Triflate, trimethylsilyl fluorination

It was found that the counteranion of the 1-fluoropyridinium salts considerably affects selective fluorinations.54 For example, fluorination with 1-fluoropyridinium triflate (la) was compared to that with l-fluoro-4-methylpyridinium-2-sulfonate (2b) using a steroid 10 containing an acetoxy function and a conjugated trimethylsilyl enol ether as the substrate.54... 

Diethyl 1-lithio-l-(trimethylsilyl)-1-fluoromethylphosphonate, generated in situ from diethyl 1,1-dibromo-1-fluoromethylphosphonate and -BuLi (2 eq) or from diethyl fluoromethylphosphonate and LDA (2 eq) in the presence of Me,SiCl under internal quench conditions, is readily alkylated at low temperature with alkyl and allyl iodides -" " or triflates." "- The resulting fluorinated alkylphosphonates can be desilylated with EtOLi in EtOH at 0°C to give diethyl a-fluoroalkylphos-phonates in excellent yields (83-96%, Scheme 3.58). 

In spite of electron withdrawing action of four fluorine atoms, 4,5,6, 7-tetrafluoroindole 132 reacts with electrophiles under quite mild conditions to give products of substitution at the 3-position. For example, reaction of 132 with AT-(carbobenzyloxy)piperidin-4-one 168 in the presence of trimethylsilyl triflate and tiiethylsilane afforded the corresponding piperidine derivative 169 [62], Using sulfur ttioxide-pyridine complex indolyl sulfonic acid 170 was obtained, which was further converted into sulfonyl amide 172 by reaction with phosphoms(V) oxychloride, followed by treatment with derivative of piperazine [2a],... 

Konno reported a detailed study of the reaction of fluorine-containing propargyl acetates under Nicholas conditions with a variety of nucleophiles. Allylstannanes and allylsilanes provided moderate yields of the desired products, while enamines, silyl enol ethers, and silyl ketene acetals furnished the target compounds with excellent efficiency. In one example, cobalt-alkyne complex 25 reacts with silyl ketene acetal 26 in the presence of trimethylsilyl triflate to yield, after cobalt decomplexation, ester 27. ... 

Ichikawa and co-workers have examined the Nazarov cyclization of difluorodienone 107 (Scheme 19.28) and have shown that it leads selectively to cross-conjugated dienone product 110. The Nazarov cyclization was initiated by exposure of 107 to trimethylsilyl triflate in a mixed solvent of dichloromethane and hexafluoroisopropanol (HFIP), leading to the resonance tautomers 108 and 109 of the intermediate cyclic allylic cation. Tautomer 109 is disfavored because of the electrostatic effect of the two fluorines on the adjacent carbon atom therefore, the process is terminated through proton loss from the methyl group rather than from the n-butyl group. Elimination of one of the (3 fluorine atoms leads to 110 as the observed final product. 

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