Super Bronsted acid catalyst

We have developed a fluorous super-Bronsted acid catalyst, 4-(lH,lH-perfluoro-tetradecanoxy)-2,3,5,6-tetra Luorophenylbis(trifluoromethanesulfonyl)methane (8), which can be recycled by applying liquid/solid phase separation without fluorous solvents [8] and an organic-solvent-swellable resin-bound super-Br0nsted acid, polystyrene-bound tetrafluorophenylbis(trifluoromethanesulfonyl)methane (9) [9]. 

Yamamoto had earlier reported that Lewis acid activation of valine-derived oxazaborolidine 60 yielded a highly reactive and moisture-tolerant LLA catalyst 61 for the Diels-Alder reaction (Scheme 5.76) [145]. In later studies, activation of 60 with the super Bronsted acid, C,sF5CHTf2, was found to produce the even more reactive catalytic species BLA 62. During studies toward an enantioselective route to Platensimycin [146], BLA 62 was found to catalyze the Diels-Alder reaction between various monosubstituted dienes and ethyl acrylate to afford adducts... 

As a result of the preliminary examples described in Sect. 3.1 and the quest for automation techniques in solution-phase synthesis, various examples of continuous flow processes appeared in the literature lately which utilized solid-phase-bound chemical catalysts. In a simple example, Yamamoto and coworkers studied the use of super Bronsted acids loaded on polystyrene beads 5 for use in a single-pass column system (Fig. 4) [30]. It was shown that these columns are suited for the acetylation of alcohols, acetalization of carbonyl compounds, Sakurai-Hosomi allylation reactions, and Mukaiyama aldol reactions. 

Together with the well-accepted threshold DPE value for superacidity (250 kcal/mol) [18], it is noteworthy that only marginal variation in was observed for the TMPH" " adduct as DPE decreases from ca. 320 (weak acidic strength) to 250 kcal/mol (super strong acidic strength) [18]. As such, it is obvious that the P-TMP NMR approach is inferior for differentiating Bronsted acidic strengths in soHd acid catalysts. 

Heteropolyacids are well known to be strong Bronsted acids and their acidity has been quantitatively characterized and compared with the acidity of mineral acids such sulfuric acid [10]. Differently of H2SO4 which has only one totally ionizable proton, heteropolyacids when in aquous solution are completely dissociated at the first three steps because of the solvent leveling effect [11]. Several works described the H3PW12O40 heteropolyacid as an efficient super-acid, which has been used either as soluble catalyst in polar solvents, or in the heterogeneous phase, supported silica or on active coal as solid matrix. 

Combination of super B rousted acid with simple Bronsted acid or Lewis acid would be a new system for acid catalysis in organic synthesis. The example of super BLA would be an interesting tool for selective organic transformations. We have already shown excellent examples in Scheme 1.3 using the Tf2NH-chiral Lewis acid combined system. Scheme 1.36 exemplifies some additional recent reactions based on pentafluorophenyl bis (triflyl)methane and chiral Lewis acid catalyst [38]. 

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