Examples triflic acid

No other adducts are formed, and the endo/exo diastereomeric ratio is essentially the same for all of these methods. Further, the existence of an acid catalyzed mechanism for cycloaddition can be explicitly excluded by using an excess of a hindered amine base (2,6-di-tert-butylpyridine, DTBP) in the aminium salt induced reaction and by examining the results of an authentic acid catalyzed reaction (using, for example, triflic acid). In the former case, the same endo and exo adducts are formed in virtually the same relative amounts, but in the latter case neither of these adducts is formed. It is worth noting that acid catalyzed reactions have indeed sometimes been observed under typical aminium salt conditions [70], but these have never been observed, nor would they be expected, under PET conditions. Finally, in the instance where cation radicals are generated by the aminium salt method, the intervention of substrate cation radicals can usually be verified by the addition of the reduced form of the catalyst, i.e., the neutral triarylamine, to the reaction mixture. 

Molecular examples of trivalent molybdenum are known in mononuclear, dinuclear, and tetranuclear complexes, as illustrated in Figure 5. The hexachloride ion, MoCk (Fig- 5a) is generated by the electrolysis of Mo(VI) in concentrated HCl. Hydrolysis of MoCP in acid gives the hexaaquamolybdenum(III) ion, Mo(H20) g, which is obtainable in solution of poorly coordinating acids, such as triflic acid (17). Several molybdenum(III) organometaUic compounds are known. These contain a single cyclopentadienyl ligand (Cp) attached to Mo (Fig. 5d) (27). 

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)... 

Trifluoromethanesulfonic (triflic) anhydride is commercially available or can be prepared easily by the reaction of triflic acid with phosphorus pentoxide [66] This moderately hygroscopic colorless liquid is a useful reagent for the preparation of various organic derivatives of triflic acid A large variety of organic ionic triflates can be prepared from triflic anhydride A recent example is the preparation of unusual oxo-bridged dicatiomc salts of different types [SS, 89, 90, 91, 92, 93] (equations 38-44) Stabilized dication ether salts of the Huckel aromatic system and some other systems (equations 38 and 39) can be prepared in one step by the... 

Silver(I) triflate is widely applied to the preparation of various derivatives of triflic acid, both covalent esters [66] and ionic salts For example, it can be used for the in situ generation of iodine([) triflate, a very effective lodinatmg reagent for aromatic and heteroaromatic compounds [130] (equations 65 and 66)... 

On the other hand polysilylalkynes with phenyl or allyl substituents are converted with triflic acid into polymeric alkynylsilyltriflates. These polymers react with many acidic element hydrogen compounds or lithium element compounds with formation of silicon element bonds. Thus we found an easy approach to numerous new functional substituted alkynes [12], Eq.(9) shows selected examples of this reaction type. 

An example of an alcohol that can undergo rapid skeletal rearrangement is 3,3-dimethyl-2-phenyl-2-butanol (Eq. 29). Attempts to reduce this alcohol in dichloromethane solution with l-naphthyl(phenyl)methylsilane yield only a mixture of the rearranged elimination products 3,3-dimethyl-2-phenyl-l-butene and 2,3-dimethy 1-3-phenyl-1 -butene when trifluoroacetic acid or methanesulfonic acid is used. Use of a 1 1 triflic acid/triflic anhydride mixture with a 50 mol% excess of the silane gives good yields of the unrearranged reduction product 3,3-dimethyl-2-phenylbutane, but also causes extensive decomposition of the silane.126 In contrast, introduction of boron trifluoride gas into a dichloromethane solution of the alcohol and a 10 mol% excess of the silane... 

There are relatively few entries in the non-fused dioxepin area, and most of these focus on reactions of these systems. For example the triflic acid-initiated polymerisation of 1,3-dioxepane in the presence of acetic acid and hexanedicarboxylic acid has been studied and mechanistic aspects discussed <00JPS(A)1232>. Biodegradable microspheres for the controlled delivery of drugs have been made from copolymers and homopolymer blends of L-lactide and l,5-dioxepan-2-one <00PP1628>. Ring contraction of 5-methylene-l,3-dioxepanes (eg. Ill) on reaction with trimethylsilyl trifluoromethanesulfonate in the presence of base afforded the exo tetrahydropyrans, in good yields <00TL2171>. 

To suppress the side reaction (348=350). which hinders the generation of cations (349) and thus their involvement in C,C-coupling reactions, it is advantageous to add Bronsted bases, for example, 2,6-bis(ferf-butyl)-4-methylpyridine (Scheme 3.206) (478). These bases can efficiently bind triflic acid. 

Condensahon of N-tritylglycin with donor or acceptor subshtuted aldehydes results in fulleropyrrolidines whose amino functionality can be easily deprotected with triflic acid and further funchonalized with, e.g., another donor [236]. Thus, access to various triads is provided. An example for a donor-acceptor-donor triad is given in Scheme 4.38. 

Section V contains specific examples of procedures for Inflate synthesis some general comment about these procedures is worthwhile. Triflic anhydride is the reagent of choice for preparing triflates derived from sugars, whereas triflyl chloride is used more often when uucleosides are involved. Triflate formation with triflic anhydride requires addition of a base (usually pyridine) to the reaction mixture to neutralize the triflic acid produced [Eq. (4) 7]. Some triflates are reactive enough that pyridine can function as a nucleophile in the substitution process. In these instances, replacement of pyridine with a non-nucleophilic base, such as 2,6-di-r-butyl-4-methylpyridine, avoids this undesired reaction (Scheme 1) [7]. 

Harman and co-workers738 reported the synthesis of arenium cations stabilized by Os complexation (for example, 415) via protonation with triflic acid of [Os(NH3)5(Ar)](OTf)2 complexes (Ar = benzene, toluene, xylenes, naphthalene, anthracene). 

In all cases, superelectorophilic dicationic intermediates3 5 were suggested to be involved in the activation of carbonyl compounds based on the observation that protonated /V-heterocycles significantly enhance the reactivity of adjacent carbo-cationic centers. For example, cyclohexanone and acetophenone are unreactive toward benzene in triflic acid, whereas 4-piperidones252 and acetylpyridines254 react readily. Likewise, 3-pyridinecarboxaldehyde is able to alkylate deactivated... 

A range of diols and cyclic ethers were used to carry out alkylation of aromatics (benzene, toluene, xylenes, trimethylbenzenes, naphthalene) in the presence of triflic acid.204 310 In a recent study,311 various methyl-substituted benzene derivatives were alkylated with 1,4-diols [Eq. (5.117)] to form substituted tetralin derivatives in high yields. The transformations involve an intermolecular alkylation step followed by intramolecular alkylation (cyclialkylation). 2,2,5,5-Tetramethyltetrahydrofuran is similarly effective. For example, it alkylates benzene to give octamethyloctahydroan-thracene (98% yield) and reacts with naphthalene to yield octamethyloctahydrote-tracene [Eq. (5.118)]. 

Formylation of the less reactive phenol and anisole with CO in HF-BF3 was found to require at least stoichiometric amount of the acid for effective transformation (50 equiv. of HF, 2 equiv. of BF3, 50 bar CO, 45°C).445 Conversion increases with increasing reaction time but results in decreasing paralortho ratios suggesting a change from kinetic control to thermodynamic control and the reversibility of formylation. Furthermore, the amount of byproducts (mainly diphenylmethane derivatives) originating from reactions between substrates and products also increases. Additional studies in ionic liquids showed that imidazolium cations with increased chain lengths—for example, l-octyl-3-methylimidazolium salts—are effective in the formylation process. This was attributed to the enhanced solubility of CO in the ionic liquid medium. Tris(dichloromethyl)amine, triformamide, and tris (diformylamino)methane have recently been applied in the formylation of activated aromatic compounds in the presence of triflic acid at low temperature (— 10 to 20°C) albeit yields are moderate.446... 

Barluenga et al.565 have reported the selective monoiodination of arenes with bis (pyridine)iodonium(I) tetrafluoroborate [I(py2)BF4] in excess superacids (2 equiv.) [Eq. (5.210)]. Comparable results were found for activated compounds with both HBF4 and triflic acid, whereas triflic acid was more effective in the iodination of deactivated aromatics. For example, nitrobenzene and methyl benzoate are unreactive in HBF4 but give the corresponding iodo derivatives in triflic acid (83% and 84% yields, respectively, in 14 h). Iodination of phenol required low temperature (-60°C). 

Anhydrous hydrogen fluoride has been successfully applied in the structural analysis of polysaccharides to cleave glycosidic linkages since the 1980s. Recently, triflic acid has been found to be a more potent and more selective agent.774 In structural studies, for example, polysaccharides isolated from bacteria were stable toward... 

The allyl-transfer reaction based on 2-oxonia Cope rearrangement allows highly stereocontrolled chirality transfer. Triflic acid has been shown to induce the rearrangement of the 251 allyl sterols into 22-homoallylic sterols with high stereoselectivity without side reactions86 [Eq. (5.316)]. The protocol, however, is not effective for syn substrates (for example, 251, R = H, R = COOEt). 

Triflic acid has become a widely applied catalyst in various polymerization processes, and a few selected characteristic examples are discussed here. Additional basic information, examples of practical significance, and recent trends of cationic polymerization can be found in books and monographs.942 945... 

A rare example of cationic polymerization of emulsified epoxy resins has been reported by Walker et al.973 Polymerization of water emulsion of epoxy resins with a variety of superacids (triflic acid, HCIO4, HBF4, HPF6) results in polyols with two glycidyl units (294) in contrast to commercial epoxy resins with one unit separating the aromatic moieties. The level of residual glycidyl ether and Bisphenol-A units is also much lower than in conventional epoxy resins. 

Although electrophilic reactions involving dications with deactivated arenes may suggest the formation of superelectrophilic intermediates, there are a number of well-known examples of monocationic electrophiles that are capable of reacting with benzene or with deactivated aromatic compounds. For example, 2,2,2-trifluoroacetophenone condenses with benzene in triflic acid (eq 12).13 A similar activation is likely involved in the H2SO4 catalyzed reaction of chloral (or its hydrate) with chlorobenzene giving DDT (eq 13). 

Such an example has been demonstrated by Johnson and Sames, who chose a platinum-mediated dehydrogenation as a key step in the synthesis of the antimitotic rhazinilam 33 (Scheme 6) [20], The key intermediate 27 was converted into the imine 28, which was allowed to react with Me Pt(//-SMe2)]2 to afford the platinum complex 29. Subsequent treatment with triflic acid resulted in elimination of methane and furnished the cationic complex 30. Upon thermolysis in trifluoroethanol, the complex lost a second methane molecule, which resulted in the activation of the ethyl group. A subsequent /1-hydride elimination gave the hydrido-Pt(n) complex 31. Treatment with aqueous KCN followed by hydrox-ylamine removed the platinum and yielded the liberated amine 32. Johnson and Sames added a homologization and a macrolactamization and completed the total synthesis of rhazinilam (33) by removal of the carboxyl group. 

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