Pyridinium trifluoromethanesulfonate

Trimethylsilyl esters. Trimcthylsilyl esters are obtained in 65-90% yield by reaction ol carboxylic acids with hexamethyldisiloxane in refluxing toluene containing a catalytic amount of sulfuric acid. The actual catalyst is probably his(trimcthylsilyl) sulfate, [(CH3)3Si]2SO. Pyridinium trifluoromethanesulfonate also can serve as catalyst. 

Interestingly, while chirality may be found either in anion or cation, a chiral structure can arise even when neither anion nor cation is chiral. For instance, the (high-melting) pyridinium trifluoromethanesulfonate crystallises [662] in the chiral space group brought about by... 

Tsuge O, Kanemasa S, Kuraoka S, Takenaka Shigeori (1984) N-(Trimethylsilylmethyl)-pyridinium trifluoromethanesulfonates as facile precursors for nonstabihzed pyridinium methyhdes. Chem Lett 13 279-280... 

The values of heat capacities of 24 ILs, pyridinium-based, imidazolium, and ammonium ILs were presented by the same laboratory a year later [195]. The high value (766 J mol K at 298 K) was observed for l-hexyl-2-propyl-3,5-diethylpyridinium salt, [lCg-2C3-3,5C2py][Tf2N]. It was found that heat capacity increases linearly with increasing molar mass for these compounds that are comprised of a limited number of different atoms. A series of pyri-dinium and imidazolium-based ILs mainly with trifluoromethanesulfonate anion, [TfO], have been measured by Diedrichs and Gmehling [124]. The values between 300 and 800 J mol K were observed for different ILs. 

Bis(trimethylsilyl) peroxide, (CH3)3SiOOSi(CH3)3, is prepared from trimethylsilyl chloride, l,4-diaza[2,2,2]bicyclooctane, and Dabco s complex with 2 mol of hydrogen peroxide [127]. It is used alone [228] or in the presence of catalysts such as pyridinium dichromate [236] trimethylsilyl trifluoromethanesulfonate, CF3S03Si(CH3)3 [228, 237] or tris-(triphenylphosphine)ruthenium dichloride, [(C6H5)3P]3RuCl2 [236]. This reagent oxidizes primary alcohols to aldehydes (in preference to the oxidation of secondary alcohols to ketones [236]), ketones to esters or lactones Baeyer-Villiger reaction) [238], and nucleoside phosphites to phosphates [228]. All these oxidations require anhydrous conditions. 

Treatment of 90 with sodium methoxide results in the deprotonation of the NH group of the heterocycle to give the pyrido[l,2-a]pyrimidinyl derivative Ru(ri5-C5H5) 2,2-diphenyl-2//-pyrido[l,2-a]pyrimidin-4-yl (CO)(P Pr3) (92), which reacts with methyl trifluoromethanesulfonate to afford [Ru(T 5-C5H5) 1-methyl-2,2-diphenyl-2//-pyridinium[ 1,2-a]pyrimidin-4-yl (CO)(P Pr3)]CF3SOj (93). The deprotonation of 90 produces a high electronic perturbation in the bycycle of this compound, as is shown in Scheme 26. 

Reaction with Alcohols and Phenols. The reaction of alcohols and phenols with triflic anhydride (Tf20) at 0°C in the presence of a base (usually Pyridine) in an inert solvent (usually dichloromethane) for 2-24 h affords the corresponding reactive trifluoromethanesulfonate esters (triflates). When triflic anhydride and pyridine are combined, the pyridinium salt forms immediately and normally precipitates out from the reaction mixture. Nevertheless, the salt is an effective esterifying agent, reacting with the added alcohol to give triflates in high yields (eq 1). ... 

Since it became obvious that liquid HF is not at all the detachment reagent of choice (see in contrast [35, p. 380]), the search for a similarly efficient reagent resulted recently in the utilization of boron tris (trifluoroacetate) [172], which in trifluoroacetic acid is reported to smoothly liberate peptides bound to the polymer support by benzyl ester type linkages. All above-mentioned protecting groups stable against trifluoroacetic acid are simultaneously cleavable. Trifluoromethanesulfonic acid [173] and pyridinium polyhydro-genfluoride are proposed for the same purpose [174] (Fig. 52). 

Addition of a proton to a carbyne ligand may transform it into a carbene. The proton can be provided by an external source or by transfer from another ligand. Examples of the first land are provided by the addition of acids to complex 18 (see Scheme 7).l l The use of pyridinium salts yields more stable pyridine adducts. This reaction can be reversed by the addition of a strong base (see Section 2.6.2). When an acid containing a noncoordinating anion is used, e.g. trifluoromethanesulfonic add, cationic derivatives may be obtained, Depending on the nature of the coligands, the proton may preferentially add to another position in the molecule (see Section 2.6.2). 

SCHEME 2 Synthesis of intermediate 13. Tf, trifluoromethanesulfonate PCC, pyridinium chlor-ochromate DMSO, dimethyl sulfoxide PPTS, pyridinium 4-toluenesulfonate EE, ethoxyethyl. 

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