis of α-silyl biaryl atropisomers. The reaction features a dynamic kinetic resolution of C(Ar)-Si bond cleavage, whose stereochemistry was controlled by a phosphoramidite ligand derived from (S)-3-methyl-1-((2,4,6-triisopropylphenyl)sulfonyl)piperazine. This work is a demonstration of an aryl-Narasaka acylation, where the C(Ar)-Si bond cleavage is promoted by the torsional strain of α, α’-disubstituted silafluorene.

Methods to access atropisomers bearing silanol groups are of value in organic synthesis. Here, the authors report a rhodium-catalyzed torsional strain-promoted asymmetric ring-opening reaction for the synthesis of α-silyl biaryl atropisomers.

Catalytic asymmetric oxo-Diels-Alder reactions with chiral atropisomeric biphenyl diols

New chiral atropisomeric biphenyl diols 3, 4 and 6 containing additional peripheral chiral centers with different steric bulkiness and/or electronic properties were synthesized. The X-ray crystal structure of 3 shows the formation of a supramolecular structure whereas that of 6, containing additional CF₃ substituents, shows the formation of a monomeric structure. Diols 1–6 were found to be active organocatalysts in oxo-Diels–Alder reactions in which 2 recorded a 72% ee with trimethylacetaldehyde as a substrate.

Catalysis by Networks of Cooperative Hydrogen Bonds

The main paradigm of today's chemistry is sustainability. In pursuing sustainability, we need to learn from chemical processes carried out by Nature and realize that Nature does not use either strong acids, or strong bases or fancy reagents to achieve outstanding chemical processes. Instead, enzyme activity leans on the cooperation of several chemical entities to avoid strong acids or bases or to achieve such an apparently simple goal as transferring a proton from an NuH unit to an E unit (NuH + E → Nu–EH). Hydrogen bond catalysis emerged strongly two decades ago in trying to imitate Nature and avoid metal catalysis. Now to mount another step in pursuing the goal of sustainability, the focus is upon cooperativity between the different players involved in catalysis. This chapter looks at the concept of cooperativity and, more specifically, (a) examines the role of cooperative hydrogen bonded arrays of the general type NuH⋯(NuH)n⋯NuH (i.e. intermolecular cooperativity) to facilitate general acid–base catalysis, not only in the solution phase but also under solvent-free and catalyst-free conditions, and, most important, (b) analyzes the capacity of designer chiral organocatalysts displaying intramolecular networks of cooperative hydrogen bonds (NCHBs) to facilitate enantioselective synthesis by bringing conformational rigidity to the catalyst in addition to simultaneously increasing the acidity of key hydrogen atoms so to achieve better complementarity in the highly polarized transition states.

Synthesis of paracyclophanes with planar and central chirality: kinetic resolution of [2.2]paracyclophane aldimines via palladium-catalyzed addition of arylboronic acids

Kinetic resolution of [2.2]paracyclophane aldimines was achieved through a palladium-catalyzed enantioselective arylation, providing access to chiral paracyclophanes with planar and central chirality as well as the recovered chiral aldimines.

NMR Quantification of Hydrogen-Bond-Activating Effects for Organocatalysts including Boronic Acids

The hydrogen-bonding activation for 66 organocatalysts has been quantified using a ³¹P NMR binding experiment with triethylphosphine oxide (TEPO). Diverse structural classes, including phenols, diols, silanols, carboxylic acids, boronic acids, and phosphoric acids, were examined with a variety of steric and electronic modifications to understand how the structure and secondary effects contribute to hydrogen-bonding ability and catalysis. Hammett plots demonstrate high correlation for the Δδ ³¹P NMR shift to Hammett parameters, establishing the ability of TEPO binding to predict electronic trends. Upon correlation to catalytic activity in a Friedel-Crafts addition reaction, data demonstrate that ³¹P NMR shifts correlate to catalytic activity better than p K_a values. Boronic acids were investigated, and ³¹P NMR binding experiments predicted strong hydrogen-bonding ability, for which catalytic activity was confirmed, resulting in the greatest rate enhancement observed in the Friedel-Crafts addition of all organocatalysts studied. A detailed investigation supports that boronic acid activation proceeds through hydrogen-bonding interactions and not coordination with the Lewis acidic boron center. Using ³¹P NMR spectroscopy offers a simple and rapid tool to quantify and predict hydrogen-bonding abilities for the design and applications of new organocatalysts and supramolecular synthons.

Reactivity of the Sterically Demanding Siloxanediol Mes2 Si(OH)(μ-O)Si(OH)Mes2 Towards Water and Ether Molecules

A series of isotopologues of the siloxanediol Mes₂ Si(OH)(μ-O)Si(OH)Mes₂ (3 a) (Mes=2,4,6-trimethylphenyl) were synthesized by reactions of the corresponding disiloxane precursors Mes₂ Si(μ-O)₂ SiMes₂ (2 a), Mes₂ Si(μ-¹⁷ O)₂ SiMes₂ (2 b) or Mes₂ Si(μ-¹⁸ O)₂ SiMes₂ (2 c) with an excess of H₂ O, H₂¹⁷ O or H₂¹⁸ O. NMR and IR signal assignments for the siloxanediols in benzene are supported by quantum-chemical calculations, which indicate small energy differences between trans and cis conformers, the latter of which exhibits an intramolecular hydrogen bond. ¹ H NMR as well as IR data suggest the presence of a mixture of both conformers in C₆ D₆ . Hydrogen-bonded adducts of Mes₂ Si(OH)(μ-O)Si(OH)Mes₂ with ethers such as diethylether, dimethoxyethane or dioxane were observed in the solid state, where they form polymeric chain-like structures. The latter appear to be stable only in the crystal. ¹⁷ O{¹ H} NMR and IR data in THF solution suggest an interaction of 3 a with at least one THF molecule, whereas diethylether appears not to interact. Water adducts form neither in solution nor in the solid state as indicated by NMR and ATR IR data. ¹⁷ O{¹ H} NMR and ESI-MS experiments illustrate the remarkably high stability of the siloxanediols towards water and show no evidence for intra- or intermolecular oxygen-exchange reactions. In marked contrast, a stepwise exchange of all three oxygen atoms-including the one in the Si-O-Si bridge-occurred in the gas phase, when [Mes₂ Si(¹⁸ OH)(μ-¹⁸ O)Si(¹⁸ O)Mes₂ ]^- was treated with H₂ O in the hexapole of an ESI FT-ICR mass spectrometer. The scrambling between the bridging and the other oxygen atoms likely proceeds through cyclic Si₂ O₂ intermediates.

Planar chiral desymmetrization of a two-layered cyclophane and control of dynamic helicity through the arrangement of two nonstereogenic centers

Dynamic helicity generated in a configurationally stable cyclophane is controlled through the arrangement of two nonstereogenic centers with different dimensions.

A hydrogen bonded molecular capsule versus a 3D network of tripodal organopolysilanols

1,3,5-Triethylbenzene based tripodal trisilanols were synthesized. X-ray, NMR and ESI-mass spectra disclosed the first example of an organotrisilanol molecular capsule based on a 1,3,5-triethylbenzene scaffold which showed encapsulation of H₂O or halogen anions in both the crystalline and solution states.

Reductive arene ortho-silanolization of aromatic esters with hydridosilyl acetals

The design and application of a single-pot, reductive arene C–H bond silanolization of esters for synthesis of ortho-formyl arylsilanols.

Quantification of electrophilic activation by hydrogen-bonding organocatalysts

A spectrophotometric sensor is described that provides a useful assessment of the LUMO-lowering provided by catalysts in Diels–Alder and Friedel–Crafts reactions. A broad range of 33 hydrogen-bonding catalysts was assessed with the sensor, and the relative rates in the above reactions spanned 5 orders of magnitude as determined via ¹H- and ²H NMR spectroscopic measurements, respectively. The differences between the maximum wavelength shift of the sensor with and without catalyst (Δλ_max^–1) were found to correlate linearly with ln(k_rel) values for both reactions, even though the substrate feature that interacts with the catalyst differs significantly (ketone vs nitro). The sensor provides an assessment of both the inherent reactivity of a catalyst architecture as well as the sensitivity of the reaction to changes within an architecture. In contrast, catalyst pK_a values are a poor measure of reactivity, although correlations have been identified within catalyst classes.

Exploring the potential of diarylacetylenediols as hydrogen bonding catalysts

In the course of a search for new classes of hydrogen bonding catalysts, we have examined diarylacetylenediols as potential catalysts for the Diels-Alder reaction. General and efficient methods have been developed for the preparation of these diols. Their structures were systematically modified, and increased activity was observed for those possessing an electron-withdrawing group on the aryl groups. The electron-deficient diarylacetylenediol catalysts, while more active, undergo spontaneous cyclization to the corresponding benzo[b]furans. A mechanism is postulated to explain this facile transformation. Computational studies performed on 2-ethynylphenol help to explain the effect of the alkyne on the conformation and hydrogen bond donating ability of the adjacent OH group. Finally, the crystal structure of one of the diols is reported, and it displays an intricate network of intermolecular hydrogen bonds.