Triflate counterions

Fig. 20 A plot of the observed pseudo-first-order rate constants (kobs) for the methanolysis of HPNPP (4 x 10 5moldm ) as a function of [35 2Zn(II)] in the presence of 1 equivalent of added CH30 per complex giving jpH = 9.5, T = 25 + 0.1 °C. Dotted line is presented as a visual aid directed through all actual data collected at 280 nm ( ) or 320 nm (O) which are the wavelengths for disappearance of HPNPP and appearance of /j-nitrophenol solid line is a linear fit of the data corrected for inhibition by triflate counterions at 280 nm ( ) or 320 nm ( ). Reproduced with permission from ref. 95.

Stoichiometric ionic hydrogenation of the C=C bond of a,/ -unsaturated ketones by HOTf and [Cp(CO)3WH] results in the formation of -ketone complexes of tungsten [32]. As exemplified in Eq. (17), hydrogenation of methyl vinyl ketone gives a 2-butanone complex of tungsten. The bound ketone is displaced by the triflate counterion, giving the free ketone. Similar reactions were reported for hydrogenation of the C=C bond of a,/ -unsaturated aldehydes. 

These Mo catalysts with a C2-tether connecting the phosphine and cyclopenta-dienyl ligand provide an example of the use of mechanistic principles in the rational design of improved catalysts, in this case based on information about a decomposition pathway for the prior generation of catalysts. The new catalysts offer improved lifetimes, higher thermal stability, and low catalyst loading. The successful use of a triflate counterion and solvent-free conditions for the hydrogenation are additional features that move these catalysts closer to practical utility. 

Figure 4. Two views of the X-ray crystal structure of polymeric 55c-CuOTf (triflate counterions omitted for clarity). [Adapted from (36).]...

It was assumed that the triflate counterions of 269c are fully dissociated in solution, at least in the presence of the dicarbonyl substrate. However, an examination of the influence of the counterion revealed that SbF6 provides a much more active catalyst than the triflate counterpart (199). Whereas the triflate catalyst 269c requires 10 h for the reaction to proceed to completion at -78°C, the SbF6 catalyst 271c induces complete conversion to cycloadduct in 4 h under identical conditions, albeit with slightly eroded diastereoselectivity (96 4 vs 98 2) (200). Enantioselec-tivity is identical for the two catalysts (>98% ee, endo diastereomer). 

Systematic studies of the radiofluorination of 2-substituted [201] and 3,6-dis-ubstituted pyridines [202-204] were described (Scheme 46). The trimethylammonium leaving group with the triflate counterion appears the precursor of choice in the preparation of F-fluoroanalogues of epibatidine (Scheme 47). 

The popular activation method of pyridine rings by reaction with chloroformates was not observed for reaction of N-acylated quinoline with allyltrimethylsilane until a catalytic amount of silver triflate was added. It was shown that the triflate counterion increases the electrophilicity of the iV-acylquinolinium salt (Equation 54) < 1997TL403, 2001T109>. 

Reactions of complexes of 1,2-cycloheptadienes have received only cursory attention. 1,2-cycloheptadiene is readily displaced from bisftriphenyl-phosphine)platinum(O)118 [Eq. (54)], but no reagent has been found that will displace the allene from iron.119 Reaction of the iron complex with alcohol in the presence of base (e.g., 312 — 322) is typical of Fp+ complexes of acyclic allenes.131132 The thermal chemistry of 312 is unusual in its decomposition to 324 (Scheme 41). This is probably attributable to the presence of the triflate counterion, since the corresponding fluoroborate salt is stable when warmed to 40°C for 16 h.119 A mechanism to 324 via carbene complex 321 appears likely. 

Polymerization was initiated with alkylating agents such as CH3I, CeHsC Br, or CH3OSO2CF3. Depending on the nucleophilicity of the counterion, propagation proceeds on phosphonium ion active species (triflate counterion) or on covalent alkyl bromide species. 

Similarly to Bianchini s approach, De Rege [26] also immobilized cationic [((R,R)-Me-duphos (26))Rh-(COD)]OTf complex noncovalently by the hydrogenbonding interaction of triflate counterion with surface silanols ofMCM-41 support. In contrast to the results obtained by Bianchini et al. [25c], the catalytic activity and selectivities of the immobilized 26-Rh complex on MCM-41 were equal to or greater than the homogeneous counterparts (Scheme 2.7). Moreover, the catalysts were recyclable (up to four times, with no loss of activity) and did not leach. Here again, the counteranion was very important for the successful immobilization of the catalyst onto MCM-41. Whereas, the DuPhos-Rh complex with triflate anion was effectively immobilized (6.7 wt% based on Rh), tlie analogous complex with the lipophilic BArp anion [BArp = R(3,l-((. i )2-C J I i was not loaded onto the support. 

Nitronium triflate is approximately 150-fold more reactive than the "more-ordered" nitronium tetrafluoroborate and hexafluorophosphate salts in chlorinated solvents and this is attributed to increased solubility conferred to the nitronium ion by the triflate counterion. (Ref. 27a)... 

Crystal structures have also been obtained for the corresponding chloronium and iodonium ions and for the bromonium ion with a triflate counterion. Each of these structures is somewhat unsymmetrical, as shown by the dimensions below. The significance of this asymmetry is not entirely clear. It has been suggested that the bromonium ion geometry is affected by the counterion and it can be noted that the triflate salt is more symmetrical than the tribromide. On the other hand, the dimensions of the unsymmetrical chloronium ion, where the difference is considerably larger, has been taken as evidence that the bridging is inherently unsymmetrical. Note that the C- C bond lengthens considerably from the double-bond distance of 1.35 A. 

In the H NMR spectrum, the OH of the alcohol appeared as a doublet at 87.34. This chemical shift is at least 5 ppm downfield of the corresponding OH resonance of the free alcohol, suggesting that the alcohol was hydrogen bonded in solution to the triflate counterion. A crystal structure of the complex of isopropyl alcohol showed that the OH was strongly hydrogen-bonded in the solid state as well, with an O O distance of 2.63(1) A. While alcohol complexes can be isolated, solutions of these complexes release free alcohol, forming the metal triflate an approximate half-life of 14 h at room temperature was found for the alcohol complex shown in Equation 3.13. 

The reaction has also been successfully applied to the use of furan (8.69) as the dienophile, where replacement of the triflate counterion with SbFe"... 

A cationic 7r-complex is fonned when aryl triflates are employed with dppp, ° a strongly coordinating difunctional phosphine (Scheme 11). Thus, ionization occurs by dissociation of the weakly coordinating triflate counterion.Alternatively, a neutral TT-complex is generated after phosphine dissociation of one of the arms of a bidentate ligand (Scheme 12). ... 

A regioselective transition metal-catalyzed cycloisomerization reaction of boron-containing alkynyl epoxides toward C-2- and C-3-borylated furans was developed. It was found that the copper catalyst as well as the gold catalyst with a relatively more basic triflate counterion favored boryl migration toward C-3-borylated furans, whereas employment of the cationic gold hexafluoroantimonate afforded C-2-borylated furans via a formal 1,2-hydrogen shift (14JA13146). 

Consideration of Scheme 6 and of (1) leads directly to the hypothesis that the explanation of any factors affecting the stereochemistry of glycosylation reactions can be found in the manner in which these factors influence the equilibrium between the contact and solvent separated ion pairs. For example, polar solvents support charge separation better than nonpolar solvents and so are likely to shift the equilibrium toward the solvent separated ion pair and increase the proportion of a-glycoside formation. The difference in selectivity noted earlier between the use of diethyl ether and dichloromethane as solvent [14], as well as the increased p-selectivity with weaker nucleophiles in toluene solution (see Sect. 2.1) [80, 81], are thus readily understood. The importance of the concentration of the alcohol on selectivity is also apparent from (1) as is the expected influence of the concentration of the triflate counterion. To favor p-mannoside formation it is necessary to shift the contact ion pair-solvent separated ion pair equilibrium as far as possible toward the contact ion pair. However, any factors favoring the contact ion pair over the solvent separated ion pair are also likely to favor the covalent glycosyl triflate over the... 

The reason for the exceptional ability of triflate counterion to mediate the copolymerization in favor of m-DMB enchainment is difficult to ascertain. A recent study of Hasegawa and Higashimura on the cationic polymerization of divinylbenzene showed reduced propagation tendency relative to chain transfer when triflic acid initiation was used. This implies that increased m-DMB incorporation from triflic acid initiation is a result of reduced propagation rather than enhanced alkylation. 

In applications for which greater solubility is desired the trifluoromethanesulfo-nate (triflate) counterion is a better choice than tetrafluoroborate. Although (CF3SO3) is more highly coordinating than (BF4), it is quite labile and displays high thermal stability. Triflate salts are used widely as anions for highly cationic clusters in order to increase their solubility and volatility for mass spectrometric analysis. 

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