Triflates mechanism

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... 

The original Sonogashira reaction uses copper(l) iodide as a co-catalyst, which converts the alkyne in situ into a copper acetylide. In a subsequent transmeta-lation reaction, the copper is replaced by the palladium complex. The reaction mechanism, with respect to the catalytic cycle, largely corresponds to the Heck reaction.Besides the usual aryl and vinyl halides, i.e. bromides and iodides, trifluoromethanesulfonates (triflates) may be employed. The Sonogashira reaction is well-suited for the synthesis of unsymmetrical bis-2xy ethynes, e.g. 23, which can be prepared as outlined in the following scheme, in a one-pot reaction by applying the so-called sila-Sonogashira reaction ... 

For aryl halides and sulfonates, even active ones, a unimolecular SnI mechanism (lUPAC Dn+An) is very rare it has only been observed for aryl triflates in which both ortho positions contain bulky groups (fe/T-butyl or SiRs). It is in reactions with diazonium salts that this mechanism is important ... 

The reaction between aryl halides and cuprous cyanide is called the Rosenmund-von Braun reactionP Reactivity is in the order I > Br > Cl > F, indicating that the SnAt mechanism does not apply.Other cyanides (e.g., KCN and NaCN), do not react with aryl halides, even activated ones. However, alkali cyanides do convert aryl halides to nitrilesin dipolar aprotic solvents in the presence of Pd(II) salts or copper or nickel complexes. A nickel complex also catalyzes the reaction between aryl triflates and KCN to give aryl nitriles. Aromatic ethers ArOR have been photochemically converted to ArCN. 

Solvolyses of these cyclic vinyl triflates at 100 in 50% aqueous ethanol, buffered with triethylamine, lead exclusively to the corresponding cyclo-alkanones. Treatment of 176 with buffered CH3COOD gave a mixture of cyclohexanone (85%) and 1-cyclohexenyl acetate (15%). Mass spectral analysis of this cyclohexanone product showed that the amount of deuterium incorporation was identical to that amount observed when cyclohexanone was treated with CH3COOD under the same conditions. This result rules out an addition-elimination mechanism, at least in the case of 174, and since concerted elimination is highly unlikely in small ring systems, it suggests a unimolecular ionization and formation of a vinyl cation intermediate in the solvolysis of cyclic triflates (170). The observed solvent m values, 174 m =. 64 175 m =. 66 and 16 m =. 16, are in accord with a unimolecular solvolysis. 

The mechanism of reaction of primary vinyl triflates and the possibility of solvolytic generation of primary vinyl cations needs further exploration. Along these lines, an examination of the behavior of the simplest vinyl system CH2=CHOTf and the possibility of generating the parent vinyl cation needs to be done. 

Gel electrolytes were also prepared by Allcock [605] from co-substituted polyphosphazenes with various ratios of methoxyethoxyethoxy and trifluo-roethoxy side groups, lithium triflate and propylene carbonate. These gel electrolyte systems have a better mechanical stability than MEEP. The highest ionic conductivity obtained was 7.7x10" S cm" at 25 °C for a gel containing 37.5% of polymer with 80% and 20% of methoxyethoxyethoxy and trifluoro ethoxy... 

Alkyl sulfonates are very effective cationic initiators of e-caprolactone, although only the more reactive methyl triflate and methyl fluorosulfate result in a high conversion. The mechanism of polymerization in the presence of these initiators is believed to involve methylation of the exocyclic carbonyl oxygen, followed by partial ring opening of the activated lactone by the counteranion (Fig. 

Treatment of suitably protected 2-triflates 220 and 222 of methyl ) -d-talopyranoside with Et4NF (MeCN, 50°, 30-50 min) gave, respectively, the 2-fluoro-y -D-galactopyranoside 221 (50%) and the unsaturated product 223 (74%). The mechanism was discussed. Acid-catalyzed hydrolysis of 221 (5... 

The reaction proceeds with isolated double bonds and electron-rich alkynes. Electron-withdrawing groups in the acetylene moiety decelerated the reaction. A plausible mechanism implies the activation of the olefin by coordination of the metal triflate followed by nucleophilic attack of the acetylene or acetylide (Scheme 31). 

A very simplified but general scheme for the mechanism of all these transformations is shown below (Scheme 6.1). The first step of the catalytic cycle is the oxidative addition of the organo-hahde or -triflate B to produce the species C. Transmetallation of the appropriate organometalhc reagent D forms E which, upon reductive elimination, provides the desired product and regenerates the catalyst A. 

The carbonyl group can be deprotected by acid-catalyzed hydrolysis by the general mechanism for acetal hydrolysis (see Part A, Section 7.1). A number of Lewis acids have also been used to remove acetal protective groups. Hydrolysis is promoted by LiBF4 in acetonitrile.249 Bismuth triflate promotes hydrolysis of dimethoxy, diethoxy, and dioxolane acetals.250 The dimethyl and diethyl acetals are cleaved by 0.1-1.0 mol % of catalyst in aqueous THF at room temperature, whereas dioxolanes require reflux. Bismuth nitrate also catalyzes acetal hydrolysis.251... 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

Copper(II) triflate is quite inefficient in promoting cyclopropanation of allyl alcohol, and the use of f-butyl diazoacetate [164/(165+166) = 97/3%] brought no improvement over ethyl diazoacetate (67/6 %)162). If, however, copper(I) triflate was the catalyst, cyclopropanation with ethyl diazoacetate increased to 30% at the expense of O/H insertion (55%). As has already been discussed in Sect. 2.2.1, competitive coordination-type and carbenoid mechanisms may be involved in cyclopropanation with copper catalysts, and the ability of Cu(I) to coordinate efficiently with olefins may enhance this reaction in the intramolecular competition with O/H insertion. 

A new approach to the synthesis of 2,4,5-trisubstituted and 2,5-disubstituted oxazoles, 97 and 98, used l-(methylthio)acetone 95 with nitriles in the presence of trifluoromethanesulfonic anhydride. The proposed mechanism involves an unstable 1-(methylthio)-2-oxopropyl triflate 96 which was detected using NMR spectroscopy <06JOC3026>. 

Direct high-temperature microwave-assisted aminations of a variety of aryl halides [257] or aryl triflates [258] under transition metal-free conditions have also been reported, probably involving a benzyne mechanism. 

An unexpected one-pot two-step transformation of 3,4-diphenyl-l,2,5-thiadiazole 8 via the intermediate trimethylsi-lylmethylated 3,4-diphenyl-1,2,5-thiadiazolium triflate 20 gave the l-trimethylsilylmethyl-4,5-diphenylimidazole 139. The proposed reaction mechanism invokes desilylation of thiadiazolium 20 with CsF to afford the methide 138 (Scheme 21) <1999J(P1)1709>. 

Interaction of simple acyclic dications with acetylenes proceeds anomalously. Instead of unsaturated disulfonium salts, the reaction leads to the corresponding vinyltriflates 96.107 It was shown earlier108 that interaction of trifluoromethylsulfonyloxysulfonium triflate (the dimethylsulfide ditriflate complex) with certain acetylenes leads to analogous vinyl triflates. The drastic differences between the reactivity of cyclic and acyclic dications toward acetylenes provide another evidence for a stepwise mechanism of this process (Scheme 37). 

Figure 8-5. Proposed mechanism for the catalytic arylation of 2,3-dihydrofuran with phenyl triflate in the presence of Pd(OAc)2-(i )-BINAP catalyst.

The proposed mechanism is illustrated in Figure 8-5.60a Oxidative addition of the phenyl triflate to the palladium(0)-BINAP species A gives phenylpalla-dium triflate B. Cleavage of the triflate and coordination of 2,3-dihydrofuran on B yields cationic phenyl palladium olefin species C. This species C bears a 16-electron square-planar structure that is ready for the subsequent enantio-selective olefin insertion to complete the catalytic cycle (via D, E, F, and G). The base and catalyst precursor have profound effects on the regioselectivity and enantioselectivity. 

The Heck reaction, first disclosed by the Mori and Heck groups in the early 1970s [65, 66], is the Pd-catalyzed coupling reaction of organohalides (or triflates) with olefins. Nowadays, it has become an indispensable tool for organic chemists. Inevitably, many applications to heterocyclic chemistry have been pursued and successfully executed. In one case, Ohta et al. reacted 2-chloro-3,6-dimethylpyrazine (49) with styrene to furnish ( )-2,5-dimethyl-3-styrylpyrazine (50) [67]. Here, only the E isomer was observed. The outcome will become apparent during the ensuing discussions on the mechanism. 

Methods for indirect oxidation have also been developed. The combination of KF/ wCPBA in acetonitrile and water has been used to generate KOF CH3CN reagent, a mild and selective oxidant that reacts at 0 °C with no overoxidation [78]. This reagent functions by providing a fluorosulfonium ion intermediate, which is hydrolyzed in the presence of water to the desired sulfoxides. As a result of the indirect oxidation method, the typical stereoselectivity of mCPBA-type oxidations is not observed here. The KOFCH3CN oxidant is similar in scope and mechanism to 1-fluoropyridinium triflates, Selectfluor [302] and the more classical t-butyl hypochlorite [288]. 

In the critical area of (1-mannoside synthesis [317-321], the evidence strongly suggests that a-mannosyl triflate serves as a reservoir for a transient contact ion pair (CIP), which is the glycosylating species (Scheme 4.37), although the possibility of an SN2-like mechanism with an exploded transition state cannot be completely excluded [135]. In view of the probable operation of the contact ion-pair mechanism... 

This class of donor is activated by soft Lewis acids, such as copper triflate at room temperature, and despite their hydrolytic instability, they appear inert to conditions of sulfoxide activation, TMSOTf or Tf20 (Scheme 4.53). Activation is achieved with stoichiometric promoter in the presence of the acceptor alcohol, and although the mechanism has not been investigated, presumably it proceeds via coordination followed by collapse to a stabilized oxacarbenium ion. The method is compatible with standard glycosidation solvents such as dichloromethane, acetonitrile and diethyl ether, and ester-directed couplings do not lead to orthoesters, perhaps as a result of the presence of the Lewis acid promoter [303,304]. 

---