Silyl triflates, production

The silyl ketene acetal rearrangement can also be carried out by reaction of the ester with a silyl triflate and tertiary amine, without formation of the ester enolate. Optimum results are obtained with bulky silyl triflates and amines, e.g., f-butyldimethylsilyl triflate and (V-methyl-Af, /V-dicyclohcxylaminc. Under these conditions the reaction is stereoselective for the Z-silyl ketene acetal and the stereochemistry of the allylic double bond determines the syn or anti configuration of the product.243... 

The reaction is performed at —94°C in dichloromethane and, generally, in the presence of catalytic amounts of silyl triflate. Although the reaction can be performed with trimethylsilyl derivatives as well, it is advantageous to use hydrolytically more stable SilV Bu derivatives. The reaction of 1,4-dinitrobutane can afford both mono- and bis-C,C-coupling products. 

Two recently developed coupling reactions of an alkene (R1CH2CH=CH2), an aldehyde (R2CHO), and a silyl triflate (R33SiOTf) yield an allylic (66) or homoallylic (67) alcohol (in protected form).188 Employing nickel-phosphine catalysts, either product can be selected by small changes in the phosphine component. A mechanism distinct from that of Lewis acid-catalysed carbonyl-ene reactions is proposed and discussed. 

More recently, trifluoromethanesulfonic acid (triflic acid, TfOH) has been used to functionalize silanes by electrophilic substitution of aryl substituents62,63 (equation 35). The silyl triflates formed in this reaction are useful building blocks for a wide variety of products. Chlorosilanes can be obtained by treatment with lithium chloride (equation 36). 

A large number of different a,(o-bis[(trifluoromethyl)sulfonyloxy]-substituted organosilicon compounds can be obtained by relatively simple methods from the corresponding amino-, allyl-, or phenylsilanes. Moreover, it is remarkable that these silyl triflate derivatives are often easily formed, when the synthesis of the corresponding chloro- or bromosilanes is difficult or does not appear to have been attempted. Eq. 2 and Eq. 3 show selected examples of this synthesis [10-12]. The products were prepared in high purities and yields. The resulting triflates should be used for the polycondensation without further purification, because they often cannot be destilled without decomposition. 

The reaction of tri-O-benzoylgalactal with methyl benzenesulfenate in the presence of trimethyl-silyl triflate and molecular sieves at —15 °C for 30 minutes gives a 68 20 12 mixture of products. 

The highly reactive silyl ttiflates and silyl bis(triflates) are obtained by reaction of the corresponding aminosilanes with triflic acid in diethylether as shown in Schemes 1 and 2 [12]. The substitution patterns at the silicon atoms are variable. Functional substituted silanes, e.g., vinyl, allyl, or hydrogen derivatives can also be obtained. (Diethylamino)diphenylsilyllithium is formed from (diethylamino)diphenylchlorosilane with lithium in THF. Yields of both reactions are high (90-95%). The products can be used for following reaction without further purification. Silyl triflates and (aminosilyl)lithium compounds react to give the amino-substituted trisilanes la - Ic (Scheme 1). 

Mukaiyama aldol reactions using a catalytic amount of a Lewis acidic metal salt afford silylated aldols (silyl ethers) as major products, but not free aldols (alcohols). Three mechanistic pathways which account for the formation of the silylated aldols are illustrated in Scheme 10.14. In a metal-catalyzed process the Lewis acidic metal catalyst is regenerated on silylation of the metal aldolate by intramolecular or intermolecular silicon transfer (paths a and b, respectively). If aldolate silylation is slow, a silicon-catalyzed process (path c) might effectively compete with the metal-catalyzed process. Carreira and Bosnich have concluded that some metal triflates serve as precursors of silyl triflates, which promote the aldol reaction as the actual catalysts, as shown in path c [46, 47]. Three similar pathways are possible in the triarylcarbenium ion-catalyzed reaction. According to Denmark et al. triarylcarbenium ions are the actual catalysts (path b) [48], whereas Bosnich has insisted that hydrolysis of the salts by a trace amount of water generates the silicon-based Lewis acids working as the actual catalysts (path c) [47]. Otera et al. have reported that 10-methylacridinium perchlorate is an efficient catalyst of the aldol reaction of ketene triethylsilyl acetals [49]. In this reaction, the perchlorate reacts smoothly with the acetals to produce the actual catalyst, triethylsilyl perchlorate. 

The standard Ireland conditions for the ester enolate rearrangement (lithium diisopropylamine, tetrahydrofuran) give a retro-Michael addition product in this ease. However, silyl ketene acetal 15 is successfully obtained by the silyl triflate/triethylamine protocol539 for the preparation of ketene acetals which proceeds via a silyladon and then deprotonation mechanism560-563. 

Enones can be reacted with tm-butyldimethylsilyl triflate and triphenylphosphine to produce the regiospecifically generated enol er -butyldimethylsilyl ether of /toriphenyl-phosphonium ketones57 (equation 50). In this reaction the silyl triflate is not only present in order to form the enol silyl ether, but to catalyze the addition of the phosphine to the enone. The resultant products can be further used to prepare dienol silyl ethers and / -substituted enones. 

The silyl triflate 8 (Scheme 3) shows an ambireactive behavior of the cation. Reaction with a methylmagnesium chloride or water leads to the methyl-substituted product and to the siloxane, respectively, as expected for an electrophilic silicon center. An oxonium ion reactivity is observed in the reaction with neutral Lewis bases such as triethylamine and trimethylphosphine. 

The endIcT-selective hetero Dids-Alder reaction of chiral oxazolidones 12 with (Z)-l-acetoxy-2-ethoxyethene yields the 1-substituted glycals 13 as the major products when catalysed with dimethylaluminium chloride, and 14 as the major products when catalysed with trimeth silyl triflate. Compounds 13 (R = Et)and 14 (R = Et) were subsequently converted into ethyl P-D-mannopyranoside and ethyl 3-L-mannopyrannoside, respectively. ... 

A novel silyl triflate-promoted Payne rearrangement of silyloxy epoxides was reported by Jung et al When the ethyl substituted epoxy silyl ether 13 was treated with silyl triflate in the presence of a base, a mixture of four ketones, 16a-d and four aldehydes 17a-d were obtained. It has been assumed that two ketones and two aldehydes could be formed via a non-aldol process and an epoxide rearrangement, whereas the other four products through 14 and 15, a silyl triflate promoted Payne rearrangement. 

Figure 36 Production of silyl triflates and silanol intermediates.

Treatment of ketone (69) with excess amounts of t-BuMe2SiOTf and bis[(k)-l-phenylethyl]amine ((i )-BPEA) gives tricyclic silyl aldolate (70) with moderate enantioselectivity [104]. The formation of (70) can be explained by the enol silylation to (71) followed by a tandem Michael-aldol reaction. The asymmetric induction by the chiral amine occurs in the enol silylation (Scheme 9.40). The combined use of silyl triflates and amines has been applied to an intramolecular aldol reaction for natural product synthesis [105]. 

Scheme XLL Synthesis of dl-perhydrogephyrotoxin (277).Reaction B (iii) with LiAlILt affords 1 part of the 2-epimer and 8 parts of the desired isomer, while with NaBR the major product is the 2-epimer. Enol silylation is effected with the triflate in reaction C (iii). Reaction D is best conducted with the dimethylacetal which is formed in reaction D (i). This reaction yield some of the 1-epimer. A i) Heat, ii) Lithium dimethyl 2-oxo-7-(ethylenedioxy)-heptyl phosphonate. B i) Pd/C, CF3CO2H, H2, ii) NaOH, iii) LiAlH4. C i) CICO2CH2CCI3, ii) HCl, iii) Trimethyl-silyl triflate, iv) Pb(OAc)2. D i) Pyridinium toluenesulphonate, CH3OH, ii) Zn/PbO, iii) HCl,...

The 2+2 cycloadditions of benzyne to cis- and trani-propenyl ether gave cis- and fran -benzocyclobntanes as the main products, respectively [ 117,118], Stereospecific [2+2] cycloaddition reactions were observed between the benzyne species generated by the halogen-Uthium exchange reaction of ort/io-haloaryl triflates and the ketene silyl acetals (Scheme 23) [119],... 

The use of the enolsilyl ether of 1-menthone [16, 19, 21-23] and of some free triflic acid favors the formation of the thermodynamically controlled products as with free 2,2 -dihydroxydiphenyl [22] and only subsequently added HMDS 2 [22]. On reacting silylated alcohols and carbonyl compounds with pure trimethylsilyl triflate 20 under strictly anhydrous conditions no conversion to acetals is observed [24]. Apparently, only addition of minor amounts of humidity to hydrolyze TMSOTf 20 to the much stronger free triflic acid and hexamethyldisiloxane 7 or addition of traces of free triflic acid [18-21, 24, 26] or HCIO4 [25] leads to formation of acetals. 

Entries 4 and 9 are closely related structures that illustrate the ability to control stereochemistry by choice of the Lewis acid. In Entry 4, the Lewis acid is BF3 and the (3-oxygen is protected as a f-butyldiphenylsilyl derivative. This leads to reaction through an open TS, and the reaction is under steric control, resulting in the 3,4-syn product. In Entry 9, the enolate is formed using di-n-butylboron triflate (1.2 equiv.), which permits the aldehyde to form a chelate. The chelated aldehyde then reacts via an open TS with respect to the silyl ketene acetal, and the 3,4-anti isomer dominates by more than 20 1. 

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