Silyl Esters Research Articles

Ring Opening Reduction of Cyclic Anhydrides Catalyzed by Tris(pentafluorophenyl)borane Using Hydrosilanes as a Hydride Source.

Hydrosilanes are widely used as reducing agents in the reduction of carbonyl groups, and various catalysts have been developed for the activation of hydrosilanes, the majority of them being transition metal-based. A main-group-based Lewis acid tris(pentafluorophenyl)borane (BCF) has gained increasing attention due to its Lewis acidity and versatility, along with being nonmetal. Herein, we describe the BCF-catalyzed ring opening reduction of cyclic anhydrides using hydrosilanes as a source of hydrides. The reduction affords unsymmetrical bis(silyl) protected hydroxy acids, leading to an efficient way for the synthesis of silyl ester functionalities. The capability of forming protected hydroxy acids under mild conditions with high yields in one step is also advantageous. A range of hydrosilanes and cyclic anhydrides can be employed with quantitative conversion, high yields, relatively fast reaction time, and mild reaction conditions. NMR spectroscopy is used in the characterization of the products, along with gaining insight into the potential mechanism.

Combined Computational and Experimental Study Reveals Complex Mechanistic Landscape of Brønsted Acid-Catalyzed Silane-Dependent P═O Reduction.

The reaction mechanism of Brønsted acid-catalyzed silane-dependent P═O reduction has been elucidated through combined computational and experimental methods. Due to its remarkable chemo- and stereoselective nature, the Brønsted acid/silane reduction system has been widely employed in organophosphine-catalyzed transformations involving P(V)/P(III) redox cycle. However, the full mechanistic profile of this type of P═O reduction has yet to be clearly established to date. Supported by both DFT and experimental studies, our research reveals that the reaction likely proceeds through mechanisms other than the widely accepted "dual activation mode by silyl ester" or "acid-mediated direct P═O activation" mechanism. We propose that although the reduction mechanisms may vary with the substitution patterns of silane species, Brønsted acid generally activates the silane rather than the P═O group in transition structures. The proposed activation mode differs significantly from that associated with traditional Brønsted acid-catalyzed C═O reduction. The uniqueness of P═O reduction originates from the dominant Si/O═P orbital interactions in transition structures rather than the P/H-Si interactions. The comprehensive mechanistic landscape provided by us will serve as a guidance for the rational design and development of more efficient P═O reduction systems as well as novel organophosphine-catalyzed reactions involving P(V)/P(III) redox cycle.

Palladium-catalyzed siloxycarbonylation of alkenes to synthesize silyl esters

Synthesis of stable acyl intermediates and their transformation for the synthesis of carbonyl derivatives is highly desirable. We have developed a method for the synthesis of stable acyl intermediates, silyl esters, through siloxycarbonylation.

Synthesis of Cs-symmetrical C60 tetra-adducts via reactions of C60Cl6 with CH-acids and enol silyl ester.

Here we report the synthesis of novel fullerene derivatives with attached aliphatic residues based on the previously unknown reactions of chlorofullerene C60Cl6 with CH-acids and silyl enol ether.

SYNTHESIS AND EVALUATION OF THE CYTOTOXICITY OF FURANOLABDANOIDS CONTAINING A 3-AMINOPROPANOYL SUBSTITUENT IN THE FURAN RING

15-Acetyl- and 16-acetylfuranolabdanoids were synthesised by the acetylation of the methyl ester of lambertianic acid with acetic anhydride in the presence of Mg(ClO4)2 and used as starting compounds for the preparation of 15- or 16-(3-aminopropanoyl)methyllambertianates. The proposed approach involved silylation of 15-acetyl- or 16-acetylmethyllambertianate and the Mannich reaction of the resulting enol silyl ester with N,N-disubstituted methyleneiminium salts. The synthesized compounds possessed cytotoxicity on tumour cell lines CCRF CEM, MCF7 and PC-3 (MTT test). The compounds containing a 3-morpholinopropanoyl or 3-pyrrolidinopropanoyl substituent at the C-16 position of methyl lambertianate selectively inhibited the growth of human T-cell leukemia cells (GI50 was 5.8-6.1 μM) and exhibited cytotoxicity an order of magnitude higher than the parent compound lambertianic acid.

(Invited) Tether-Directed Regioselective Synthesis of Bispyrazolinofullerenes

The problematic regioselective synthesis of fullerene bisadducts was finely solved by Diederich with the introduction of the tether-directed functionalization.[1] Since then, many other examples have been described, most of them using double Bingel addition or Diels-Alder cycloaddition reactions. Surprisingly, although 1,3-dipolar cycloaddition reactions are the most popular procedure for functionalization of C60, the use of this reaction for regioselective synthesis of fullerene bisadducts.Here we present our results of selective synthesis of trans-3 or e-bispirazolinofullerenes using bisnitrilimine tethered by di-tertbutylsilylbis(trifluoromethanesulfonate) group [3][4]. Further rupture of the silyl ester group allows the preparation of new derivatives for selected applications.[1] L.Isaacs, R.F.Haldimann,F.Diederich,Angew.Chem.Int.Ed.Engl.1994,33,2339. [2] F. Diederich, R. Kessinger. Acc. Chem. Res. 1999, 32, 537. [3] V. Cuesta, M. Urbani, P. de la Cruz, L. Welte, J.F. Nierengarten, F. Langa, Chem. Comm. 2016, 52, 13205 [4] R. Caballero, M. Barrejón, J. Cerdá, J. Aragó, S. Seetharaman, P. de la Cruz, E. Ortí, F. D’Souza, F. Langa. J. Am. Chem. Soc. 2021, 143, 11199. Figure 1

Molecular simulation of selective coordination for various silyl ester Lewis base donors with MgCl2 in Ziegler–Natta catalysis

ABSTRACT In this article, we present the influence of substituents with different electron-donating (ED) powers on the coordination properties of bis(benzoyloxy)dimethylsilane (BDMS) on the MgCl2 support of Ziegler–Natta (ZN) catalysts through molecular simulation of different adsorption adducts. Thermodynamically, the most favourable adsorption structure was found with the diaminoborane substituent in a bridging mode on the (110) crystalline facet, representing a 37.63% increase at PBEh/TZVP in the adsorption strength of the original silyl ester. Generally, the higher the ED strength of the substituent, the higher the exergonicity of donor coordination. All of the new donors except those containing the nitro and formyl groups are preferably chemisorbed on the (110) surface. While the nitro, formyl, vinyl, and hydroxyl substitutions deteriorated the isotacticity of the polymer, a diaminoborane group was predicted to enhance the isotactic index by 306% relative to the original donor. Particularly, nitrophenyl could improve the isotactic index by 69% while its complexation remained in the thermodynamically controlled regime. Overall, the present DFT study demonstrated how a systematic exploration of substituents over a wide range of ED power can help in the design of new Lewis base donors for ZN catalysis.

Silyl Esters as Reactive Intermediates in Organic Synthesis

AbstractSilyl esters have been exploited as metastable reaction intermediates, both purposefully and unintentionally, since at least the 1960s. Their reactivity is broadly related to the substituents on the silicon, and in this way their properties can be readily modulated. Silyl esters have unique reactivity profiles that have been used to generate downstream products of a range of functionalities, and because of this many excellent methods for the synthesis of a variety of value-added chemicals have been developed. Furthermore, because of the frequent use of hydrosilanes as terminal reductants in catalytic processes, silyl ester intermediates are likely more commonly utilized by synthetic chemists than currently realized. This review comprehensively summarizes the reactions known to take advantage of reactive silyl ester intermediates and discusses examples of catalytic reactions that proceed in an unanticipated manner through silyl ester intermediates.1 Introduction2 Synthesis of Silyl Esters3 Making Amides from Silyl Esters3.1 Amidation Using Chlorosilanes3.2 Amidation Using Azasilanes3.3 Amidation Using Oxysilanes3.4 Amidation Using Hydrosilanes3.5 Amine Formation via Amidation/Reduction3.6 Miscellaneous4 Mechanistic Investigations of Amidation4.1 Mechanism of Amidation Using Chlorosilanes4.2 Mechanism of Amidation Using Hydrosilanes4.3 Mechanism of Amidation Using Oxy- or Azasilanes5 Making Esters from Silyl Esters6 Making Aldehydes, Alcohols, Amines, and Alkanes via Reduction6.1 Aldehyde Synthesis by Metal-Free Reduction6.2 Aldehyde Synthesis by Metal-Mediated Reduction6.3 Alcohol Synthesis by Metal-Mediated Reduction6.4 Amine Synthesis6.5 Alkane Synthesis by Metal-Free Reduction7 Making Acid Chlorides from Silyl Esters8 In Situ Generated Silyl Esters and Ramifications for Catalysis9 Conclusion

Unraveling the polishing processes and mechanisms of polyacrylate silyl ester based antifouling coating: A molecular dynamic study

Self-polishing coatings (SPCs) can renew its surface and consequently prevent the adhesion of marine fouling. In order to unravel the molecular mechanism, in this work, molecular dynamics (MD) simulations were conducted to simulate the polishing process of typical triisopropyl silyl acrylate (TIPSA). The results show that the poor static antifouling performance of TIPSA is due to the spontaneous adsorption of hydrolyzed side groups (silyl groups) onto the coating surface undertheir strong Van der Waals (vdW) nonbonding interactions. The infiltration of water molecules at the interface between hydrolyzed silyl groups and coating surface promotes the polishing process. However, when the number of permeated water molecules is not enough to screen the nonbonding interactions, the hydrolyzed silyl groups can be adsorbed on the resin matrix again. The formation of water channel in the hydrolyzed silyl layer further restrains the polishing process. In addition, the uneven distribution of hydrophilic groups in resin matrix surface also plays an important role in the polishing process of the TIPSA coating. This work deepens our understanding of the polishing process and mechanism of SPCs and might be able to open a new direction to further comprehend the antifouling mechanism.

Remolding and Deconstruction of Industrial Thermosets via Carboxylic Acid-Catalyzed Bifunctional Silyl Ether Exchange

Convenient strategies for the deconstruction and reprocessing of thermosets could improve the circularity of these materials, but most approaches developed to date do not involve established, high-performance engineering materials. Here, we show that bifunctional silyl ether, i.e., R'O-SiR2-OR'', (BSE)-based comonomers generate covalent adaptable network analogues of the industrial thermoset polydicyclopentadiene (pDCPD) through a novel BSE exchange process facilitated by the low-cost food-safe catalyst octanoic acid. Experimental studies and density functional theory calculations suggest an exchange mechanism involving silyl ester intermediates with formation rates that strongly depend on the Si-R2 substituents. As a result, pDCPD thermosets manufactured with BSE comonomers display temperature- and time-dependent stress relaxation as a function of their substituents. Moreover, bulk remolding of pDCPD thermosets is enabled for the first time. Altogether, this work presents a new approach toward the installation of exchangeable bonds into commercial thermosets and establishes acid-catalyzed BSE exchange as a versatile addition to the toolbox of dynamic covalent chemistry.

Aromatic 1,2-Azaborinin-1-yls as Electron-Withdrawing Anionic Nitrogen Ligands for Main Group Elements.

The 2-aryl-3,4,5,6-tetraphenyl-1,2-azaborinines 1-EMe3 and 2-EMe3 (E=Si, Sn; aryl=Ph (1), Mes (=2,4,6-trimethylphenyl, 2)) were synthesized by ring-expansion of borole precursors with N3 EMe3 -derived nitrenes. Desilylative hydrolysis of 1- and 2-SiMe3 yielded the corresponding N-protonated azaborinines, which were deprotonated with nBuLi or MN(SiMe3 )2 (M=Na, K) to the corresponding group 1 salts, 1-M and 2-M. While the lithium salts crystallized as monomeric Lewis base adducts, the potassium salts formed coordination polymers or oligomers via intramolecular K⋅⋅⋅aryl π interactions. The reaction of 1-M or 2-M with CO2 yielded N-carboxylate salts, which were derivatized by salt metathesis to methyl and silyl esters. Salt metathesis of 1-M or 2-M with methyl triflate, [Cp*BeCl] (Cp*=C5 Me5 ), BBr2 Ar (Ar=Ph, Mes, 2-thienyl), ECl3 (E=B, Al, Ga) and PX3 (X=Cl, Br) afforded the respective group 2, 13 and 15 1,2-azaborinin-2-yl complexes. Salt metathesis of 1-K with BBr3 resulted not only in N-borylation but also Ph-Br exchange between the endocyclic and exocyclic boron atoms. Solution 11 B NMR data suggest that the 1,2-azaborinin-2-yl ligand is similarly electron-withdrawing to a bromide. In the solid state the endocyclic bond length alternation and the twisting of the C4 BN ring increase with the sterics of the substituents at the boron and nitrogen atoms, respectively. Regression analyses revealed that the downfield shift of the endocyclic 11 B NMR resonances is linearly correlated to both the degree of twisting of the C4 BN ring and the tilt angle of the N-substituent. Calculations indicate that the 1,2-azaborinin-1-yl ligand has no sizeable π-donor ability and that the aromaticity of the ring can be subtly tuned by the electronics of the N-substituent.

In-vitro assessment of biological properties and antioxidant activity of leaf extracts of Moringa oleifera L.

Moringa oleifera is native to India and being used as medicinal and nutritive plant for thousands of years. The plant is an important source for essential nutrients and biochemical constituents. The present study is carried out to investigate the biochemical constituents through FT-IR, GC-MS analysis. The antimicrobial and antioxidant activity of the M. oleifera leaves extract are studied using DPPH assay and antimicrobial activity is observed by selecting various indicator microorganisms. In the present study, the methanolic extract is evaluated for its antioxidant activity and free radical scavenging activity was found through DPPH assay. The IC50 value is found to be 275μg/ml. The antimicrobial activity is studied through well diffusion assay and highest inhibitory activity is observed for E. coli and K. pneumoniae. Further, the biochemical constituents are identified through FTIR and GC-MS analysis and the presence of fatty acids such as dodecanoic acid, n-hexadecanoic acid, 11-octadecanoic acid, hexadecanoic acid trimethyl silyl ester and α-Linolenic acid trimethylsilyl ester are found. The fatty acids occupied major portion of the leaves and the other compounds identified through this analysis are diterpenoids, carotenoid pigments, silanes and diterpene alcohol. The leaves consist of a rich source of bioactive components with antimicrobial and antioxidant activity. The carotenoid pigments and omega fatty acids are considered as food supplements for boosting the immunity and also for scavenging of free radicals.

Silica‐Mediated Monohydrolysis of Dicarboxylic Esters

AbstractA new method for the monohydrolysis of dicarboxylic esters is presented, involving as key step a silanolysis at elevated temperatures at the silica gel surface. In the second step, the surface bound silyl esters are cleaved off under mild conditions, giving a straightforward and fast access to half esters. Based on recovered starting material generally yields well above 70 % are achieved, both, with stiff aromatic as well as flexible aliphatic substrates, as long as the ester groups involved are remote enough from each other. Otherwise competing reactions are becoming determinative, anhydride formation in the case of phthalates and decarbonylative fragmentation in the case of malonates. The new method was also successfully tested on a multigram scale with a minimalistic apparatus setup.

Siloxy Esters as Traceless Activators of Carboxylic Acids: Boron‐Catalyzed Chemoselective Asymmetric Aldol Reaction**

AbstractThe catalytic asymmetric aldol reaction is among the most useful reactions in organic synthesis. Despite the existence of many prominent reports, however, the late‐stage, chemoselective, catalytic, asymmetric aldol reaction of multifunctional substrates is still difficult to achieve. Herein, we identified that in situ pre‐conversion of carboxylic acids to siloxy esters facilitated the boron‐catalyzed direct aldol reaction, leading to the development of carboxylic acid‐selective, catalytic, asymmetric aldol reaction applicable to multifunctional substrates. Combining experimental and computational studies rationalized the reaction mechanism and led to the proposal of Si/B enediolates as the active species. The silyl ester formation facilitated both enolization and catalyst turnover by acidifying the α‐proton of substrates and attenuating poisonous Lewis bases to the boron catalyst.

Siloxy Esters as Traceless Activators of Carboxylic Acids: Boron-Catalyzed Chemoselective Asymmetric Aldol Reaction*.

The catalytic asymmetric aldol reaction is among the most useful reactions in organic synthesis. Despite the existence of many prominent reports, however, the late-stage, chemoselective, catalytic, asymmetric aldol reaction of multifunctional substrates is still difficult to achieve. Herein, we identified that in situ pre-conversion of carboxylic acids to siloxy esters facilitated the boron-catalyzed direct aldol reaction, leading to the development of carboxylic acid-selective, catalytic, asymmetric aldol reaction applicable to multifunctional substrates. Combining experimental and computational studies rationalized the reaction mechanism and led to the proposal of Si/B enediolates as the active species. The silyl ester formation facilitated both enolization and catalyst turnover by acidifying the α-proton of substrates and attenuating poisonous Lewis bases to the boron catalyst.

Trans-Selective hydrocyanation of ynoates, ynones and ynoic acids catalyzed by nucleophilic phosphines

trans-Selective hydrocyanation of ynoates and ynones in the presence of TMSCN and an alcohol additive are catalyzed by nucleophilic phosphines. The trisubstituted E-olefin products of anti-addition of hydrogen cyanide to the alkyne are produced with high regio- and stereoselectivity. The alcohol additive reacts with TMSCN to produce hydrogen cyanide in situ. Ynoic acids undergo the phosphine catalyzed hydrocyanation in the presence of TMSCN under aprotic conditions only. In these reactions, TMSCN reacts with the acid to generate hydrogen cyanide and the silyl ester which, unlike the acid, undergoes phosphine catalyzed hydrocyanation and gives the stereo-defined E-2-cyano-acrylic acids after work up.

One Pot Conversion of Carboxylic Acid to Ketone Using Trimethylsilyl Chloride

Background: Ketone is abundant in many natural products and in pharmaceuticals. It is believed to be one of the important functional groups in organic chemistry. Till date,several research approaches have been made to access ketone from a readily available starting materials. One such notable transformation consists of the conversion of carboxylic acid to the corresponding ketone in a one pot manner. Objective: We aimed to develop a simple one pot reaction for the conversion of carboxylic acid to ketone. This reaction could be useful to convert all types of carboxylic acid to ketone in a facile manner. Methods: In this procedure, a carboxylic acid has been converted to the corresponding trimethylsilyl ester using trimethylsilyl chloride in the presence of a base. A suitable organometallic reagent can interact with the ester formed at 20°C to produce the corresponding ketone. Results: Under the optimized reaction conditions, various aromatic, aliphatic and heteroaromatic carboxylic acids have been converted to the corresponding ketones using organolithium reagents, in a one pot manner. Moderate to good yields of the desired ketones were observed in most of the transformations. Conclusion: Conversion of carboxylic acid to ketone has been reported in a one pot fashion, where carboxylic acid has been transformed to its silyl ester. Organolithium reagents were used as nucleophile for our reaction purpose, whereas the organomagnesium reagents were not useful for this transformation. Aliphatic, aromatic and heteroaromatic carboxylic acids have been converted to the ketones following a simple process.

Molecular-level insights into adsorption of a novel silyl ester donor on essential MgCl2 facets of supported Ziegler–Natta catalysts

This article deals with the adsorption properties of a new silyl ester donor called bis(benzoyloxy)dimethylsilane (BDMS) on the MgCl2 support of the Ziegler–Natta catalysts in both conventional and zip modes of coordination. The results confirmed the strong (by up to 47.6 kcal/mol at PBEh/TZVP) preference of BDMS for bridging over chelating or monodentate binding modes on both tetracoordinate and pentacoordinate Mg atoms. Among the spontaneous events, the lowest but appreciable preference was predicted for monodentate chemisorption on penta-coordinated Mg. The zero-point energy (ZPE) values confirmed that BDMS could be adsorbed more strongly, by 7.0 kcal/mol, on the (110) facet than on the (104) surface of the MgCl2 matrix. The enthalpy changes confirmed the ability of BDMS to effectively stabilize the lateral crystallite terminations. The observed characteristics were explained in terms of the quantum theory of atoms in molecules, natural bond orbital population, reduced density gradient, non-covalent interactions, and charge density difference. While some in-plane redistribution of charges was demonstrated, the electron transfer did not exceed 0.1 e. Overall, BDMS was shown to be more selective than the commonly used phthalate donors.

Reduction of Carboxylic Acids to Alcohols via Manganese(I) Catalyzed Hydrosilylation

The reduction of carboxylic acids to the respective alcohols, in mild conditions, was achieved using [MnBr(CO)5] as the catalyst and bench stable PhSiH3 as the reducing agent. It was shown that the reaction with the earth-abundant metal catalyst could be performed either with a catalyst loading as low as 0.5 mol %, rare with the use of [MnBr(CO)5], or on a gram scale employing only 1.5 equiv of PhSiH3, the lowest amount of silane reported to date for this transformation. Kinetic data and control experiments have provided initial insight into the mechanism of the catalytic process, suggesting that it proceeds via the formation of silyl ester intermediates and ligand dissociation to generate a coordinatively unsaturated Mn(I) complex as the active species.

Tandem Insertion/[3,3]-Sigmatropic Rearrangement Involving the Formation of Silyl Ketene Acetals by Insertion of Rhodium Carbenes into S–Si Bonds

Allyl 2-diazo-2-phenylacetates are shown to react with trimethylsilyl thioethers in the presence of rhodium(II) catalysts to generate α-allyl-α-thio silyl esters. The transformation involves a tandem process involving formal rhodium-catalyzed insertion of the carbene group into the S-Si bond to generate a silyl ketene acetal, followed by a spontaneous Ireland-Claisen rearrangement. The silyl ester products were isolated as the corresponding carboxylic acids after aqueous workup. Intramolecular cyclopropanation of the allyl fragment generally competes with addition of the heteroatom to the carbene center. The reaction occurs under mild conditions and in high yield, allowing for rapid entry into rearrangement tetrasubstituted products. Propargyl esters were shown to generate the corresponding α-allenyl products.