Super acid/base systems

Furthermore, the same sol-gel matrices have been used in a system where acid and base catalysis occur in the same pot without quenching either catalyst [29]. In this case, the acids were either entrapped Nafion (perfluorinated resin sulfonic super acid, a3) or entrapped molybdic acid (M03-Si02, a2), while the bases were two ORMOSILs (organically modified silica sol-gel materials), one with H2N (CH2)2NH(CH2)3 groups (bi) and the other guanidine base residues (b2) (Scheme 5.12). 

Streitwieser et al. [160] and BordweU et al. [161] used the lyate ions of organic solvents such as cyclohexylamine and dimethyl sulfoxide in the determination of the C—H acidity of weak organic carbon acids. Using super base systems such as alkali metal salts of cyclohexylamine [i.e. lithium and cesium cyclohexylamides) [160] and dimethyl sulfoxide (sodium dimsyl) [161] in an excess of these non-HBD solvents, relative acidity scales for weak carbon acids have been established. In this way, values for the ionization of over a thousand Bronsted acids in dimethyl sulfoxide have... 

Other highly acid-labile functionalized solid supports and handles incorporate one or two additional electron-donating substituents, such as methoxy groups in 4-alkoxybenzyl alcohol-base-resins. Among these systems, SASRIN resin (super acid sensitive resin, 11),[4 " ] 4-(4-hydroxymethyl-3-methoxyphenoxy)butanoic acid (HMPB, 12),t4 and 2,4,5-trichlorophenyl 5-(4-hydroxymethyl-3,5-dimethoxyphenoxy)valerate (13)f4 l handles are the most widely used. 

In the lithiation of fluoroanisoles (15) and (16) and their derivatives, butyllithium exclusively deprotonates the less acidic protons from the position ortho to the alkoxyl group. On the other hand, deprotonation takes place at the more acidic site, i.e. the ortho-position next to the fluorine substituent on reaction of the substrates with super bases, such as BuLi— t-BuOK or BuLi—N,N,N/,N//,N//-pentamethyldiethylenetriamine, in which lithium cation is stabilized by chelation in the combined base-system (Scheme 3.5) [ 14]. The lithium cation interacts preferentially with the more Lewis basic alkoxyl oxygen to locate butyllithium close to the position ortho to the alkoxyl group, enhancing kinetic deprotonation (see 17 in Scheme 3.5). 

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]. 

In fermenting hay, o-coumaric acid (8) is converted into dicoumarol [3,3 -methylenebis(4-hydroxycoumarin)] (13), a compound with marked anticoagulant properties (Beilis et al., 1967 Davies and Ashton, 1964), that occasionally has been responsible for the death of livestock (Sneader, 1985). The synthesis of warfarin (14), a powerful rodenticide, was based on these properties, and for a time, this compound was effective in controlling populations of rats. Recently, however, a strain of super rats has emerged which have developed an enzyme system that allows them to cope with this warfarin (Mann, 1987 Sneader, 1985). Regular low doses of warfarin can prevent strokes in humans (Singer, 1990). 

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]. 

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