Silylation Of Phenols Research Articles

Kinetic Resolution of BINOLs and Biphenols by Atroposelective, Cu-H-Catalyzed Si-O Coupling with Hydrosilanes.

A nonenzymatic kinetic resolution of monoprotected BINOL and biphenol derivatives by atroposelective Si-O coupling with hydrosilanes is described. The reaction relies on a previously unprecedented Cu-H-catalyzed silylation of phenols. The catalyst system consisting of CuCl, (R,R)-Ph-BPE, and NaOtBu enables the enantioselective coupling of the phenolic hydroxy group with a hydrosilane with moderate to good selectivity factors.

Aryne cycloaddition reactions of benzodioxasilines as aryne precursors generated by catalytic reductive ortho-C[sbnd]H silylation of phenols with traceless acetal directing groups

Diversely substituted arylsilyl triflates, as aryne precursors for aryne cycloaddition reactions, were accessed from benzodioxasilines. Catalytic reductive CH ortho-silylation of phenols with traceless acetal directing groups was exploited to prepare benzodioxasilines. Sequential addition of MeLi and then trifluoromethanesulfonic anhydride to benzodioxasilines provided arylsilyl triflates in a single pot. Notably, this approach was successfully utilized to prepare sterically hindered 1,2,3-trisubstituted arylsilyl triflates, which ultimately underwent fluoride-mediated aryne cycloaddition.

ChemInform Abstract: Catalytic Reductive ortho‐C—H Silylation of Phenols with Traceless, Versatile Acetal Directing Groups and Synthetic Applications of Dioxasilines.

A new, highly selective, bond functionalization strategy, achieved via relay of two transition metal catalysts and the use of traceless acetal directing groups, has been employed to provide facile formation of C–Si bonds and concomitant functionalization of a silicon group in a single vessel. Specifically, this approach involves the relay of Ir-catalyzed hydrosilylation of inexpensive and readily available phenyl acetates, exploiting disubstituted silyl synthons to afford silyl acetals and Rh-catalyzed ortho-C–H silylation to provide dioxasilines. A subsequent nucleophilic addition to silicon removes the acetal directing groups and directly provides unmasked phenol products and, thus, useful functional groups at silicon achieved in a single vessel. This traceless acetal directing group strategy for catalytic ortho-C–H silylation of phenols was also successfully applied to preparation of multisubstituted arenes. Remarkably, a new formal α-chloroacetyl directing group has been developed that allows catalyti...

Catalytic Reductive ortho-C-H Silylation of Phenols with Traceless, Versatile Acetal Directing Groups and Synthetic Applications of Dioxasilines.

A new, highly selective, bond functionalization strategy, achieved via relay of two transition metal catalysts and the use of traceless acetal directing groups, has been employed to provide facile formation of C-Si bonds and concomitant functionalization of a silicon group in a single vessel. Specifically, this approach involves the relay of Ir-catalyzed hydrosilylation of inexpensive and readily available phenyl acetates, exploiting disubstituted silyl synthons to afford silyl acetals and Rh-catalyzed ortho-C-H silylation to provide dioxasilines. A subsequent nucleophilic addition to silicon removes the acetal directing groups and directly provides unmasked phenol products and, thus, useful functional groups at silicon achieved in a single vessel. This traceless acetal directing group strategy for catalytic ortho-C-H silylation of phenols was also successfully applied to preparation of multisubstituted arenes. Remarkably, a new formal α-chloroacetyl directing group has been developed that allows catalytic reductive C-H silylation of sterically hindered phenols. In particular, this new method permits access to highly versatile and nicely differentiated 1,2,3-trisubstituted arenes that are difficult to access by other catalytic routes. In addition, the resulting dioxasilines can serve as chromatographically stable halosilane equivalents, which allow not only removal of acetal directing groups but also introduce useful functional groups leading to silicon-bridged biaryls. We demonstrated that this catalytic C-H bond silylation strategy has powerful synthetic potential by creating direct applications of dioxasilines to other important transformations, examples of which include aryne chemistry, Au-catalyzed direct arylation, sequential orthogonal cross-couplings, and late-stage silylation of phenolic bioactive molecules and BINOL scaffolds.

Deprotection of tert-butyldimethylsilyl (TBDMS) ethers using efficient and recyclable heterogeneous silver salt of silicotungstic acid catalyst under mild reaction condition

Deprotection of various substituted alkyl and phenolic TBDMS ethers were carried out using prepared silver exchanged silicotungstic acid (AgSTA) salt as heterogeneous catalyst in methanol at room temperature. The Brönsted acidic protons of silicotungstic acid were substituted by Lewis acidic silver molecule and prepared heterogeneous AgSTA catalyst which characterized using different spectroscopic and analytical methods. 0.2equiv. of AgSTA catalyst showed tremendous catalytic activity in the desilylation of alkyl and phenolic TBDMS ethers under mild reaction condition in methanol to produce respective alcohol in excellent yield. On screening, 100% conversion of reactants and up to 92.0% yield of respective alcohol in 40min reaction time was achieved using 0.2equiv. of AgSTA catalyst. The solid AgSTA catalyst was separated from the reaction mixture by simple filtration process at the end of reaction and reused for several times without significant loss of catalytic activity. The present eco-friendly catalytic system is found to be very convenient, high yielding, fast and clean method for desilylation of alkyl as well as phenolic silyl ethers even in existence of other sensitive organic functional groups such as aldehyde, methoxy, and acetate under mild reaction condition.

Taming of a Superbase for Selective Phenol Desilylation and Natural Product Isolation

Hydroxyl moieties are highly prevalent in natural products. We previously reported a chemoselective strategy for enrichment of hydroxyl-functionalized molecules by formation of a silyl ether bond to a resin. To generate smaller pools of molecules for analysis, we developed cleavage conditions to promote stepwise release of phenolic silyl ethers followed by aliphatic silyl ethers with a "tamed" version of the superbase 1,1,3,3-tetramethylguanadine. We demonstrate this as a general strategy for selective deprotection of phenolic silyl ethers under neutral conditions at room temperature.

Highly atom economical uncatalysed and I2-catalysed silylation of phenols, alcohols and carbohydrates, using HMDS under solvent-free reaction conditions (SFRC)

An uncatalysed silylation of phenols, regardless on the aggregate state and nature of the substituents with 0.55equiv of HMDS under solvent-free reaction conditions (SFRC) at room temperature is reported. Sterically hindered phenols, carbohydrates and most of the alcohols additionally required a catalytic amount (up to 2mol%) of iodine. The reaction protocol is very simple; obtaining a pure product, particularly of uncatalysed reactions, was frequently a completely solvent-free process.

A mild and efficient method for the selective deprotection of silyl ethers using KF in the presence of tetraethylene glycol

A mild and efficient protocol for the deprotection of silyl ethers using KF in tetraethylene glycol is reported. A wide range of alcoholic silyl ethers can be selectively cleaved in high yield in the presence of certain acid- and base-labile functional groups. Moreover, the phenolic silyl ethers were cleaved exclusively, without affecting the alcoholic silyl ethers, at room temperature.

ChemInform Abstract: Synthesis of Phytyl- and Chroman-Derivatized Photoaffinity Labels Based on α-Tocopherol.

Photoaffinity analogues of α-tocopherol have been prepared by substituting photosensitive functional groups at either the terminus of an alkyl chain of varying length mimicking the phytyl tail or on C-3 of the chroman portion of tocopherol. The alkyl chain-modified compounds 2a−d contain a hexyl to nonyl alkyl chain extending from C-2 of the chroman, terminating in a tetrafluoroazidobenzyloxy group. These compounds were prepared starting from the commercially available Trolox acid 4, followed by esterification, protection, and reduction to the silyl-protected Trolox aldehyde 7, which was coupled using Wittig chemistry to different ω-hydroxyphosphonium bromides. Reduction of the alkene product, coupling with p-azidotetrafluorobenzyl bromide, and deprotection of the phenolic silyl group gave compounds 2a−d in excellent yields. Chroman-functionalized photoaffinity labels were synthesized starting from the protected tocopherol chromene 16b which was a key intermediate for preparation of a 3-hydroxy derivative...

Simple and Practical Protocol for the Silylation of Phenol Derivatives Using Reusable NaHSO4 Dispread on Silica Gel Under Neutral Conditions

A simple and mild procedure for the trimethylsilylation of a wide variety of phenols with hexamethyldisilazane (HMDS) on the surface of silica gel dispersed with NaHSO4 at r.t. in a few minutes with excellent yields under neutral conditions is reported. This procedure also allows an excellent selectivity for the silylation of phenols in the presence of amine and CO2H groups.

ChemInform Abstract: Potassium Exchanged Layered Zirconium Phosphate as Base Catalyst for the Desilylation of Phenol Silyl Ethers.

Abstract Layered zirconium hydrogen phosphonate exchanged with the potassium ion was found to be an efficient heterogeneous catalyst for the hydrolysis of sterically hindered phenolic silyl ethers.

Potassium Exchanged Layered Zirconium Phosphate as Base Catalyst for the Desilylation of Phenol Silyl Ethers

Layered zirconium hydrogen phosphonate exchanged with the potassium ion was found to be an efficient heterogeneous catalyst for the hydrolysis of sterically hindered phenolic silyl ethers.

Synthesis of phytyl- and chroman-derivatized photoaffinity labels based on alpha-tocopherol.

Photoaffinity analogues of alpha-tocopherol have been prepared by substituting photosensitive functional groups at either the terminus of an alkyl chain of varying length mimicking the phytyl tail or on C-3 of the chroman portion of tocopherol. The alkyl chain-modified compounds 2a-d contain a hexyl to nonyl alkyl chain extending from C-2 of the chroman, terminating in a tetrafluoroazidobenzyloxy group. These compounds were prepared starting from the commercially available Trolox acid 4, followed by esterification, protection, and reduction to the silyl-protected Trolox aldehyde 7, which was coupled using Wittig chemistry to different omega-hydroxyphosphonium bromides. Reduction of the alkene product, coupling with p-azidotetrafluorobenzyl bromide, and deprotection of the phenolic silyl group gave compounds 2a-d in excellent yields. Chroman-functionalized photoaffinity labels were synthesized starting from the protected tocopherol chromene 16b which was a key intermediate for preparation of a 3-hydroxy derivative, either by reduction of epoxides produced directly with Jacobsen's catalysts or by treatment with NBS in wet DME to give two stereoisomeric bromohydrins which were cyclized and reduced to give the phenol-protected C-3 alcohols 19a,b. These alcohols were then converted to diazoacetate esters, and the protecting group was removed to give 3-diazoacetoxy alpha-tocopherols 3a,b.

An Investigation on Desilylation of Alkyl and Phenolic Silyl Ethers Using FeCI3

An Investigation on Desilylation of Alkyl and Phenolic Silyl Ethers Using FeCI3

ChemInform Abstract: Mild Base‐Mediated Desilylation of Various Phenolic Silyl Ethers.

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Mild base mediated desilylation of various phenolic silyl ethers

A variety of phenolic silyl ethers are easily desilylated in good to excellent yields by treatment with 1.1 equivalents of K 2CO 3, and ethanol containing 5 equivalents of water.

Utility of Weitz' Aminium Salt for Obtaining Quinones as Potential Synthetic Precursors of Quassinoids.

Reactions of phenols and phenolic silyl ethers 5, 6, and 7 using Weitz' aminium salt, tris(4-bromophenyl)aminium hexachloroantimonate (BAHA), gave the corresponding quinone derivatives 8, 9, and 10, which may be useful synthons for obtaining quassinoids, the bitter principle of Simaroubaceous plants. Treatment of 10a with silica gel powder resulted in the intramolecular conjugate addition to afford the indanone 11 in good yield.

Mixed silyl ether-perfluoroacyl ester derivatives for gas chromatography/mass spectrometry of oestrogens. Application to the quantitative determination of oestriol in human plasma.

The conversion of oestrogens into mixed derivatives comprising a phenolic silyl ether and one or more alcoholic perfluoroactyl ester(s) is described. They are easily formed in quantitative yield and have excellent gas chromatographic and mass spectrometric properties, making them suitable for analysis by selected ion monitoring. Reaction mechanisms are examined which propose explanations for the experimentally observed optimal reaction conditions. The 3-t-butyldimethylsilyl ether-16 alpha 17 beta-bis(pentafluoropropionate) has been used for the quantitative determination of oestriol in plasma by gas chromatography/mass spectrometry using a stable isotope internal standard. Comparison with the pertrimethylsilyl ether of oestriol indicates higher specificity and better precision for low-level (less than 0.5 ng ml-1) estimations.

Trimethylsilyl trichloroacetate: a new reagent for salt-free silylations

Trimethylsilyl trichloroacetate ( 1 ) is a convenient reagent for the silylation of phenols, carboxylic acids, mercaptans, amides, acetylenes, and β-keto esters, while the reaction of 1 with aldehydes and ketones affords silylated trichloromethyl carbinols ( 5 ).

Selective deprotection of alcoholic and phenolic silyl ethers

Hydrofluoric acid and tetrabutylammonium fluoride will selectively deprotect alcoholic and phenolic silyl ethers respectively.