Trialkylsilyl triflates

For products (507), the choice of reaction conditions is governed by the nature of the C,C double bond. For terminal enamines (507a,b), trialkylsilyl triflates with a pyridine additive is the reagent of choice (535) (Table 3.35, entries 1,2). By... 

Reviews. Simchen et al.6 have reviewed use of trialkylsilyl triflates for silylation and as catalysts in Friedel-Crafts acylations (191 references). The review includes a method for recovery of triflic acid from the triethylammonium triflates formed in silylation in the presence of triethylamine. 

Transfer hydrogenation, 255 Trialkylaluminum, 539-542 Trialkylsilyl triflates, 567 Triaryl phosphates, 542 1,2,3-Triazoles, 573 2,2,2-Tribromoethanols, 111-112 Tri-/i-butylcrotyltin, 542-543 Tributylfluorophosphonium bromide, 543 Tri-rt-butyl(iodomethyl)tin, 543-544... 

This method provides a general route to trialkylsilyl triflates and has the advantage of circumventing use of the expensive silver triflate. Triisopropylsilyl triflate in combination with a base, preferably 2,6-lutidine, is effective for silylation of alcohols. 

Doubly acceptor-activated imines with an intramolecular alkene moiety such as 29 can cyclize according to an electrophilic mechanism to give pyrrolidine, piperidine or azepine derivatives. This reaction is induced by stoichiometric amounts of Lewis acids, preferably trialkylsilyl triflates [38]. In certain cases FeCl3 can also be used, for example for the preparation of azepinolactone 30 from imine 29 (Scheme 8.10) [39]. Catalytic amounts of FeCl3 are required for the addition of diethyl phosphite to an... 

Conjugate addition of alkynylzinc bromides.2 Alkynylzinc bromides in combination with 1 equiv. of a trialkylsilyl triflate undergo conjugate addition to cyclic and acyclic a,P-enones at -40° in THF or ether. A mixture of 1,2- and 1,4-adducts is formed from 3 disubstituted a,p-enones. 

It is advantageous to employ trialkylsilyl triflates as additives these conditions improve the yields of the subsequent addition reactions, for example, in forming 293 and 294 <2000JOC9272>. 

Diazoacetic acid silyl esters can be prepared by fra t-esterification of tert-butyl diazoacetate with trialkylsilyl triflate <1985JOM33>. Analogously prepared (alkenyloxy)silyl 203 and (alkynyloxy)silyl diazoacetates 206 underwent silicon-tethered 1,3-dipolar cycloaddition reactions as shown in Scheme 37 and Equation (38). Compound 205 resulted from a lateral criss-cross cycloaddition of the intermediate azine 204, which was formed from two molecules of 203 by diazo + diazo or diazo + carbene reaction <2000T4139>. On the other hand, when silyl diazoacetates 206 were kept in xylene at 142 °C for 1 h, bicyclic pyrazoles 207 were obtained (Equation 38). 

Carbon-centered nucleophiles can also be used to advantage in the reaction with epoxides. For example, the lithium enolate of cyclohexanone 96 engages in nucleophilic attack of cyclohexene oxide 90 in the presence of boron trifluoride etherate to give the ketol 97 in 76% yield with predominant syn stereochemistry about the newly formed carbon-carbon bond <03JOC3049>. In addition, a novel trimethylaluminum / trialkylsilyl triflate system has been reported for the one-pot alkylation and silylation of epoxides, as exemplified by the conversion of alkenyl epoxide 98 to the homologous silyl ether 99. The methyl group is delivered via backside attack on the less substituted terminus of the epoxide <03OL3265>. 

Since introduction of the Ireland-Claisen rearrangement in 1972, the Ireland variant has become increasingly popular in organic synthesis [66]. Although excess Ic with appropriate base is often employed as selective silylating agent, the role of residual trialkylsilyl triflate and base has not been detailed. Illustrated here are some examples of silyl triflate-mediated rearrangement which can be conducted under milder conditions. 

In addition, amine N-oxides (82) can be treated with trialkylsilyl triflates to generate, after methyl-lithium treatment, a-siloxyamines (83) which, when allowed to react with Grignaid reagents (or trialkyl-aluminum), generate tertiary amines (M) in modest yields (Scheme IS). ... 

Tsunoda, T., Suzuki, M., Noyori, R. Trialkylsilyl triflates. III. Trimethylsilyl trifluoromethanesulfonate as a catalyst of the reaction of allyltrimethylsilane and acetals. Tetrahedron Lett. 1980, 21,71-74. 

The alkoxy- and silyloxyiminium salts 81 and 84 are obtained from chiral acrylamides by heating with triethyloxonium tetrafluoroborate or trialkylsilyl triflate. These very active dienophiles add to cyclopentadiene (2) with excellent diastereoselectivity, generating norbor-nenyl amides after hydrolysis (see table below)22,23. The best results are obtained by triflate activation, while tetrafluoroborate activation generally leads to a mixture of products22. 

In the rearrangement of allyl fluoroacetates, trialkylsilyl triflates have been introduced as a new reagent for the Z-selective generation of silyl ketene acetals485. Thus, when (T)-crotyl fluoroacetates are treated at ambient temperatures with a trialkylsilyl triflate in the presence of a tertiary amine, rearranged products with a svn relationship are preferentially obtained. The ketene acetal intermediates cannot be isolated and the geometry has been deduced from the stereochemistry of the products. The selectivity of this process improves in the order triisopropyl > ferf-butyldimethylsilyl > rerf-hexyldimethylsilyl > trimethylsilyl a triethylsilyl (see Table 11). 

When using iminium salts derived from chiral acrylamides and trialkylsilyl triflate at low temperature, moderate to high stereoselectivities were observed. 

An asymmetric intramolecular Michael-aldol reaction which leads to nonracemic tricyclic cyclobutanes is performed by using TMSOTf andbis[(/ )-l-phenylethyl]amine as chiral amine, but only moderate enantioselectivities are reached (eq 68). A similar reaction sequence can also be carried out with TMSOTf and HMDS as base, with (—)-8-phenylmenthol as the chiral auxiliary however, the iodotrimethylsilane-HMDS system is more efficient in terms of yield and diastereoselectivity. The combination EtsN/TMSOTf (or some other trialkylsilyl triflates) has been used to accomplish an intramolecular Michael reaction, which was the key step for the synthesis of sesquiterpene (=E)-ricciocarpin A. ... 

Epoxide Ring Opening. One-pot alkylation-O-silylation reactions of epoxides take place in excellent yields when the epoxides are treated with trialkylaluminum-TMSOTf and EtsN (1.5 equiv of each one) in methylene chloride at —50 °C (eq 92). Other trialkylsilyl triflates also undergo this reaction, which is stereospecific awft -compounds are obtained from fran -epoxides while cis-epoxides yield 5yn-adducts. ... 

Solubility readily sol hydrocarbons, dialkyl ethers, halogenated solvents. CH2CI2 is employed most commonly. Reactions in 1,2-dichloroethane proceed faster than those in CCLj or Et20. Protic solvents and THF react with trialkylsilyl triflates and are therefore not suitable. 

Handling, Storage, and Precautions trialkylsilyl triflates are generally corrosive and moisture sensitive. Appropriate precautions should be taken to ensure that the reagent is handled and stored under rigorously anhydrous conditions. 

Unsymmetrical ketones form the silyl ether of the thermodynamically more stable enol on treatment with trialkylsilyl triflates and triethylamine. The same transformation may be carried out when the trifiate is prepared in situ by the reaction of allyltrimethylsilane with a catalytic amount of trifluoromethylsul-phonic acid. ° ... 

C-Silylation. 2-Silyloxazoles can be prepared by deprotonation and quenching the resulting ambident oxazole anion (isocyano enolate) chemoselectively with trialkylsilyl triflates (eq 46). Trialkylsilyl chlorides generally give the product of O-silylation. ... 

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