Silyl triflat reactions

Recently, Ni-catalyzed allylic substitutions of simple alkenes for the formation of 1,4-dienes 174 have been described by Jamison and Matsubara. Allylic alcohol derivatives 172, bearing a variety of leaving groups, can be coupled with a wide range of simple alkenes 173 including gaseous ethylene and propylene. The key for the reaction was the use of the appropriate nickel-phosphine complex and a stoichiometric amount of a silyl triflate. Reactions of 1-alkyl-substituted alkenes... 

A polymer-supported silyl triflate and subsequent functionalization synthesis and solid-phase Diels-Alder reactions of silyloxydienes [25]... 

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. 

Trialkylsilyl cations may play a key role in Lewis acid-catalyzed reactions. Trimethyl-silyl triflate itself is not a good catalyst, but in combination with other Lewis acids it generates excellent catalytic activity. 

Acetalizatioru Aldehydes and ketones are converted into acetals and ketals, respectively, by reaction with alkoxytrimethylsilanes in the presence of trimelhyl-silyl triflate as catalyst. 

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. 

The resulting propargylic alcohol is protected as TIPS ether by a standard procedure using the corresponding silyl chloride and imidazole in DMF. Optionally the more reactive silyl triflate and 2,6-lutidine may be employed in order to shorten the reaction time. Under acidic conditions TIPS and TPS are nearly stable protecting groups. Therefore the TBS ether is selectively cleaved with acetic acid even in presence of the acetal moiety.13 Subsequent reaction with iodine and triphenylphosphine, known as the Appel reaction14, provides the desired iodide 4. 

The Pummerer reaction of 3,3-dibenzoyloxyethyl-thietane 1-oxide 32a with thymine in the presence of trimethyl-silyl triflate (TMSOTf), triethylamine, and Znl2 in dichloromethane allowed the synthesis of the thietane-derived thymidine 33a in 70% yield <1996TL7569>. The treatment of thietane-1-oxide derivative 32b under similar conditions but in toluene resulted in the modified nucleoside 33b in 30% yield (Scheme 9) <1996TL7569>. 

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). 

Starting from silyl triflates, fluorosilanes are obtained by reaction with LiF in ether84,85 (equation 51). 

The addition of carbonylated electrophiles to the 2-lithio derivative of 4-oxazolinyloxazole 132 allowed the efficient preparation of 5-phenyloxazoles 134 bearing a variety of hydroxyalkyl groups at C-2 position and a carboxyl (or formyl) function at C-4. This protocol suppresses the troublesome electrocyclic ring-opening reaction and allows access to the target compounds by simple chemical transformation of the oxazoline moiety of 133 <02JOC3601>. A direct chemoselective C-2 silylation of oxazoles was performed by treatment of the lithiated parent compounds with silyl triflates <02TL935>. 

The other side of the coin is that the S 2 reaction at carbon is not much affected by partial positive [ charge (5+) on the carbon a tom. The Sn2 reaction at silicon is affected by the charge on silicon. The r most electrophilic silicon compounds are the silyl triflates and it is estimated that they react some 108-109 times faster with oxygen nucleophiles than do silyl chlorides. Trimethylsilyl triflate is, in fact, an excellent Lewis acid and can be used to form acetals or silyl enol ethers from carbonyl compounds, and to react these two together in aldol-style reactions. In all three reactions the triflate attacks an oxygen atom. 

In 2004 an indium-NHC complex was reported which was prepared from an air stable imidazolium salt precursor.29 Reaction of one equivalent of imidazolium salt with InMe3 affords (NHC)InMe2Cl 18 (Scheme 12). The indium-NHC bond distance of 2.267(2) A is shorter than that of InH3(NHC), though longer than the bis-NHC InBr3(NHC)2. Mono-triflate (19) and bis-triflate (20) complexes can also be prepared by treatment of 18 with trimethyl silyl triflate and treatment of 19 with triflic acid respectively. 

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. 

Four synthetic routes lead to silyl triflates (1) reaction of silyl chlorides with silver triflate (very expensive), (2) reaction of silyl chlorides with triflic acid, (3) reaction of tetraalkylsilanes with triflic acid, and (4) reaction of arylsilanes with triflic (trifluoromethane sulfonic) acid (24-26). The displacement of a chlorine atom and a phenyl group from trimethylsilyl chloride and trimethylphenylsilane indicate that these groups are much more reactive than the alkyl substituents for the synthesis of triflated silanes. We found that the aryl group is over 200 times more reactive than chloride in these reactions (14). We were able to displace two phenyl groups from diphenyl-dimethylsilane in a stepwise manner ... 

Compound 7 was evaluated as a Diels-Alder catalyst instead of silyl triflate (Sch. 22). As expected from Si NMR measurement, substantial rate acceleration was observed in the Diels-Alder reaction of methyl acrylate with a variety of dienes. 

Silylation with silyl chlorides or silyl triflates is performed in the presence of auxiliary bases, e.g. pyridine, TEA, imidazole, etc., in solvents such as CH2CI2, CHQ3, DMF, or As milder reagents A-methyl-G-(trimethylsilyl)acetamide and preferably A,C)-bis(trimethylsilyl)acetamide (BTMSA, 136) (Scheme 72) are used for such purposes. Since the acetamide that results from this reaction is difficult to remove from the reaction medium, cyanotrimethylsilanet or l-(trimethylsilyl)imidazole (137).t l may be used as alternatives. For quantitative silylation of amino acids containing additional side-chain functionalities, excess silylating agent is required. 

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