Triflic acid, catalysis

In general, fluorinated sulfonic acids can be used as eatalysts for various cationic cyclizations Typical examples are the triflic acid catalysis in the double cyclization of A,VV-dibenzylpropynylamine [82] (equation 30) and the fluorosulfonic acid catalyzed condensation of phenylacetaldehyde [<5J] (equation 31)... 

DBr, DC1, and CH3SO3D additions to E- and Z-2-butene proceed without diastereoisomerization, H/D exchange, or positional isomerization [108,109]. Although this suggests that carbenium ions may not develop completely, carbenium ion intermediates are apparently involved when the reaction is catalyzed by triflic acid. That is, triflic acid catalysis greatly increases the rate, and both stereo- and positional isomerization occur in its presence [110]. 

The direct 1,4-dimethylanthraquinone 17 synthesis via double acylation of para-xylene with phthalic anhydride can be performed in 15% yield at 138°C, but an equimolecular amount of triflic acid is necessary (Scheme 3.3). Better results are achieved by using phthaloyl dichloride under triflic acid catalysis (5% mol) at the same temperature anthraqui-none 17 can be synthesized in 52% yield. Moreover, product 17 is obtained in 89% yield when the same reaction is carried out in 1,2-dichloroethane and a higher amount of triflic acid (10% mol). 

The isomeric epoxy triflates 153 and 157 undergo triflate displacement-epoxide opening with the dianion of methyl propanoyl acetate. Reaction of 153 generates the epimeric bicyclic tetrahydrofurans 154 and 155, subsequent treatment with triflic acid leading to isomerization about the alkene bond to a mixture of 154-156. Isomeric 157 under similar conditions gives 158 and 159, with triflic acid catalysis leading to some of the alkene isomers 160 along with 158 (Scheme 32). ... 

Triflates of aluminum, gallium and boron, which are readily available by the reaction of the corresponding chlorides with triflic acid, are effective Fnedel-Crafis catalysis for alkylation and acylation of aromatic compounds [119, 120] Thus alkylation of toluene with various alkyl halides m the presence of these catalysts proceeds rapidly at room temperature 111 methylene chloride or ni-tromethane Favorable properties of the triflates in comparison with the correspond mg fluorides or chlorides are considerably decreased volatility and higher catalytic activity [120]... 

This type of alkoxylation chemistry cannot be performed with conventional alkali metal hydroxide catalysts because the hydroxide will saponify the triglyceride ester groups under typical alkoxylation reaction conditions. Similar competitive hydrolysis occurs with alternative catalysts such as triflic acid or other Brpnsted acid/base catalysis. Efficient alkoxylation in the absence of significant side reactions requires a coordination catalyst such as the DMC catalyst zinc hexacyano-cobaltate. DMC catalysts have been under development for years [147-150], but have recently begun to gain more commercial implementation. The use of the DMC catalyst in combination with castor oil as an initiator has led to at least two lines of commercial products for the flexible foam market. Lupranol Balance 50 (BASF) and Multranol R-3524 and R-3525 (Bayer) are used for flexible slabstock foams and are produced by the direct alkoxylation of castor oil. 

We have previously reported that when the rearrangement of trans-stilbene oxide was carried out with CF3SO3H, the solution turned red and the product diphenylacetaldehyde was less pure than that obtained with bismuth triflate. This observation points to the role of bismuth(III) triflate as a Lewis acid in the rearrangement of epoxides and not to protic acid catalysis by triflic acid released by hydrolysis of bismuth triflate. 

Work by Sigwalt, Bischoff and Cypryk22 have used this inter-and intramolecular catalysis to explain the condensation kinetics and cyclic formation processes in siloxane condensations. The kinetics show a very complex dependence on siloxane chain length, complicated by equilibria involving acid, silanol and water. They do indicate that the dominating reaction in the process is condensation and that chain disproportionation and chain scrambling are negligible The kinetics of condensation are influenced by the involvement of triflic acid in several equilibria, i.e. the formation of triflate esters, shown in equation 3, the possible involvement of triflic acid in the reaction of these esters with silanol, shown... 

It has also been shown that XIII is converted into XIV and vice versa upon contact with triflic acid in acetonitrile, thus strongly suggesting the intermediacy of the bicyclic structure in the route that leads to XIV and III. Its intermediate precursor XII would also be amenable to fragmentation to give carbocation XVI that obviously precedes carbinol XV. Finally, the slow conversion of the latter to XIV under p-toluenesulfonic acid catalysis has also been observed. 

Hydrostannation of carbonyl compounds with tributyltin hydride is promoted by radical initiation and Lewis or protic acid catalysis.The activation of the carbonyl group by the acidic species allows the weakly nucleophilic tin hydride to react via a polar mechanism. Silica gel was a suitable catalyst allowing chemoselective reduction of carbonyl groups under conditions that left many functional groups unchanged. Tributyltin triflate generated in situ from the tin hydride and triflic acid was a particularly efficient catalyst for the reduction of aldehydes and ketones with tributyltin hydride in benzene or 1,2-di-chloromethane at room temperature. Esters and ketals were not affected under these conditions and certain aldehydes were reduced selectively in preference to ketones. 

Simplified nucleoside synthesis,8 The known synthesis of nucleosides from silylated heterocycles and a protected sugar derivative in the presence of (CHjijSiClO or (CH3)3SiOTf (6, 639-640) has been adapted to a one-pot synthesis based on in situ silylation and Lewis acid catalysis. The reagent (1) is prepared in situ (equation I) and is added to the free base and acylated sugar then triflic acid, potassium nonaflate, or SnCl4 is added as catalyst. The last Lewis acid is fhe most active and allows condensation to proceed at 24°. Acetonitrile is the most useful solvent. The method is generally applicable and yields are about the same as those obtained in the two-step procedure. 

One final report of alkane activation has been reported by Moiseev. The mechanism of the reaction was not investigated, but this system might be classified as an electrophilic activation of methane, either of the Shilov type or of the concerted four-center type (Fig. lc) where X=triflate. Reaction of methane with cobalt(III)triflate in triflic acid solution leads to the formation of methyltriflate in nearly stoichiometric quantities (90% based on Co) (Eq. 18). Carbon dioxide was also observed, but not quantified. Addition of 02 led to catalysis (four turnovers) [79]. 

O-Dechloroacetylation of IV.61 by treatment with thiomea gave IV.62, which was subsequently reprotected as the hydrogenolyzable 4-methoxybenzyl ether with 4-methoxybenzyl trichloroacetimidate and triflic acid under phase transfer catalysis conditions [104]. Saponification of the benzoate and methyl esters with lithium hydroperoxide followed by methanolic sodium hydroxide and acidification then gave the acid IV.63. O-Sulfonation of IV.63 was achieved with the sulfur trioxide-tri-methylamine complex to give the disulfate IV.64 as the sodium salt. Finally, hydro-genolysis of IV.64 with Pd/C in aqueous methanol afforded the target disaccharide IV.51. 

Catalysis by Bronsted acids requires very strong concentrations (refs. 4-6), and is restricted to the more stable reagents and substrates. In this respect, anisole is not acylated with a good yield in presence of 1 % of triflic acid (ref. f 16). 

TMSOTf generation. Reactions that require catalysis of TMSOTf can be promoted by the combination of triflic acid and bis(trimethylsilyl)acetamide [or bis(trimethylsilyl)urea]. 

Tin Coulombel and coworkers have used tin(IV) triflate as catalyst in the hydroalkoxylation of unsaturated alcohols (Scheme 9a) [51]. The substrate reactivity decreases along the order trisubstituted olefins 1,1-disubstituted olefins > 1,3-disubstituted > monosubstituted olefin. Incidentally, this is a typical reactivity profile for most Lewis acid catalysts discussed in this section. The catalyst loading could be reduced down to 0.1% in favorable cases and in the absence of a solvent. As trifiic acid alone (5%) also catalyzed the reaction in Scheme 9 efficiently, and because Sn(OTf)4 is readily hydrolyzed, a control experiment with cocatalytic amounts (5% each) of Sn(OTf)4 and 2,6-lutidine as proton quencher was performed, in which catalytic activity was retained. We do not believe that this experiment is sufficient proof of tin catalysis, as Sn(OTf)4 may release more than a single equivalent of triflic acid upon hydrolysis. In any case, the selectivity profile of the tin-catalyzed reaction matches that of the trifiic acid-induced hydroalkoxylation reactions studied earlier in the same research group [45]. 

Evidence for the protiocatalytic nature of the diacetoxylation of alkenes using PhI(OAc)2 as oxidant is presented. Cu(OTf)2, Pd(Otf)2 were used as catalyst and Pd + and Cu + ions interacted with the oxidant in the initiation phase of the catalytic transformation. However, 1 equiv. of triflic acid formed in the first cycle functioned as the active catalyst. On the basis of the observed proton catalysis, the infra- and inter-molecular triflic acid-catalysed dioxygenation for a range of alkene substrates is suggested. It is pointed out that Pd-catalysed reactions performed under basic conditions are not explicable by the suggested protolytic scheme. ... 

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