Synthesis of Structurally Varied 1,3-Disiloxanediols and Their Activity as Anion-Binding Catalysts

Construction of Si-Stereogenic Silanols by Palladium-Catalyzed Enantioselective C-H Alkenylation

The construction of silicon-stereogenic silanols via Pd-catalyzed intermolecular C-H alkenylation with the assistance of a commercially available L-pyroglutamic acid has been realized for the first time. Employing oxime ether as the directing group, silicon-stereogenic silanol derivatives could be readily prepared with excellent enantioselectivities, featuring a broad substrate scope and good functional group tolerance. Moreover, parallel kinetic resolution with unsymmetric substrates further highlighted the generality of this protocol. Mechanistic studies indicate that L-pyroglutamic acid could stabilize the Pd catalyst and provide excellent chiral induction. Preliminary computational studies unveil the origin of the enantioselectivity in the C-H bond activation step.

Chiral Silanols: Strategies and Tactics for Their Synthesis

Silanols are valuable and important compounds, which have found widespread applications in the field of materials science, synthetic chemistry, and medicinal chemistry. Although a handful of approaches have been developed for the synthesis of various silanols, access to enantioenriched silicon-stereogenic silanols remains underdeveloped. This Concept article intends to summarize and highlight recent advances in the construction of silicon-stereogenic silanols and endeavors to encourage further research in this area.

Hidden silylium-type reactivity of a siloxane-based phosphonium-hydroborate ion pair

A new class of siloxane-based cations with hidden silylium-type reactivity is provided, which, in combination with an arylborate counteranion, initiates a highly selective para-C(sp²)-F defunctionalization of a perfluorinated aryl group. The hydrodefluorinated aryl borane is obtained as a crystalline solid via continuous sublimation during the reaction. The heterocyclic six-membered cation could be obtained single-crystalline after dehydrogenative anion exchange. DFT calculations give insight into the bonding within the siloxane-based cation and the mechanism of the ion pair reaction.

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.

The Elusive Au(I)···H-O Hydrogen Bond: Experimental Verification

Our long-standing interest in atypical bonding situations has recently led us to target complexes in which a metallobasic gold(I) center is hydrogen-bonded to a nearby OH functionality. Here, we report on the synthesis and characterization of two neutral gold(I) indazol-3-ylidene complexes bearing a carbinol or silanol group at the 4-position. As indicated by X-ray diffraction, ¹H NMR spectroscopy, IR spectroscopy, and extensive computational modeling, the OH group of these derivatives is engaged in a bona fide Au···H-O interaction. In addition to shedding light on an elusive bonding situation, these results also indicate that increasing the acidity of the OH functionality is not necessarily beneficial to the stability of the Au(I)···H-O interaction.

Simple acyclic molecules containing a single charge-assisted O-H group can recognize anions in acetonitrile : water mixtures

Hydroxypyridinium and hydroxyquinolinium compounds containing acidic O-H groups attached to a cationic aromatic scaffold were synthesized, i.e. N-methyl-3-hydroxypyridinium (1⁺) and N-methyl-8-hydroxyquinolinium (2⁺). These very simple compounds are capable of binding to chloride very strongly in CD₃CN and with moderate strength in 9 : 1 CD₃CN : D₂O. Comparison with known association constants reveals that 1⁺ and 2⁺ bind chloride in CD₃CN or CD₃CN : D₂O with comparable affinities to receptors containing significantly more hydrogen bond donors and/or higher positive charges. Crystal structures of both compounds with coordinating anions were obtained, and feature short O-Hanion hydrogen bonds. A receptor containing two hydroxyquinolinium groups was also prepared. While the low solubility of this compound caused difficulties, we were able to demonstrate chloride binding in a competitive 1 : 1 CD₃CN : CD₃OD solvent mixture. Addition of sulfate to this compound results in the formation of a crystallographically-characterised solid state anion coordination polymer.

Enantioselective Si-H Insertion Reactions of Diarylcarbenes for the Synthesis of Silicon-Stereogenic Silanes

We report the first example of enantioselective, intermolecular diarylcarbene insertion into Si-H bonds for the synthesis of silicon-stereogenic silanes. Dirhodium(II) carboxylates catalyze an Si-H insertion using carbenes derived from diazo compounds where selective formation of an enantioenriched silicon center is achieved using prochiral silanes. Fourteen prochiral silanes were evaluated with symmetrical and prochiral diazo reactants to produce a total of 25 novel silanes. Adding an ortho substituent on one phenyl ring of a prochiral diazo enhances enantioselectivity up to 95:5 er with yields up to 98%. Using in situ IR spectroscopy, the impact of the off-cycle azine formation is supported based on the structural dependence for relative rates of diazo decomposition. A catalytic cycle is proposed with Si-H insertion as the rate-determining step, supported by kinetic isotope experiments. Transformations of an enantioenriched silane derived from this method, including selective synthesis of a novel sila-indane, are demonstrated.

Catalytic Synthesis of Oligosiloxanes Mediated by an Air Stable Catalyst, (C6F5)3B(OH2)

The utility of (C₆F₅)₃B(OH₂) as catalyst for the simple and environmentally benign synthesis of oligosiloxanes directly from hydrosilanes, is reported. This protocol offers several advantages compared to other methods of synthesizing siloxanes, such as mild reaction conditions, low catalyst loading, and a short reaction time with high yields and purity. The considerable H₂O-tolerance of (C₆F₅)₃B(OH₂) promoted a catalytic route to disiloxanes which showed >99% conversion of three tertiary silanes, Et₃SiH, PhMe₂SiH, and Ph₃SiH. Preliminary data on the synthesis of unsymmetrical disiloxanes (Si-O-Si') suggests that by modifying the reaction conditions and/or using a 1:1 combination of silane to silanol the cross-product can be favored. Intramolecular reactions of disilyl compounds with catalytic (C₆F₅)₃B(OH₂) led to the formation of novel bridged siloxanes, containing a Si-O-Si linkage within a cyclic structure, as the major product. Moreover, the reaction conditions enabled recovery and recycling of the catalyst. The catalyst was re-used 5 times and demonstrated excellent conversion for each substrate at 1.0 mol% catalyst loading. This seemingly simple reaction has a rather complicated mechanism. With the hydrosilane (R₃SiH) as the sole starting material, the fate of the reaction largely depends on the creation of silanol (R₃SiOH) from R₃SiH as these two undergo dehydrocoupling to yield a disiloxane product. Generation of the silanol is based on a modified Piers-Rubinsztajn reaction. Once the silanol has been produced, the mechanism involves a series of competitive reactions with multiple catalytically relevant species involving water, silane, and silanol interacting with the Lewis acid and the favored reaction cycle depends on the concentration of various species in solution.

A catalytic study of water dispersed gold nanoparticles for the hydrolytic oxidation of diorganosilanes – en route formation of a Pickering catalyst and synthesis of tetraorganodisiloxane-1,3-diols

This study unfolds the formation of a AuNP-stabilized Pickering catalyst (PIC) en route to the hydrolytic oxidation of diorganosilanes. The method offers a viable route for the synthesis of disiloxane-1,3-diols.

Kinetic and Binding Studies Reveal Cooperativity and Off-Cycle Competition for H-Bonding Catalysis with Silsesquioxane Silanols

The catalytic activity, kinetics, and quantification of H-bonding ability of incompletely condensed polyhedral oligomeric silsesquioxane (POSS) silanols are reported. POSS-triols, a homogeneous model for vicinal silica surface sites, exhibit enhanced H-bonding compared with other silanols and alcohols as quantified using a ³¹ P NMR probe. Evaluation of a Friedel-Crafts addition reaction shows that phenyl-POSS-triol is active as an H-bond donor catalyst whereas other POSS silanols studied are not. An in-depth kinetic study (using RPKA and VTNA) highlights the concentration-dependent H-bonding behavior of POSS-triols, which is attributed to intermolecular association forming an off-cycle dimeric species. Binding constants provide additional support for reduced H-bond ability at higher concentrations, which is attributed to competitive association. POSS-triol self-association disrupts H-bond donor abilities relevant for catalysis by reducing the concentration of active monomeric catalyst.

Anion coordination chemistry using O-H groups

This review covers significant advances in the use of O-H groups in anion coordination chemistry. The review focuses on the use of these groups in synthetic anion receptors, as well as more recent developments in transport, self-assembly and catalysis.

Metal-Free Synthesis of 1,3-Disiloxanediols and Aryl Siloxanols

The first example of metal-free oxidative hydrolysis of hydrido-siloxanes is reported. Both base-catalyzed and organocatalytic hydrolysis methods are demonstrated to transform 1,3-dihydrido-disiloxanes into 1,3-disiloxanediols. The first example of a chemoselective silane hydrolysis is demonstrated.

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.