Trimethylsilyl trifiate

The cyclization of 306 promoted by trimethylsilyl trifiate and diisopropylethyl-amine gives cw-dihydropyrones 307. Under these conditions methoxybutenone fails to form the aldol condensation product 305 (90TL2213). 

Acetals and ethers can also be used to produce benzylic car-bocations by reacting with a Lewis acid. Treatment of acetals derived from chiral 2-substituted benzaldehyde chromium tricarbonyl complexes with trimethylsilyl trifiate followed by reaction with a silyl enol ether give diastereomerically pure alkylation products (Scheme 98). Benzylic alcohols can be... 

Tetra-n-butylammonium hydrogen sulfate s. under NaOH Trimethylsilyl trifiate s. under Mg Trifiuoromethanesulfonic acid s. under Mg Tetra-n-butylammonium bromide s. under A2CO3... 

Tanabe and coworkers reported the first instances of additions of titanium enolates to ketones [8]. They reported moderate to excellent yields for several reactions. When the two substituents on the ketone were sufficiently sterically differentiated, good to excellent syn diastereoselectivity was observed. Representative examples of these crossed-aldol reactions are shown in Table 2.1. The initial method involved a catalytic (5 mol%) amount of trimethylsilyl trifiate in the reaction mixture it was suggested this generates a trichlorotitanium trifiate species in situ which is more effective at enoli-zation. This theory was, however, called into question by reports of additions of enolates to ketones with trimethylsilyl chloride as catalyst, which also provides moderate to good yields [9]. Additions of enolates of phenyl esters or phenyl thioesters to ketones were also shown to proceed in good yield and with syn diastereoselectivity in the absence of silyl additive [10]. 

A specific combination of t vo veak acids, tin(II) chloride and chloro-trimethylsilane, is found to serve as an effective catalyst for the aldol reaction [37]. Neither chlorotrimethylsilane or tin(II) chloride has any accelerating effect at —78 °C even vhen more than one equivalent is added. In the combined presence of catalytic amounts of chlorotrimethylsilane and tin(II) chloride, hovrever, the aldol reaction gives the desired product in more than 90% yield (Eq. (29)). It is supposed that the cationic silyl species generated by coordination of the chloride to the tin(II) atom catalyzes the aldol reaction. Because a similar cationic silicon Le vis acid, trimethylsilyl trifiate, is kno vn to catalyze aldol reactions of silicon enolates [38], and several other unique Le vis acid catalysts have also been reported (e.g. lanthanide catalysts [39]). Water-stable Le vis acids such as lanthanide trifiates, vhich catalyze the aldol reaction in aqueous solvent or pure vater, have received much attention in the development of economical and environmentally benign synthetic methods [40]. 

N-Alkoxymethyl(trimethylsilyImethyl)amines are convenient precursors for C-unsubstituted azomethine ylides 43 since the N-alkoxymethyl(trimeth-ylsilylmethyl)amines are readily available by the reaction of N-alkylated silylmethylamines with formaldehyde in alcohol solvents (84CL1117). Treatment of the amines with trimethylsilyl triflate in acetonitrile or tetrahy-drofuran brings about the elimination of alkoxy group R O to form N-silylmethyliminium trifiates 42. The subsequent desilylation with cesium fluoride generates C-unsubstituted azomethine ylides 43 (84CL1117). Both steps of the alkoxy elimination and subsequent desilylation of 42 are induced by trifluoroacetic acid (85CPB896, 85CPB2762), tetrabutylammonium... 

A solution of the o-(trimethylsilyl)aryl trifiate (1 mmol) in dry acetonitrile (2 mL) is added to a suspension of finely powdered anhydrous CsF (2 mmol) and Pd(PPh3)4 (0.1 mmol) in the same solvent (10 mL). After stirring for 12 h at room temperature under argon, the solvent is evaporated and the residue purified by column chromatography. 

Kita and coworkers developed an efficient synthesis of aiyl(phenyl)iodonium tosylates from HTIB and arenes in TFE at room temperature [29, 30]. This type of iodonium salt is chemoselectively employed in metal-free syntheses of biaryls (see Sect. 4.2.1). The synthetic strategy has been employed, for example, in the synthesis of polyfluorinated At21X [81] and acid-labile boron-substituted diaryliodonium salts firom aryl boronates with HTIB or the reagent combination PIFA/AcOH (Scheme 4a) [82]. Suna and coworkers have recently described the synthesis of heteroaryl(aryl)iodonium salts from heteroarenes and [hydroxy(tosyloxy)iodo] mesitylene or DIB combined with TsOH or AcOH. The salts were either isolated or used in situ (see Sects. 3.1.2 and 3.2.2) because of their relative instability [83, 84]. (Diacetoxyiodo)arenes were recently combined with electron-donating arenes and trimethylsilyl triflate (TMSOTf) to provide unsymmetric iodonium trifiates without the need for acid addition [85]. 

Efforts to trap the carbonyl ylide intermediate by intramolecular [3 - - 2] cycloaddition to a C=C bond were unsuccessful. Rather, the decomposition of allyl (trimethylsilyl)diazoacetate (218) (equation 69) in the presence of aldehydes gave 1,3-dioxolan-4-ones 219 their formation has been explained by 1,5-cyclization of the carbonyl ylide intermediate followed by a Claisen rearrangement. With acetone as carbonyl component, the reaction proceeds analogously. Clean formation of 219 occurred only with Rh2(OOCC3p7)4 as catalyst, while the copper(I) trifiate catalyzed version led to a mixture of 219, an oxirane and the product of intramolecular carbenoid... 

Lewis adds, such as FeCls and SnCU, initiate polymerization in cooperation with protic adds, resulting from the reaction of those spedes with traces of water or other acid contaminations present in the S3 tem. Indeed, some Lewis acid-protic acid combinations are very effective as initiators. The question whether Lewis adds alone are capable of initiating ROP of siloxanes is still open. Studies of the polymerization of D3 in the presence of sterically hindered substituted pyridine used as a proton trap proved that some nonprotic spedes, such as RC(0)Cl-SbCls complex, ethylboron sesquitrifiate (Et3B2(OTf)3), and certain metal trifiates, can initiate polymerization of this monomer. Trimethylsilyl triflate was considered to be inactive without addition of free trifluoro-methanesulfonic (trifiic) add CF3SO3H (TfOH). However, Jallouli and Saam reported that, after a long induction period, TMSOTf initiated the polymerization of some... 

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