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Bis(pertrifluoromethylcatecholato)silane: Extreme Lewis Acidity Broadens the Catalytic Portfolio of Silicon.

ABSTRACT: Given its earth abundance, silicon is ideal for constructing Lewis acids of use in catalysis or materials science. Neutral silanes were limited to moderate Lewis acidity, until halogenated catecholato ligands provoked a significant boost. However, catalytic applications of bis(perhalocatecholato)silanes were suffering from very poor solubility and unknown deactivation pathways. In this work, the novel per(trifluoromethyl)catechol, H₂ cat^CF3 , and adducts of its silicon complex Si(cat^CF3 )₂ (1) are described. According to the computed fluoride ion affinity, 1 ranks among the strongest neutral Lewis acids currently accessible in the condensed phase. The improved robustness and affinity of 1 enable deoxygenations of aldehydes, ketones, amides, or phosphine oxides, and a carbonyl-olefin metathesis. All those transformations have never been catalyzed by a neutral silane. Attempts to obtain donor-free 1 attest to the extreme Lewis acidity by stabilizing adducts with even the weakest donors, such as benzophenone or hexaethyl disiloxane.

SUBMITTER: Thorwart T

PROVIDER: S-EPMC8361710 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Lewis Acidity of Bis(perfluorocatecholato)silane: Aldehyde Hydrosilylation Catalyzed by a Neutral Silicon Compound.

Project description:Bis(perfluorocatecholato)silane Si(cat(F))2 was prepared, and stoichiometric binding to Lewis bases was demonstrated with fluoride, triethylphosphine oxide, and N,N'-diisopropylbenzamide. The potent Lewis acidity of Si(cat(F))2 was suggested from catalytic hydrosilylation and silylcyanation reactions with aldehydes. Mechanistic studies of hydrosilylation using an optically active silane substrate, R-(+)-methyl-(1-naphthyl)phenylsilane, proceeded with predominant stereochemical retention at silicon, consistent with a carbonyl activation pathway. The enantiospecificity was dependent on solvent and salt effects, with increasing solvent polarity or addition of NBu4BAr(F)4 leading to a diminished enantiomeric ratio. The medium effects are consistent with an ionic mechanism, wherein hydride transfer occurs prior to silicon-oxygen bond formation.

| S-EPMC4428610 | biostudies-literature

Bis(perfluoropinacolato)silane: A Neutral Silane Lewis Superacid Activates Si-F Bonds.

Project description:Despite the earth abundance and easy availability of silicon, only few examples of isolable neutral silicon centered Lewis superacids are precedent in the literature. To approach the general drawbacks of limited solubility and unselective deactivation pathways, we introduce a Lewis superacid, based on perfluorinated pinacol substituents. The compound is easily synthesized on a gram-scale as the corresponding acetonitrile mono-adduct 1⋅(MeCN) and was fully characterized, including single crystal X-ray diffraction analysis (SC-XRD) and state-of-the-art computations. Lewis acidity investigations by the Gutmann-Beckett method and fluoride abstraction experiments indicate a Lewis superacidic nature. The challenging Si-F bond activation of Et3 SiF is realized and promising catalytic properties are demonstrated, consolidating the potential applicability of silicon centered Lewis acids in synthetic catalysis.

| S-EPMC9298387 | biostudies-literature

Silicon Tetrakis(trifluoromethanesulfonate): A Simple Neutral Silane Acting as a Soft and Hard Lewis Superacid.

Project description:A facile synthesis and isolation of pristine silicon tetrakis(trifluoromethanesulfonate), Si(OTf)4 , is reported, acting as the first neutral silicon-based Lewis superacid suitable towards soft and hard Lewis bases. Its OTf groups have a dual function: they are excellent leaving groups and modulate the degree of reactivity towards soft and hard Lewis bases. Exposed to soft Lewis donors, Si(OTf)4 leads to [L2 Si(OTf)4 ] complexes (L=isocyanide, thioether and carbonyl compounds) with retention of all Si-OTf bonds. In contrast, it can cleave C-X bonds (X=F, Cl) of hard organic Lewis bases with a high tendency to form SiX4 (X=F, Cl) after halide/triflate exchange. Most notable, Si(OTf)4 allows a gentle oxydefluorination of mono- and bis(trifluoromethyl)benzenes, resulting in the formation of the corresponding benzoylium species, which are stabilized by the weakly coordinating [Si(OTf)6 ] dianion.

| S-EPMC8252640 | biostudies-literature

Thermally Induced Silane Dehydrocoupling on Silicon Nanostructures.

Project description:Organic trihydridosilanes can be grafted to hydrogen-terminated porous Si nanostructures with no catalyst. The reaction proceeds efficiently at 80 °C, and it shows little sensitivity to air or water impurities. The modified surfaces are stable to corrosive aqueous solutions and common organic solvents. Octadecylsilane H3 Si(CH2 )17 CH3 , and functional silanes H3 Si(CH2 )11 Br, H3 Si(CH2 )9 CH=CH2 , and H3 Si(CH2 )2 (CF2 )5 CF3 are readily grafted. When performed on a mesoporous Si wafer, the perfluoro reagent yields a superhydrophobic surface (contact angle 151°). The bromo-derivative is converted to azide, amine, or alkyne functional surfaces via standard transformations, and the utility of the method is demonstrated by loading of the antibiotic ciprofloxaxin (35 % by mass). When intrinsically photoluminescent porous Si films or nanoparticles are used, photoluminescence is retained in the grafted products, indicating that the chemistry does not introduce substantial nonradiative surface traps.

| S-EPMC5074321 | biostudies-literature

Molecular Bismuth Cations: Assessment of Soft Lewis Acidity.

Project description:Three-coordinate cationic bismuth compounds [Bi(diaryl)(EPMe3 )][SbF6 ] have been isolated and fully characterized (diaryl=[(C6 H4 )2 C2 H2 ]2- , E=S, Se). They represent rare examples of molecular complexes with Bi⋅⋅⋅EPR3 interactions (R=monoanionic substituent). The 31 P NMR chemical shift of EPMe3 has been found to be sensitive to the formation of LA⋅⋅⋅EPMe3 Lewis acid/base interactions (LA=Lewis acid). This corresponds to a modification of the Gutmann-Beckett method and reveals information about the hardness/softness of the Lewis acid under investigation. A series of organobismuth compounds, bismuth halides, and cationic bismuth species have been investigated with this approach and compared to traditional group 13 and cationic group 14 Lewis acids. Especially cationic bismuth species have been shown to be potent soft Lewis acids that may prefer Lewis pair formation with a soft (S/Se-based) rather than a hard (O/N-based) donor. Analytical techniques applied in this work include (heteronuclear) NMR spectroscopy, single-crystal X-ray diffraction analysis, and DFT calculations.

| S-EPMC7818483 | biostudies-literature

Lewis Acidic Boranes, Lewis Bases, and Equilibrium Constants: A Reliable Scaffold for a Quantitative Lewis Acidity/Basicity Scale.

Project description:A quantitative Lewis acidity/basicity scale toward boron-centered Lewis acids has been developed based on a set of 90 experimental equilibrium constants for the reactions of triarylboranes with various O-, N-, S-, and P-centered Lewis bases in dichloromethane at 20 °C. Analysis with the linear free energy relationship log KB =LAB +LBB allows equilibrium constants, KB , to be calculated for any type of borane/Lewis base combination through the sum of two descriptors, one for Lewis acidity (LAB ) and one for Lewis basicity (LBB ). The resulting Lewis acidity/basicity scale is independent of fixed reference acids/bases and valid for various types of trivalent boron-centered Lewis acids. It is demonstrated that the newly developed Lewis acidity/basicity scale is easily extendable through linear relationships with quantum-chemically calculated or common physical-organic descriptors and known thermodynamic data (ΔH BF3 ). Furthermore, this experimental platform can be utilized for the rational development of borane-catalyzed reactions.

| S-EPMC7985883 | biostudies-literature

Fluxionate Lewis acidity of the Zn2+ ion in carboxypeptidase A.

Project description:Competitive inhibition constants Ki for a series of phenol-ring-substituted derivatives of alpha-(2-hydroxyphenyl)benzenepropanoic acid have been ascertained by observing their influence on the catalytic hydrolysis of a peptide substrate by the zinc enzyme carboxypeptidase A. The pH-dependence of Ki shows that binding is maximal between two pKa values: one is that of the phenol group of the inhibitor, and the other uniformly has a value of 6, the pKa of a Zn(2+)-bound water molecule on the enzyme in the absence of substrate or inhibitor. This is the dependence expected if phenolate binds to the Zn2+ displacing its bound H2O/HO-. A log-log plot of the dissociation constants for the productive forms of inhibitor plus enzyme versus the acid dissociation constants of the phenolic residues in the inhibitors yields a straight line with a slope of +0.76. This number indicates that the active-site metal ion has special capacity for dispersing negative charge, such as builds up on the oxygen atom of a carboxamide group undergoing nucleophilic addition.

| S-EPMC1132148 | biostudies-other

An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids.

Project description:A new NMR-based Lewis acidity scale is suggested and its application is demonstrated for a family of silyl Lewis acids. The reaction of p-fluorobenzonitrile (FBN) with silyl cations that are internally stabilized by interaction with a remote chalcogenyl or halogen donor yields silylated nitrilium ions with the silicon atom in a trigonal bipyramidal coordination environment. The 19 F NMR chemical shifts and the 1 J(CF) coupling constants of these nitrilium ions vary in a predictable manner with the donor capability of the stabilizing group. The spectroscopic parameters are suitable probes for scaling the acidity of Lewis acids. These new probes allow for the discrimination between very similar Lewis acids, which is not possible with conventional NMR tests, such as the well-established Gutmann-Beckett method.

| S-EPMC6899571 | biostudies-literature

Bis(2-naphthyl-meth-yl)diphenyl-silane.

Project description:The title compound, C(34)H(28)Si, was prepared as an inter-nal standard for diffusion-ordered NMR spectroscopy. The four ligands are arranged tetra-hedrally around the Si atom. The two naphthalene systems are nearly perpendicular, making an angle of 86.42?(4)° with one another. A naphthalene system and a phenyl ring are also nearly perpendicular, making an angle of 86.18?(6)° with one another. In the crystal, the mol-ecules pack in columns parallel to the a axis, and exhibit arene C-H??(arene) inter-actions both within and between columns.

| S-EPMC2980158 | biostudies-literature

A Quantum-chemical Analysis on the Lewis Acidity of Diarylhalonium Ions.

Project description:Cyclic diaryliodonium compounds like iodolium derivatives have increasingly found use as noncovalent Lewis acids in the last years. They are more stable toward nucleophilic substitution than acyclic systems and are markedly more Lewis acidic. Herein, this higher Lewis acidity is analyzed and explained via quantum-chemical calculations and energy decomposition analyses. Its key origin is the change in energy levels and hybridization of iodine's orbitals, leading to both more favorable electrostatic interaction and better charge transfer. Both of the latter seem to contribute in similar fashion, while hydrogen bonding as well as steric repulsion with the phenyl rings play at best a minor role. In comparison to iodolium, bromolium and chlorolium are less Lewis acidic the lighter the halogen, which is predominantly based on less favorable charge-transfer interactions.

| S-EPMC10092059 | biostudies-literature

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