Trimethylsilyl triflate catalyst

Some of the most useful 2,3-unsaturated C-glycosyl derivatives are the allyl compounds such as 92, made by use of allyltrimethylsilane, and a notable feature of this compound is that it can be produced directly from tri-O-acetyl-D-glucal with DDQ as promoter.111 A further surprising feature of this C-allylation procedure, however, is its applicability to glycals that are not O-substituted. Reactions are carried out in dichloromethane-acetonitrile at low temperatures with trimethylsilyl triflate as catalyst, and almost quantitative yields of 2,3-unsaturated C-allyl glycosyl compounds are recorded, with the a anomers being formed almost exclusively.112... 

Dialkoxymethyllithiums 43 3649 and 434649 651, 2-lithio-l,3-dioxolane 435649 and 2-lithio-1,3-dioxane 436649 are formyl anion equivalents, which have been prepared either by reductive lithiation of 2-(phenylsulfanyl) substituted precursors at —95 °C649 or by trans-metallation of 2-(tri-n-butylstannyl) substituted compounds at —110649 or —78°C651. The starting acyclic phenylsulfanyl precursors can be prepared from the corresponding orthoformates by reaction with (phenylsulfanyl)trimethylsilane and trimethylsilyl triflate as catalyst (for compounds 433 and 434). The cyclic derivatives (435, 436) were prepared from l,2-bis(l,3-dioxolan-2-yloxy)ethane and propane, in the same way649. 

After the readily available Friedel-Crafts catalyst, tin(IV) chloride, had been established as a catalyst for nucleoside synthesis, several other new catalysts, such as trimethylsilyl triflate (trimethylsilyl trifluoromethanesulfonate TMSOTf), etc., which are less acidic as Lewis acids... 

Silylation, for instance, providing 1-0-trimethylsilyl glycosides frOTi 1-0-unprotected sugars in an anomerically pure form, has been shown by Tietze and coworicers to be useful in phenyl glycoside and l,l -diacetal (see Section 1.2.5) synthesis with 0-silylated acceptors and trimethylsilyl triflate as catalyst. The diastereoselectivity was dependent on 0-protection. 

Scheme 2.25 shows some examples of additions of enolate equivalents. A range of Lewis acid catalysts has been used in addition to TiCl4 and SnCl4. Entry 1 shows uses of a lanthanide catalyst. Entry 2 employs LiC104 as the catalyst. The reaction in Entry 3 includes a chiral auxiliary that controls the stereoselectivity the chiral auxiliary is released by a cyclization using (V-methylhydroxylamine. Entries 4 and 5 use the triphenylmethyl cation as a catalyst and Entries 6 and 7 use trimethylsilyl triflate and an enantioselective catalyst, respectively. 

Trimethylsilyl triflate is also a powerful catalyst for acylation by anhydrides. Reactions of alcohols with a modest excess (1.5 equival) of anhydride proceed in inert solvents at 0°C. Even tertiary alcohols react rapidly.114 The active acylation reagent is presumably generated by O-silylation of the anhydride. 

Other reactions described are those of imidazolides with Grignard reagents, which are carried out in the presence of the catalysts trimethylsilyl triflate, iron trichloride, or iron trisacetylacetonate ... 

With boron trifluoride etherate instead of trimethylsilyl triflate the yield was 51%, without catalyst 40% and without catalyst and HMPA 25%.t88],[89] With CH3MgI instead of CH3MgBr the ketone is further transformed into a tertiary alcohol.[893 b) Reaction in the presence of FeCl3.[90J... 

Trimethylsilyl triflate itself can not promote allylation reactions of aldehydes with allyltri-methylsilane. By using the more highly reactive system Me3SiB(OTf)4, the reactions proceed smoothly.326 A very small amount (0.2-1 mol.%) of Me3SiB(OTf)4 is enough for the reactions (Scheme 75). Allylation of acetals can be promoted by trimethylsilyl bis(trifluoromethanesulfo-nyl)imide (Me3SiNTf2),327 which is also a reactive catalyst for Diels Alder reactions.328... 

The use of trimethylsilyl-based electrophilic catalysts with organosilicon hydrides also promotes the conversion of aldehydes into ethers and avoids the need to employ the potentially hazardous trityl perchlorate salt.314,334,338 One reagent pair that is particularly effective in the reductive conversion of aldehydes into symmetrical ethers is a catalytic amount of trimethylsilyl triflate combined with either trimethylsilane, triethylsilane, PMHS,334 or 1,1,3,3-tetramethyldisiloxane (TMDO, 64) as the reducing agent (Eq. 179).314 Either... 

Trimethylsilyl iodide can be substituted for the trimethylsilyl triflate catalyst in the reactions of aliphatic aldehydes. TMSI can be generated conveniently in situ either from trimethylsilyl chloride and sodium iodide in acetonitrile314 or from hexamethyldisilane and iodine in dichloromethane334 or pentane.338 It is noted that neither triisopropylsilane nor PMHS is an effective reducing agent for this purpose when used with TMSI under these conditions.314,334... 

Benzyne, which is generated in situ from 2-(trimethylsilyl)phenyl triflate and KF, acts as an alkyne congener in distannation in the presence of palladium-/ r/-alkyl isocyanide complex.157 A variety of substituted benzyne derivatives inserts into the Sn-Sn bond to give l,2-bis(stannyl)benzenes (Equation (59)). The reaction fails to occur in the presence of other palladium catalysts such as Pd(PPh3)4. 

C-Allyl glycosides can be prepared by the reaction of glycal epoxides with allyltributyltin in the presence of tributyltin triflate as a Lewis acid,281 and aldonitrones can be allylated with trimethylsilyl triflate as a catalyst (Equations (101) and (102)).282... 

Allenyltrimethylsilanes add to ethyl glyoxalate in the presence of a chiral pybox scandium triflate catalyst to afford highly enantioenriched homopropargylic alcohols or dihydrofurans, depending on the nature of the silyl substituent (Tables 9.39 and 9.40) [62]. The trimethylsilyl-substituted silanes give rise to the alcohol products whereas the bulkier t-butyldiphenylsilyl (DPS)-substituted silanes yield only the [3 + 2] cycloadducts. A bidentate complex of the glyoxalate with the scandium metal center in which the aldehyde carbonyl adopts an axial orientation accounts for the observed facial preference ofboth additions. 

In large measure, the problem associated with the execution of a stereoselective aldol condensation has been reduced to the generation of a specific enolate geometry. The recent results of Kuwajima (66a), which demonstrate that enolsilanes may be transformed into boryl enolates without apparent loss of stereochemistry (eq. [53]), should enhance the utility of vinyloxyboranes in stereoselective synthesis. The only current drawback to this procedure is associated with the presence of trimethylsilyl triflate (69), which must be removed from the reaction medium before the aldol condensation. It has recently been established that 69 is an effective catalyst for the aldol process (4). 

A bulky methylaluminium diphenoxide has been used as a co-catalyst with trimethylsilyl triflate to effect diastereoselective Mukaiyama aldols, including cases with less reactive aldehydes, and with ketones. ... 

For some condensations with silylated substrates as starting compounds, trimethylsilyl triflate can be used as a catalyst [103, 104, 705] A typical example of such a reaction is the aldol type condensation of silyl enol ethers and acetals catalyzed by 1-5 mol% of trimethylsilyl triflate [705] (equation 53)... 

Similarly, trimethylsilyl triflate can be used as a catalyst for the alkylation of 2 methoxy 1,3-oxazolidines [704] or 1-acetoxyadamantane [705] with allylsilane and for the reduction of acetals to ethers with tnalkylsilanes [106]... 

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