Syntheses of Cyclopropyl Silyl Ketones

The synthesis of the cyclopropyl silyl ketones 1 – 4 is described. The trimethylsilyl ketone 1 was prepared from geraniol ((E)- 5 ) in ca. 10% overall yield by cyclopropanation leading to 6 , CrO₃ oxidation to the aldehyde 8 , reaction of the latter with trimethylsilyl anion to 14A + B , and CrO₃ oxidation to 1 . Also for the (t-butyl)dimethylsilyl ketones 2 – 4 , an efficient four-step synthesis with overall yields of 48%, 85%, and 13%, respectively, was elaborated, starting from the allylic alcohols (E)- 5 , and 23 . The method of preparation involves as the key step a Wittig rearrangement of the silylallyl ethers ((E/Z)- 20 , 24 ) to the silyl alcohols ((E/Z)- 21 , 25 ), subsequent cyclopropanation ( 19A + B , 22A + B , 26 ), and oxidation to the cyclopropyl silyl ketones 2 – 4 .     

Solvent-Free Esterification of Carboxylic Acids and Alcohols in the Presence of Silphos [PCl3-n(SiO2)n] as a Heterogeneous Phosphine Reagent

An efficient solvent-free method for the preparation of esters from various aromatic and aliphatic acids with primary, secondary, and tertiary alcohols using a heterogeneous phosphine reagent, silphos [PCl_3-n(SiO₂)_n], in good yields is reported.     

A Mild and Highly Efficient Method for the Preparation of Silyl Ethers using Fe(HSO4)3/Et3N by Chlorosilanes

A very efficient and mild procedure for preparation of silyl ethers from benzylic, allylic, propargilic alcohols, phenols, naphtoles and some of phenolic drugs with trimethylsilylchloride (TMSCl), triethylsilylchloride (TESCl) and t‐buthyldimethylsilyl chloride (TDSCl) ethers in the presence of Fe(HSO₄)₃/Et₃N in room temperature in excellent yields is reported. This procedure also allows the excellent selectivity for silylation of alcohols and phenols.     

Stereoselective glycosylation of Alcohols and Silyl Ethers Using Glycosyl Fluorides and Boron Trifluoride Etherate

The stereoselective glycosylation of alcohols and their silyl ethers has been achieved using O-alkyl-, O-acyl-, and acetal-protected glycosyl fluorides of the pyranose and furanose series and boron trifluoride etherate in CH₂Cl₂.     

Convenient Preparations of 2,3-Dihydro-4H-pyran-4-ones from D-Glucal Triacetate : Selective Oxidations of Alyllic Acetates and Allylic Silyl Ethers Using N-Bromosuccinimide

N-Bromosuccinimide (1.1 eq) in the presence of potassium carbonate (2 eq) and a catalytic amount of dibenzoyl peroxide converts the allylic acetates and the allylic silyl ethers (O-TBDMS or O-SiEt₃) of secondary allylic alcohols, derived from D-glucal 3a into corresponding dihydro γ-pyrones 2.     

Rhodium‐Catalyzed Dehydrogenative Silylation of Acetophenone Derivatives: Formation of Silyl Enol Ethers versus Silyl Ethers

A series of rhodium–NSiN complexes (NSiN=bis (pyridine‐2‐yloxy)methylsilyl fac‐coordinated) is reported, including the solid‐state structures of [Rh(H)(Cl)(NSiN)(PCy₃)] (Cy=cyclohexane) and [Rh(H)(CF₃SO₃)(NSiN)(coe)] (coe=cis‐cyclooctene). The [Rh(H)(CF₃SO₃)(NSiN)(coe)]‐catalyzed reaction of acetophenone with silanes performed in an open system was studied. Interestingly, in most of the cases the formation of the corresponding silyl enol ether as major reaction product was observed. However, when the catalytic reactions were performed in closed systems, formation of the corresponding silyl ether was favored. Moreover, theoretical calculations on the reaction of [Rh(H)(CF₃SO₃)(NSiN)(coe)] with HSiMe₃ and acetophenone showed that formation of the silyl enol ether is kinetically favored, while the silyl ether is the thermodynamic product. The dehydrogenative silylation entails heterolytic cleavage of the Si?H bond by a metal–ligand cooperative mechanism as the rate‐determining step. Silyl transfer from a coordinated trimethylsilyltriflate molecule to the acetophenone followed by proton transfer from the activated acetophenone to the hydride ligand results in the formation of H₂ and the corresponding silyl enol ether.     

Chemoselective Protection of Hydroxyl Groups and Deprotection of Silyl Ethers

 Trimethylsilylation of alcohols and phenols is carried out using hexamethyldisilazane and LiClO₄ under microwave irradiation and neutral conditions. The deprotection of silyl ethers is carried out similarly using natural kaolinitic clay and a few drops of water.     

Synthesis,Properties, and Crystal Structure of Silyl Nitronates (Silyl Esters of aci-Nitroalkanes): Towards the SN2 Reaction Path with Retention of Configuration at Silicon

An efficient and flexible method for the preparation of silyl nitronates is described (see 1–10 ). NMR. spectral investigations indicate a rapid 1,3-silyl migration process, with an activation energy of about 10 kcal mol^?1. X-ray crystallographic studies on the silyl nitronates 3 and 8 show structures that lean towards an S_N2 retention pathway at silicon.     

Chemoselective Protection of Hydroxyl Groups and Deprotection of Silyl Ethers

Summary.  Trimethylsilylation of alcohols and phenols is carried out using hexamethyldisilazane and LiClO₄ under microwave irradiation and neutral conditions. The deprotection of silyl ethers is carried out similarly using natural kaolinitic clay and a few drops of water. Received March 1, 2001. Accepted (revised) April 17, 2001     

A Simple and Convenient Method for Cleavage of Silyl Esthers

Cleavage of silyl ethers can be efficiently effected using FeCl₃, SnCl₂, Cu(NO₃)₂ and Ce(NO₃)₃ at room temperature.     

合成硅氧醚的新方法：Silyltriflimides与醇或与醚反应

首次利用silyltriflimides［双－（三氟甲磺酰）－亚胺基硅烷］与醇或醚反应合成了一系列非环链状或环状的硅氧醚,反应产率较好。其中反应物silyl triflimides很容易由相应的苯基硅烷或丙烯基硅烷与HNTf2通过质子脱硅化反应得到。合成的新化合物的结构用1H NMR, 13C NMR, MS, IR 和HRMS等进行了表征.     

Activation of Carbon Dioxide by Silyl Triflate‐Based Frustrated Lewis Pairs

Silyl triflates of the form R_4?nSi(OTf)_n (n=1, 2; OTf=OSO₃CF₃) are shown to activate carbon dioxide when paired with bulky alkyl‐substituted Group 15 bases. Combinations of silyl triflates and 2,2,6,6‐tetramethylpiperidine react with CO₂ to afford silyl carbamates via a frustrated Lewis pair‐type mechanism. With trialkylphosphines, the silyl triflates R₃Si(OTf) reversibly bind CO₂ affording [R′₃P(CO₂)SiR₃][OTf] whereas when Ph₂Si(OTf)₂ is used one or two molecules of CO₂ can be sequestered. The latter bis‐CO₂ product is favoured at low temperatures and by excess phosphine.     

Intramolecular Halo Stabilization of Silyl Cations—Silylated Halonium- and Bis-Halo-Substituted Siliconium Borates

The stabilizing neighboring effect of halo substituents on silyl cations was tested for a series of peri-halo substituted acenaphthyl-based silyl cations 3 . The chloro- ( 3 b ), bromo- ( 3 c ), and iodo- ( 3 d ) stabilized cations were synthesized by the Corey protocol. Structural and NMR spectroscopic investigations for cations 3 b – d supported by the results of density functional calculations, which indicate their halonium ion nature. According to the fluorobenzonitrile (FBN) method, the silyl Lewis acidity decreases along the series of halonium ions 3 , the fluoronium ion 3 a being a very strong and the iodonium ion 3 d a moderate Lewis acid. Halonium ions 3 b and 3 c react with starting silanes in a substituent redistribution reaction and form siliconium ions 4 b and 4 c . The structure of siliconium borate 4 c ₂[B₁₂Br₁₂] reveals the trigonal bipyramidal coordination environment of the silicon atom with the two bromo substituents in the apical positions.     

Coupling Reaction of Enol Derivatives with Silyl Ketene Acetals Catalyzed by Gallium Trihalides

A cross‐coupling reaction between enol derivatives and silyl ketene acetals catalyzed by GaBr₃ took place to give the corresponding α‐alkenyl esters. GaBr₃ showed the most effective catalytic ability, whereas other metal salts such as BF₃?OEt₂, AlCl₃, PdCl₂, and lanthanide triflates were not effective. Various types of enol ethers and vinyl carboxylates as enol derivatives are amenable to this coupling. The scope of the reaction with silyl ketene acetals was also broad. We successfully observed an alkylgallium intermediate by using NMR spectroscopy, suggesting a mechanism involving anti‐carbogallation among GaBr₃, an enol derivative, and a silyl ketene acetal, followed by syn‐β‐alkoxy elimination from the alkylgallium. Based on kinetic studies, the turnover‐limiting step of the reaction using a vinyl ether and a vinyl carboxylate involved syn‐β‐alkoxy elimination and anti‐carbogallation, respectively. Therefore, the leaving group had a significant effect on the progress of the reaction. Theoretical calculations analysis suggest that the moderate Lewis acidity of gallium would contribute to a flexible conformational change of the alkylgallium intermediate and to the cleavage of the carbon?oxygen bond in the β‐alkoxy elimination process, which is the turnover‐limiting step in the reaction between a vinyl ether and a silyl ketene acetal.     

Chemo- and diastereoselective Bi(OTf)3-catalyzed benzylation of silyl nucleophiles

The direct alkylation of silyl enol ethers with para-methoxybenzylic alcohols or their corresponding acetates was efficiently catalyzed by Bi(OTf)₃ in CH₃NO₂ as the solvent. The reaction provided the α-benzylated carbonyl compounds in high yields after short reaction times using 1-2.5 mol % of the catalyst. Benzylic acetates other than para-methoxybenzylic acetates also underwent the reaction. High facial diastereoselectivities were observed with acetates derived from chiral α-branched para-methoxybenzylic alcohols. In addition, a catalytic reduction with Et₃SiH as the reducing agent is reported.     

Chlorodiphenylphosphine as Highly Selective and Efficient Reagent for the Conversion of Alcohols,Tetrahydropyranyl and Silyl Ethers to Thiocyanates and Isothiocyanates

Abstract

A simple, highly selective and efficient method is described for the conversion of primary alcohols, tetrahydropyranyl and silyl ethers to thiocyanates by use of chlorodiphenylphosphine and ammonium thiocyanate. Secondary substrates produce both the two isomeric products, thiocyanate and isothiocyanate, while tertiary ones give isothiocyanates as the only products by this new method. In contrast to previously reported methods based on trivalent phosphorus for this transformation, the present method does not require an electrophile in the presence of trivalent phosphorus (ClPPh₂). The order of activity of these substrates is silyl ether> alcohol > tetrahydropyranyl ether. The present method not only interestingly distinguishes between primary, secondary and tertiary substrates but also converts them to the corresponding thiocyanates with excellent chemoselectivity in the presence of several other functional groups.     

A novel and highly selective conversion of alcohols, thiols, and silyl ethers to azides using the triphenylphosphine/2,3-dichloro-5,6-dicyanobenzoquinone(DDQ)/n-Bu4NN3 system

Alcohols, thiols, and silyl ethers are converted into alkyl azides in good to excellent yields by treatment with PPh₃/DDQ/n-Bu₄NN₃ in CH₂Cl₂ at room temperature. The method is highly selective for 1° alcohols in the presence of 2° and 3° ones, and also thiols and silyl ethers.     

An Efficient Regio and Stereoselective Synthesis of Silyl Enol Ethers

Ketones and aldehydes on treatment with NaBr-Me₃SiCl-Et₃N in DMF at ambient temperature yield silyl enol ethers with high regio- and stereoselecti-vity.     

Unusual Structure,Fluxionality, and Reaction Mechanism of Carbonyl Hydrosilylation by Silyl Hydride Complex [(ArN)Mo(H)(SiH2Ph)(PMe3)3]

The reactions of bis(borohydride) complexes [(RN?)Mo(BH₄)₂(PMe₃)₂] ( 4 : R=2,6‐Me₂C₆H₃; 5 : R=2,6‐iPr₂C₆H₃) with hydrosilanes afford new silyl hydride derivatives [(RN?)Mo(H)(SiR′₃)(PMe₃)₃] ( 3 : R=Ar, R′₃=H₂Ph; 8 : R=Ar′, R′₃=H₂Ph; 9 : R=Ar, R′₃=(OEt)₃; 10 : R=Ar, R′₃=HMePh). These compounds can also be conveniently prepared by reacting [(RN?)Mo(H)(Cl)(PMe₃)₃] with one equivalent of LiBH₄ in the presence of a silane. Complex 3 undergoes intramolecular and intermolecular phosphine exchange, as well as exchange between the silyl ligand and the free silane. Kinetic and DFT studies show that the intermolecular phosphine exchange occurs through the predissociation of a PMe₃ group, which, surprisingly, is facilitated by the silane. The intramolecular exchange proceeds through a new non‐Bailar‐twist pathway. The silyl/silane exchange proceeds through an unusual Mo^VI intermediate, [(ArN?)Mo(H)₂(SiH₂Ph)₂(PMe₃)₂] ( 19 ). Complex 3 was found to be the catalyst of a variety of hydrosilylation reactions of carbonyl compounds (aldehydes and ketones) and nitriles, as well as of silane alcoholysis. Stoichiometric mechanistic studies of the hydrosilylation of acetone, supported by DFT calculations, suggest the operation of an unexpected mechanism, in that the silyl ligand of compound 3 plays an unusual role as a spectator ligand. The addition of acetone to compound 3 leads to the formation of [trans‐(ArN)Mo(OiPr)(SiH₂Ph)(PMe₃)₂] ( 18 ). This latter species does not undergo the elimination of a Si? O group (which corresponds to the conventional Ojima′s mechanism of hydrosilylation). Rather, complex 18 undergoes unusual reversible β‐CH activation of the isopropoxy ligand. In the hydrosilylation of benzaldehyde, the reaction proceeds through the formation of a new intermediate bis(benzaldehyde) adduct, [(ArN?)Mo(η²‐PhC(O)H)₂(PMe₃)], which reacts further with hydrosilane through a η¹‐silane complex, as studied by DFT calculations.     

A highly efficient synthesis of (Z)‐1‐aryl‐2‐silyl‐1‐ stannylethenes and their conversion to (E)‐2‐ arylethenyl‐, (Z)‐2‐(2‐pyridyl)ethenyl‐ and allenyl‐silanes

A Pd(dba)₂–P(OEt)₃ combination allowed the silastannation of arylacetylenes, 1‐hexyne or propargyl alcohols with tributyl(trimethylsilyl)stannane to take place at room temperature, producing (Z)‐2‐silyl‐1‐stannyl‐1‐substituted ethenes in high yields. Novel silyl(stannyl)ethenes were fully characterized by ¹H‐, ¹³C‐, ²⁹Si‐ and ¹¹⁹Sn‐NMR as well as infrared and mass analyses. Treatment of a series of (Z)‐1‐aryl‐2‐silyl‐1‐stannylethenes and (Z)‐1‐(3‐pyridyl)‐2‐silyl‐1‐stannylethene with hydrochloric acid or hydroiodic acid in the presence of tetraethylammonium chloride (TEACl) or tetrabutylammonium iodide (TBAI) led to the exclusive formation of (E)‐trimethyl(2‐arylethenyl)silanes with high stereoselectivity. A similar reaction of (Z)‐1‐(2‐anisyl)‐2‐silyl‐1‐stannylethene also produced E‐type trimethyl[2‐(2‐anisyl)ethenyl]silane, while (Z)‐trimethyl [2‐(2‐pyridyl)ethenyl]silane was produced exclusively from (Z)‐1‐(2‐pyridyl)‐2‐silyl‐1‐stannylethene. Protodestannylation of (Z)‐1‐[hydroxy(phenyl)methyl]‐2‐silyl‐1‐stannylethene with trifluoroacetic acid took place via the β‐elimination of hydroxystannane, providing trimethyl(3‐phenylpropa‐1,2‐dienyl)silane quite easily. The destannylation products were also fully characterized. Copyright © 2007 John Wiley & Sons, Ltd.