Superacidic triflic acid

Boron tristriflate and the conjugate superacids triflic acid-SbFs and triflic acid-B(0S02CF3)3 were also found to be highly effective in subsequent studies.117 118 They induce the isomerization of enrfo-trimethylenenorbornane to exn-trimethylenenorbornane in near-quantitative conversion followed by the isomerization of the latter to adamantane in high yields [Eq. (5.49)]. Sonication was shown to accelerate isomerization. Furthermore, the same catalysts could be... 

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. 

Subsequently McMurry and Lectka" successfully designed an appropriate system to observe stoichiometric hydrogen formation with liquid superacidic triflic acid. They found that when a highly stable carbocation is formed, weaker acids may be applied in protolysis, eliminating the need to use conjugate superacids. Indeed, when the bicyclic compound [Eq. (6.14)] was treated with triflic acid, 90% of the theoretical amount of hydrogen evolved. 

When aromatic pinacols are reacted with an acid, products often arise from dehydration and rearrangement.5 This general conversion is known as the pinacol rearrangement. The pinacol rearrangement may be promoted by both Brdnsted and Lewis acids.6 In the procedure described here, superacidic triflic acid is reacted with an aryl pinacol and a dehydrative cyclization occurs to give the substituted phenanthrene product. Related to this conversion, the chemistry of benzopinacol in sulfuric acid and triflic acid is contrasted in Scheme 1. We have proposed that the superacidic triflic acid causes the formation of diprotonated intermediates which promote the dehydrative cyclization.4... 

Sulfuric acid, fuming Sulfuric acid, mixt. with sulfur trioxide (8014-95-7), 76, 47 Superacidic triflic acid, 76, 296... 

Benzaldehyde reacts readily with benzene under superacid conditions to give tri-phenylmethane in high yields.427,428 The reaction is slow in triflic acid, giving a yield of 90% in 24 h, whereas with the strong superacid CF3SO3H2 B(0S02CF3)4 the product is formed instantaneously in 83% yield.427 Experimental evidence supports the involvement of diprotonated benzaldehyde in the reaction [Eq. (5.76)]. Theoretical calculations show that the 44 0,C(aromatic)-diprotonated cation is the reactive intermediate and not the 45 0,0-diprotonated cation ... 

Superacid systems, such as HF—BF3 and CF3S03H, allow further improvement of the Koch reaction.109 Triflic acid proved to be far superior to 95% H2S04 at atmospheric pressure.110 This was attributed to the higher acidity of triflic acid and the higher solubility of CO in this acid. FS03H—SbF5 combined with Cu(I) is also effective.111... 

The same indicators have been used to compare the relative acidity of the three most used superacids. As shown on Figure 1.7, the half-protonation of indicator 3 necessitates 40, 25, and 8mol% SbF5, respectively, in triflic acid, fluorosulfuric acid, and hydrogen fluoride systems. These results show the supremacy of the HF-SbF5 system in which small concentrations of SbF5 induce a dramatic increase in acidity (see also Section 2.2.2.7). 

In a subsequent study Devynck and co-workers81,82 studied the electrochemical oxidation of alkanes and alkenes in triflic acid monohydrate. The acidity of CF3SO3H H20 was found to be intermediate between that of aqueous acid media and superacidity. Alkanes undergo two-electron oxidation, whereas alkenes are protonated to yield carbenium ions in this medium. In addition to various transformations characteristic of carbenium ions [Eqs. (5.36)—(5.38)], they undergo a reversible disproportionation to give an alkane and an aldehyde [Eqs. (5.40)]. 

Superacid-catalyzed alkylation of adamantane with lower alkenes (ethene, propene, isomeric butenes) has been investigated by Olah et al.151 in triflic acid and triflic acid-B(0S02CF3)3. Only trace amounts of 1 -ferf-butyladamantane (37) were detected in alkylation with 1- and 2-butenes, whereas isobutylene gave consistently relatively good yield of 37. Since isomerization of isomeric 1-butyladamantane under identical conditions did not give even traces of 37, its formation can be accounted for by (r-alkylation, that is, through the insertion of the ferf-butyl cation into the C—H bond (Scheme 5.22). This reaction is similar to that between ferf-butyl cation and isobutane to form 2,2,3,3-tetramethylbutane discussed above (Scheme 5.21). In either case, the pentacoordinate carbocation intermedate, which may also lead to hydride transfer, does not attain a linear geometry, despite the unfavorable steric interactions. 

Data for the use of alcohols as alkylating agents in superacids are scarce. A study of the alkylation of phenol and naphthols with ferf-butyl alcohol has shown198 that triflic acid adsorbed on aminopropyl-modified silica is the most selective to yield monoalkylated products compared to solid acids (triflates immobilized in silica). 

Olah et al.242 and Shudo and co-workers243 have shown that benzaldehyde reacts with benzene in various superacidic systems [triflic acid, triflic acid-SbF5,... 

Quinoline and isoquinoline react in an analogous manner with benzene in conjugated superacids (HCI-AICI3, HBr-AlBr3) to yield predominantly cis-5,1-diphenyl-5,6,7,8-tetrahydroquinoline and <7,v-6,8-diphenyl-5,6,7,8-tetrahydroisoqui-noline, respectively (75-94% yield).297 The regioselectivity observed corresponds to the most stable dicationic intermediates 73 and 74. Unsaturated imides298 exhibit similar behavior in triflic acid (Scheme 5.33). 

Olah et al.450 have recently described a new, highly efficient superelectrophilic formylation-rearrangementofisoalkanes. Branched ketones are formed in high yields and with high selectivity with no detectable branched acids (Koch products) in the presence of moderately strong superacids such as HF-BF3 or triflic acid-BF3. Carbonylation of isobutane under such conditions gives isopropyl methyl ketone in high yield [Eq. (5.164)] The transformation was interpreted with the involvement of... 

---