Triflates anion nucleophilicity

An example of cleavage ol the sulfur-oxygen bond in trifluoromethane-sulfonic ester has been reported Tnfluororaethyl triflate reacts with neutral or anionic nucleophiles with elimination of carbonyl difluoride and formation of trifluoromethanesulfonyl fluoride [57] (equation 32) The mechanism of this reaction involves elimination of fluoride ion, which is a chain carrier in the substitution of fluorine for the trifluoromethoxy group... 

A large variety of salts of triflic acid formed both from metals and nonmetals are known Many of these salts are versatile reagents for organic synthesis because of such properties of the tnflate anion as very low nucleophilicity and low coordinating ability However, despite low nucleophilicity, the triflate anion can combine with carbocationic intermediates under appropriate conditions to form triflate esters [116, 117, II8. ... 

The pareitropone project began quite by accident after an unexpected observation expanded our thinking about potentially accessible targets for alkynyliodonium salt/alkylidenecarbene chemistry (Scheme 18). Treatment of the tosylamide iodonium salt 125 with base under standard conditions was designed to provide no more than routine confirmation of the aryl C-H insertion capabilities, which were first exposed in indoleforming reactions using tosylanilide anion nucleophiles and propynyl(phenyl)iodonium triflate,5b of the intermediate carbene 126. However, this substrate did not perform as expected, since only trace amounts of the 1,5 C-H insertion product 127 was detected. One major product was formed, and analysis of its spectral data provided yet another surprising lesson in alkynyliodonium salt chemistry for us. The data was only consistent with the unusual cycloheptatriene structure 129. 

Employing the lactone 279, the trimethylsilyl cation generates a positive charge at the anomeric carbon position of 280. The corresponding triflate anion recombines with one of the silyl ethers in 281 and yields the alk oxide 282 and a molecule of catalyst. After nucleophilic attack, 283 is formed, and by a subsequent intramolecular reaction, the orthoester 284 is obtained simultaneously releasing a molecule of hexamethyldisil-oxane. 

P selectivity. Crich and coworkers proposed that, under preactivation conditions, the oxocarbenium ion is trapped by a triflate anion to form the more stable a-triflate 65. After addition of the acceptor, the a-triflate intermediate can then be displaced in an SN2-like manner to afford a p-mannoside product (68). The formation of a-glycosyl triflates was confirmed by II, 13C, and 19F NMR analyses of the activated mannosyl donor recorded at low temperature [37], The experimentally determined KIE is approximately 1.12, which is consistent with an oxocarbenium-like TS [38], It was hypothesized that the a-triflate converts into the contact ion pair 66 in which the triflate anion remains at the a face or that an exploded TS is formed where the nucleophile is loosely associated with the oxocarbenium ion as the triflate departs [39,40], The a product 69 can be explained by the formation of the solvent-separated ion pair 67 where the counterion is solvated and facial selectivity is lost. 

Oxoalkyl- and 3-oxoalkyl-triphenylbismuthonium salts transfer the alkyl groups to various hetero nucleophiles such as piperidine, triphenylphosphane, arylsulfinates, alcohols, arylthiolates, dimethyl sulfide, and metal halides under mild conditions (Equation (132)).214 215 Methyl- and allyl-triphenylbismuthonium salts also alkylate some heteronucleophiles.216 The leaving ability of the triphenylbismuthonio group has been found to be higher than that of the triflate anion. 

When the acetal and the silyl enol ether are mixed with the same Lewis acid catalyst, Noyori found that an efficient aldol-style condensation takes place with the acetal providing the electrophile. The reaction is successful at low temperatures and only a catalytic amount of the Lewis acid is needed. Under these conditions, with no acid or base, few side-reactions occur. Notice that the final desilylation is carried out by the triflate anion to regenerate the Lewis acid Me3Si-OTf. Triflate would be a very poor nucleophile for saturated carbon but is reasonable for silicon because oxygen is the nucleophilic atom. 

Scheme 5. Examples of nucleophiles often present in glycosylation reactions that can compete with the nucleophilic hydroxyl group (A) 1,6-anhydro formation, (B) thio exchange, (C) leaving-group return, (D) promoter-derived components such as triflate anions, (E) additives such as added bases, (F) solvents.

Even extremely weak nucleophiles, such as perchlorate and triflate anions, can participate in the oxidatively assisted nucleophilic substitution of iodine [666-668]. The oxidation of alkyliodides in the presence of lithium perchlorate or appropriate tetrabutylammonium salts in non-nucleophilic solvents affords the respective alkyl perchlorates or alkylsulfonates 639 as principal products (Scheme 3.250) [669]. 

Summary Binuclear hexacoordinate silicon complexes with two N-+Si dative bonds (per Si atom) have been prepared. The crystal stracture conforms to a distorted octahedron, with the N— Si bonds trans to each other. The binuclear complex undergoes Si-Cl dissociation of one Si Cl bond in CD2CI2 and CDCI3 solution, to form a binuclear, monosiliconium chloride salt, in reversible equilibrium with its precursor. The dissociation and equilibrium reactions are observed by variable-temperature Si NMR spectra. The extent of ionic dissociation increases as the temperature is decreased. The equilibrium population ratio is shifted completely to the di-ionic side at room temperature, by replacement of the chloride by the less nucleophilic triflate anion. The crystal structure of a disiliconium ditriflate shows a) well separated ions, b) that the geometry about silicon is almost an exact square pyramid, and c) that the N- Si bonds are among the shortest coordination bonds of this kind. 

The efQcient one-pot two step procedure through intermediate triflate derivative 194 was elaborated for substitution of chloride in 176 with less reactive O- and N-nucleophiles [108] (Scheme 50). On the first stage [(Cl)BSubPcFi2] is treated with Me3SiOS02CF3 at 100-150 °C or with Ag0S02CF3 at 20 °C to form reactive species 194 - [ BSubPcFn] cation with weakly coordinated triflate anion. 

In subsequent studies, the scope of the Heck reaction/ anion-trapping cascade was further extended using soft car-banionic nucleophiles as illustrated in the asymmetric synthesis of (—)-D -capnellene 17. Treatment of prochiral vinyl triflate 15 with Pd(OAc>2, (5)-BINAP, and NaBr, as weU as the sodium enolate of diethyl (2-((rert-butyldiphe-nylsilyl)oxy)ethyl)malonate, gave the cyclic product 16 in 87% ee and 77% yield as the sole product. The use of NaBr as an additive improved the optical yields and was critical in preventing counteranion exchange between the triflate anion and the enolate anion by complexing with sodium enolate (Scheme 13.6). Compound 16 was then advanced through several steps to complete the total synthesis (—)-D -capnellene 17. 

The fluoraza reagents consist of two types of compounds one in which a fluorine atom is bound to the nitrogen atom of an amide or, more often, a sulfonamide and one in which a fluorine atom is bound to the nitrogen atom of a tertiary amine such as pyridine, quinuclidine, or triethylenediamine 1,4-diaza-bicyclo[2 2.2]octane. The positive charge on the nitrogen is counterbalanced by a non-nucleophilic anion such as triflate or tetrafluoroborate. 

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