Silylating reagents silyl triflates

Among these groups (siloxy, amino, benzotriazolyl, thiolate, cyanide) the methoxy group has been the subject of a large number of studies. To generate the azomethine ylid, essentialy three reagents have been utilized silyl triflate (or iodide), trifluoroacetic acid and lithium fluoride (sometime combined with sonication). Thermal rearrangement has also proven to be efficient. 

Some qualitative information indicates that perchlorates might be more reactive than iodides (24). Thus, three silylating compounds are much more reactive than the others iodide, perchlorate, and triflate (trifluorometh-anesulfonate). Iodides and perchlorates have considerable disadvantages as light-sensitive and explosive reagents, respectively. Triflates, however, are stable for long periods in the absence of nucleophiles (moisture included). 

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. 

Alternatively, the NM2 silyl triflate 7 can be used as a very powerful silylating reagent capable of silylating even tertiary alcohols under mild reaction conditions (2,6-lutidine as base, ambient temperature, 2 h, aqueous work-up). The NM2 silyl triflate can be simply prepared by reacting neat silane NM2 3 with triflic acid 6 at 60 °C for 10 h. The triflate is a colorless liquid that can be purified by direct distillation (bp 100 - 105 °C/1 mbar) from the reaction flask in 83% yield. The transformation from chloride to triflate does not alter the exo/endo ratio according to GC/NMR. 

In conjunction with the study of alkyl-substituted silyl triflates, (2) and (3) have been prepared from the corresponding alkynyl-lithium reagents and di-f-butylsilyl bis(trifluoromethanesul-fonate). ... 

In the late 1970s, Enders pioneered an elegant method for ketone and aldehyde alkylation involving the use of metalated chiral hydrazones [92, 93). Extensive studies with the (S)-l-amino-2-methoxymethylpyrrolidine (SAMP, 150, Scheme 3.24) auxiliary and its enantiomer RAMP established these as superb chiral auxiliaries with numerous applications. In a typical alkylation sequence, a RAMP/SAMP hydrazine is condensed with an aldehyde or a ketone to form the corresponding hydrazone, such as 152. This can subsequently be deprotonated and the resulting enolate trapped with a variety of electrophilic reagents including alkyl halides, aldehydes, Michael acceptors, silyl triflates, and disulfides. The RAMP/SAMP hydrazine auxiliary may be removed by acidic hydrolysis or ozonolysis to reveal the alkylated... 

Trimethylsilyl trifluoromethanesulfonate (trimethylsilyl triflate) is the most synthetically useful representative of the family of trialkylsilyl perfluoroalkane-sulfonates (for a review, see reference 101) This reagent is commercially available or can be prepared easily by the reaction of chlorotrimethylsilane and triflic acid [101] It has wide application in organic synthesis as an excellent silylating reagent... 

Difunctional reagents, for example the very cheap dimethyldichlorosilane 48, which is produced on a large technical scale, and the much more reactive and expensive dimethylsilyl bis(O-triflate) 49 [65-67] (Scheme 2.8) convert alcohols or phenols 11 in the presence of bases, for example triethylamine or DBU, into the silylated compounds 50. Thus 48 and 49 and other bifunctional reagents such as di-tert-butyldichlorosilane [68] or di(tert-butylsilyl)-bis(0-triflate) [69] and the subsequently described 51 and 52 combine two alcohols to silicon-tethered molecules 50, which can undergo interesting intramolecular reactions [70-74]. 

Trimethylsilyl triflate is also a powerful catalyst for acylation by anhydrides. Reactions of alcohols with a modest excess (1.5 equival) of anhydride proceed in inert solvents at 0°C. Even tertiary alcohols react rapidly.114 The active acylation reagent is presumably generated by O-silylation of the anhydride. 

Silylating reagents such as TMSI and TMS triflate have only a modest catalytic effect, but the still more powerful silylating reagent (CH3)3SiB(03SCF3)4 does induce addition to aldehydes.90... 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

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