Brpnsted acid activation

The combination of PdCOCOCFjjj, (/ )-C4-TunePhos (217), and ethyl sulfonic acid (Brpnsted acid, activator) catalysed the partial hydrogenation of 2,5-disubstituted... 

The carboxyhc acid plays a dual role in the Ugi four-com-ponait reaction (see Schane 7.14) they act as Brpnsted acid activating the imine intermediate and they are also incorporated in the final Ugi product However, some research groups have showed that acids can be replaced by other activators. 

Chen, X.-H., Zhang, W.-Q., Gong, L.-Z. (2008). Asymmetric organocatalytic three-component 1,3-dipolar cycloaddition Control of stereochemistry via a chiral Brpnsted acid activated dipole. Journal of the American Chemical Society, 130, 5652-5653. 

Fig. 3 Application of strong Brpnsted acid-activated oxazaborolidine catalysts, and Corey s pretransition state assembly models...

Fig. 7 Carbon-based Brpnsted acid activated oxazaborolidine catalyst for asymmetric Diels-Alder reactions...

Ruthenium/Br0nsted Acid System The tandem isomerization/Friedel-Crafts reaction of aUylamides reported by Sorimachi and Terada was another early example of a cascade catalysis procedure using Brpnsted acid and metal salts as catalysts (Scheme 2.93) [127]. With the promotion of the ruthenium catalyst 342, the aUylamides isomerized to enamide X32, which was then tautomerized by a Brpnsted acid (343 or 344) to form imine X33 then the electron-enriched aromatic substrates 341 attacked the Brpnsted acid-activated imine intermediates to afford Friedel-Crafts products 345 in up to 91% yield. 

Vaccari (1983,1999) has given a state-of-the-art account of the preparation and catalytic properties of cationic and anionic clays. Some examples of industrial importance have also been reported. Clays exhibit many desirable features, such as low cost, wide range of preparation variables, ease of set-up and wOrk-up, high selectivity, and environmental friendliness. Cationic clays are widespread in nature, whereas anionic clays are rarely found in nature, but they can be synthesized cheaply. Cationic clays are prepared from the minerals but industrial anionic clays are generally synthetic. Smectite clays exhibit both Brpnsted and Lewis acid sites on the edges of the crystals. Hammet s acidity function values are as follows Na -montmorillonite (M), -3 to t- 1.5 NH4VM -3 to 1.5 H M -8.2 to -5.6 acid activated clay less than -8.2. Laporte also has a synthetic version of cationic clays, Laponite. The acid... 

In conclusion, dehydrated TS-1 (and presumably other titanosilicates) most likely does not have Brpnsted acid centers. The observed activity for acid-catalyzed reactions that yield undesired side products is, therefore, inferred to be created under reaction conditions in the presence of aqueous H2O2 (vide infra). 

In order to exclude simple proton catalysis, this study also examined the catalytic activity of Brpnsted acids. It was noted that a 10 mM solution of hydrochloric acid has only a small catalytic effect (second-order rate constant k2 = 7.62 x 10-2M-1 s 1 compare Table 24). Another dienophile derivative also showed changes in rate (Table 25) and in the endo/exo selectivity (Table 26)302. A dramatic acceleration and an increase in the selectivity in 1,1,1-trifluoroethanol was observed in the presence of Cu2+ (Table 25). 

The water-soluble palladium complex prepared from [Pd(MeCN)4](Bp4)2 and tetrasulfonated DPPP (34, n=3, m=0) catalyzed the copolymerization of CO and ethene in neutral aqueous solutions with much lower activity [21 g copolymer (g Pd) h ] [53] than the organosoluble analogue in methanol. Addition of strong Brpnsted acids with weakly coordinating anions substantially accelerated the reaction, and with a catalyst obtained from the same ligand and from [Pd(OTs)2(MeCN)2] but in the presence of p-toluenesulfonic acid (TsOH) 4 kg copolymer was produced per g Pd in one hour [54-56] (Scheme 7.16). Other tetrasulfonated diphosphines (34, n=2, 4 or 5, m=0) were also tried in place of the DPPP derivative, but only the sulfonated DPPB (n=4) gave a catalyst with considerably higher activity [56], Albeit with lower productivity, these Pd-complexes also catalyze the CO/ethene/propene terpolymerization. 

Figure 11.4. Hydrogen-bonding and Br0nsted acid complexation modes for the LUMO-lowering activation of substrates inherent to the field of Brpnsted acid catalysis.

A second, even more worrying problem is the side reaction, the formation of condensation products. This process is essentially irreversible in most cases. The condensation products can arise either from the aldol product or directly through a Knoevenagel-Mannich type reaction where the enamine reacts with an imininm ion [26, 81, 82]. The condensation process requires only an external Brpnsted acid, whereas the aldol process appears to require simultaneous activation of the carbonyl electrophile by an internal Brpnsted acid/hydrogen bond donor (Scheme 15). 

Bifunctional catalysts have proven to be very powerful in asymmetric organic transformations [3], It is proposed that these chiral catalysts possess both Brpnsted base and acid character allowing for activation of both electrophile and nucleophile for enantioselective carbon-carbon bond formation [89], Pioneers Jacobsen, Takemoto, Johnston, Li, Wang and Tsogoeva have illustrated the synthetic utility of the bifunctional catalysts in various organic transformations with a class of cyclohexane-diamine derived catalysts (Fig. 6). In general, these catalysts contain a Brpnsted basic tertiary nitrogen, which activates the substrate for asymmetric catalysis, in conjunction with a Brpnsted acid moiety, such as urea or pyridinium proton. 

The authors proposed that the Brpnsted base interaction on the catalyst is imperative for reactivity. Catalysts lacking a basic amine moiety, specifically mono- and bis-ureas, did not promote the asymmetric catalytic addition well, if at all. In screening a variety of amine bases and bis-ureas, it became apparent that presence of a Brpnsted base was necessary for catalytic activity (Scheme 61) [113]. The reactivity was extremely low in absence of Brpnsted base (Table 2, entry 2), but slightly improved with presence of NEtj (Table 2, entry 1). Combined, a chiral Brpnsted acid and Brpnsted base increase conversion and showed some enantiose-lectivity (Fig. 8). 

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