Silyl-protected Alcohols Research Articles

Practical method for hydroxyl-group protection using strontium metal and readily available silyl chlorides.

We have found that the etherification of silyl-protected secondary alcohols proceeds smoothly in the presence of strontium metal using silyl chloride instead of the expensive, yet more reactive, and commonly used silyl triflate. The reaction occurred almost completely with various alcohols.

Normal-phase (temperature gradient) interaction chromatography – A powerful tool for the characterisation of high molecular weight chain-end functionalised polymers

We report here, for the first time, quantitative analysis of end-group functionalisation and the extent of end-group modification of polymers with molar mass up to 200,000gmol−1, using a combination of isothermal and temperature gradient interaction chromatography. At such high molecular weights, other common analytical techniques such as MALDI-ToF-MS and NMR spectroscopy are simply unable to offer any quantitative insight into the end-group functionality of polymers. Thus, normal phase isothermal interaction chromatography (NP-IIC) has been used to characterise a series of polystyrene samples, with identical molar mass (c. 90,000gmol−1), each carrying a single chain-end functionality of varying polarity, introduced via a series of end-group transformation reactions – from silyl-protected alcohol→primary alcohol→alkyl bromide. It is shown that the polarity of the functional group dictates the elution volume of the polymers and that NP-IIC could be used to monitor the quantitative end-group transformation. Thus, rather than estimating the success of the transformation reaction from the very small peaks observed in 1H NMR spectra, NP-IIC gives clear chromatographic peaks, whose retention volumes change dramatically only as a consequence of each new chain-end functional group. We also describe the analysis, by a combination of normal phase isothermal and temperature gradient interaction chromatography, of a series of linear polybutadiene samples of varying molar mass (28,000–200,000gmol−1) each carrying a single primary alcohol functionality at the chain-end. The primary alcohol functionality enabled the separation of functionalised and non-functionalised polymers, which eluted at different retention volumes despite identical molecular weights. It is all the more remarkable that complete (baseline) resolution can be achieved for two polymers with an identical molar mass (<200,000gmol−1), which differ only in that one sample is functionalised with a single primary alcohol group.

ChemInform Abstract: Ruthenium Complexes of the General Formula [RuCl2(PHOX)2] and Their Catalytic Activity in the Mukaiyama Aldol Reaction.

Abstract New ruthenium phosphinooxazoline (PHOX) complexes were synthesized and applied in the Mukaiyama aldol reaction. Four ruthenium complexes of the general formula [RuCl2(PHOX)2] were synthesized from [RuCl2(dmso)4] and the corresponding PHOX ligands through thermal ligand exchange. Two of the complexes were characterized structurally. Achiral PHOX ligands gave the ruthenium complexes as single isomers, whereas chiral PHOX ligands gave a mixture of isomers and also some incomplete substitution. After activation by chloride abstraction, one of the new ruthenium complexes was applied as catalyst in the Mukaiyama aldol reaction to give silyl-protected β-hydroxyl alcohols in 74–92% isolated yields (room temperature, 18–24 h reaction time, 1 mol % catalyst loading).

One-step chemoselective conversion of tetrahydropyranyl ethers to silyl-protected alcohols

A novel chemoselective one-pot transformation of acetals to silyl ethers is reported. Free hydroxyls, double bonds and triple bonds are unaffected in optimal reaction conditions. This practical, inexpensive protocol allows the selective replacement of acetal-forming protecting groups with silyl groups in a single step under mild conditions.

Kinetic versus Thermodynamic Stereoselectivity in the Hydrostannylation of Propargylic Alcohol Derivatives Using AIBN and Et3B as Promotors

Et(3)B versus AIBN: Equally radical? The stereoselectivity of hydrostannylation of internal propargyl alcohol derivatives has been studied by using both 2-2'-azobisisobutyronitrile (AIBN) and Et(3)B as promoters. With silyl-protected alcohols, complete Z selectivity of the resultant vinylstannane has been achieved with Et(3)B, whereas AIBN shows zero stereoselectivity. Evidence suggests that, despite decades of acceptance, the hydrostannylation mechanisms employing Et(3)B and AIBN appear to be mechanistically distinct.

Chemoselective Activation of Trimethylsilyl Enol Ether Functionalities in the Presence of Silyl-Protected Alcohols by Trimethylsilyl-Nonaflyl Exchange

Chemoselective Activation of Trimethylsilyl Enol Ether Functionalities in the Presence of Silyl-Protected Alcohols by Trimethylsilyl-Nonaflyl Exchange

Hydrosilylation of epoxides catalyzed by a cationic η1-silane iridium(iii) complex

Cationic silane complex 2, catalyzes the hydrosilylation of epoxides and cyclic ethers to give the silyl-protected alcohols, regioselectively. A mechanistic study shows that the epoxide undergoes isomerization to the ketone, followed by hydrosilylation.

Nickel-catalyzed enantioselective hydrovinylation of silyl-protected allylic alcohols: An efficient access to homoallylic alcohols with a chiral quaternary center

Asymmetric hydrovinylation of silyl-protected allylic alcohols catalyzed by nickel complexes of chiral spiro phosphoramidite ligands was developed. A series of homoallylic alcohols with a chiral quaternary center were produced in high yields (up to 97%) and high enantioselectivities (up to 95% ee). The reaction provides an efficient method for preparing bifunctional compounds with a chiral quaternary carbon center.

Generally Applicable Organocatalytic Tetrahydropyranylation of Hydroxy Functionalities with Very Low Catalyst Loading

This paper presents the first acid-free, organocatalytic tetrahydropyran and 2-methoxypropene protection of alcohols, phenols, and other ROH derivatives utilizing privileged N,N′-bis[3,5-bis(trifluoromethyl)phenyl]thiourea and a polystyrene-bound analogue. The reactions are broadly applicably (also on preparative scale), in particular, to acid-sensitive substrates such as aldol products, hydroxy esters, acetals, silyl-protected alcohols, and cyanohydrins. The catalytic efficiency is truly remarkably with turnover numbers of 100,000 and turnover frequencies of up to 5700 h-1 at catalyst loadings down to 0.001 mol%. The computationally supported mechanistic interpretation emphasizes the hydrogen bond assisted heterolysis of the alcohol and concomitant preferential stabilization of the oxyanion hole in the transition state.

Catalytic Hydrosilylation of Carbonyl Compounds with Cationic Oxorhenium(V) Salen

High-valent cationic oxorhenium(V) salen complexes efficiently catalyze the reduction of carbonyl compounds by organosilanes under mild, open-flask conditions. Good to excellent yields of silyl-protected alcohols are obtained in one step, and a variety of functional groups are compatible.

Efficient Chemoselective Deprotection of Silyl Ethers Using Catalytic 1‐Chloroethyl Chloroformate in Methanol.

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Efficient chemoselective deprotection of silyl ethers using catalytic 1-chloroethyl chloroformate in methanol

Fast and chemoselective desilylation of silyl-protected alcohols was achieved using a catalytic amount of 1-chloroethyl chloroformate in methanol. With a minimal amount of 1-chloroethyl chloroformate as the source for anhydrous HCl, extremely efficient cleavage of silyl ethers of primary and secondary alcohols was accomplished, and chemoselective deprotection of one silyl ether in the presence of another silyl or other acid-labile group was possible through controlling the amount of the chloroformate and reaction time.

Selective deprotection of silyl-protected phenols using solid NaOH and a phase transfer catalyst

Aryl silyl ethers can be deprotected to yield phenols in good to excellent yields using a biphasic system of 10 equivalents of NaOH and catalytic Bu 4NHSO 4 in 1,4-dioxane. Alkyl silyl ethers prepared using a variety of silyl protecting groups survive under these conditions, allowing selective deprotection of silyl-protected phenols in the presence of silyl-protected alcohols.

Preparation of Novel (Fluoroaryl)silanes and Their Applications in the Organoyttrium-Catalyzed Hydrosilylation of Alkenes and Oxidation to Alcohols

The synthesis of novel (fluoroaryl)silanes from the respective aryl Grignard reagents and silyl triflates is reported. The new silanes were used in the hydrosilylation of alkenes catalyzed by the organoyttrium complex Cp*2YMe·THF (THF = tetrahydrofuran). (Fluoroaryl)silanes were found to react faster than phenylsilane and in good to near quantitative yields. The resulting [(fluoroaryl)silyl]alkane products were found to be labile under very mild oxidative conditions, with both disubstituted alkenes and silyl-protected alcohol functionalities tolerating the oxidation.

P(RNCH2CH2)3N: Superbasic and Catalytic Cages

In contrast to acyclic P(NR2)3, the title pro-azaphosphatrane cages (first reported from our laboratories) are exceedingly strong nonionic bases that protonate to give the extraordinarily weak acids HP(RNCH2CH2)3N+ (pKa in MeCN, ∼41). Thus we have found that commercially available P(MeNCH2CH2)3N is a superior nonionic base for the synthesis of porphyrins, alkenes, C-monoalkylation of esters and β-hydroxy nitriles. We have also discovered that P(RNCH2CH2)3N cages function as superior catalysts for the synthesis of β-hydroxy nitriles, α,β-unsaturated nitriles and silyl-protected alcohols. Protective acylations of alcohols are very efficiently promoted by P(MeNCH2CH2)3N.

Convenient Methods for the Direct Conversion of Tetrahydropyranyl Ethers into Silyl-protected Alcohols and for the Removal of Tetrahydropyranyl Group

A reagent system of trialkylsilyl trifluoromethanesulfonate and triethylamine cleaves readily tetrahydropyranyl ethers to give directly the corresponding trialkylsilyl ethers in good yields under very mild conditions. Dialkylsilylene derivatives of 1,3-diol can be obtained directly from the corresponding bis-tetrahydropyranyl ethers of 1,3-diol. And furthermore, alcohol tetrahydropyranyl ethers can be easily deprotected by treatment of trimethylsilyl trifluoromethanesulfonate alone to afford parent free alcohols in good yields.

Direct Conversion of p-Methoxybenzyl Ethers into Silyl-protected Alcohols by the Action of Trialkylsilyl Trifluoromethanesulfonate and Triethylamine

The combined use of t-butyldimethylsilyl trifluoromethanesulfonate and triethylamine cleaves readily p- or o-methoxybenzyl ethers to give directly the corresponding t-butyldimethylsilyl ethers in high yields thus affording an expedient way to replace the benzyl ether-type protective group with the silyl ether-type one.

A one-step and chemoselective conversion of silyl-protected alcohols into the corresponding acetates

A reagent system of acetyl bromide combined with a catalytic amount of tin(II) bromide cleaves readily trialkylsilyl ethers to give the corresponding acetates in high yields under very mild conditions.

Homogeneous Ziegler-Natta polymerization of functionalized monomers catalyzed by cationic Group IV metallocenes

Ziegler-Natta catalysts are remarkable in their ability to polymerize {alpha}-olefins to high molecular weight, stereoregular polyolefins. One of the major limitations of conventional Ziegler-Natta catalysts is their intolerance to Lewis bases; catalysts based on titanium halides and alkylaluminum cocatalysts are poisoned by most types of monomers containing ethers, esters, amines, and carboxylic acids. The absence of functionality in hydrocarbon polymers seriously affects their adhesive properties, affinity for dyes, permeability, and compatibility with more polar polymers. Previous attempts to polymerize sterically hindered amines, esters and amides, alkyl halides, and carboxylic acids using catalysts derived from TiCl{sub 3} and AlR{sub 3-n}Cl{sub n} have achieved limited success due to the severe loss of catalytic activity in the presence of these monomers. This work reports that cationic, group four metallocenes are active catalysts for the homo-polymerization of {alpha}-olefins containing silyl-protected alcohols and tertiary amines. Employing different monomers and conditions, a table shows the starting monomer, reaction time and temperature, and spectroscopic analysis of the end products. A major advanatage of these metallocene-based catalysts is that the ligand system can be modified to proved the optimal combination of catalystic activity, stereospecificity, and tolerance to functionality. 32 refs., 1 tab.

Tetrafluorosilane is a mild and very selective reagent for the cleavage of silyl-protected alcohols

Silyl ethers are cleaved by SiF 4 in CH 2cl 2 or CH 3CN at a rate which depends strongly on the steric screening of the ether oxygen, providing a promising method for selective desilylation under mild conditions.