Super Bronsted acid

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. 

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]. 

In the 1960 s, Gillespie suggested calling a Bronsted acid stronger than 100% sulfuric acid (Ho = —12) super Bronsted acid and this definition is now commonly used. It should be mentioned, however, that in 1927, Conant used super acid to denote acids such as perchloric acid, which is stronger than conventional mineral acids and capable of protonating weak bases such as carbonyl compounds [3]. 

The single-pass reaction column system was realized by Yamamoto et al. based on this super Bronsted acid loaded resin and it was proved that polar and nonpolar organic solvent swellable polystyrene-bound tetrafluorophenylbis(triflyl)methane is much superior to Nafion SAC-13 as a Bronsted acid loaded resin packed in the column (Figure 2.2, Table 2.4, Table 2.5) [13]. 

Dicationic Carbonyl Compound In general, stable carbocations do not react with non activated benzene. In 1991, Shudo et al demonstrated that multiply positively charged (dicationic and tricationic) species have sufficient electrophilicity toward benzene. In the presence of super Bronsted acids (TfOH, TfOH-SbFs), Friedel-Crafts type reaction of benzene was carried out with cinnamaldehyde, its derivatives, and nitromethane (Scheme 2.13) [34]. 

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