Bronstedt acid

The strong Bronstedt acid nature of some hexacoordinated phosphorus derivatives, [7",H ] (Et20)4 in particular, was recently used within the context of an industrial application [36]. The conjugated acid of tris(oxalato)phosphate anion 7 was found to effectively catalyze the ring-forming reaction of trimethyl-hydroquinone 63 with isophytol 64 to give (all rac)-a-tocopherol 65 (ethylene-carbonate/heptane 1 1,100 °C, 90%, Scheme 19). This process is particularly... 

Classical Friedel-Crafts catalysts such as BF3 etherate or Bronstedt acids, which are applied in stoichiometric amounts or even in excess, still have their value, because they are relatively inexpensive yet powerful catalysts for C-H transformation at arenes and, most importantly, they are often successful when modern catalysts fail. Gold(III) chloride, at less than 1 %, has impressive reactivity under very moderate reaction conditions and its selectivity is exceptionally good. Friedel-Crafts type reactions with this catalyst are, however, restricted to very electron-rich arenes. Further studies should concentrate on increasing the electrophilicity of gold catalysts. New catalysts have emerged, for instance based on ruthenium and rhenium, which promise broad applicability based on alternative mechanisms. Catalysts based on rare earth metals are discussed in the next chapter. 

Another important means of mediation of metal-free catalytic enantioselective Mannich-type reactions is via electrophilic activation of the preformed imines by chiral Bronstedt acids [7, 8, 46], By using this strategy Terada and coworkers performed chiral phosphoric acid-catalyzed direct asymmetric Mannich-type reactions between Boc-protected imines and acetoacetone that furnished aryl /3-amino... 

For quite some time, it was not clear whether the catalytic cracking process of these aluminosilicates involves the concerted action of both Lewis and Bronstedt acid sides, or only Lewis sites, or even the dissociation of the adsorbed hydrocarbons by surface metal ions (impurities).16,17,18 19 The need for oxide systems that allow evidence concerning the above catalytic theories was one of the driving forces for the efforts that have been done to coat a silica surface with aluminium compounds. One of the earlier studies, concerning both the reaction mechanisms of the aluminium... 

Figure 4.27 shows that by the ethoxylation of an intermediary propoxylated triol (MW of 4500 daltons) in the presence of a Lewis acid (BF3) or a Bronstedt acid (HBF4), a primary hydroxyl of around 10-15% higher than in anionic catalysis is obtained, at the same EO content. 

Sulfonylation of arenes is normally performed using sulfonyl chloride and a stoichiometric amount of a Lewis or Bronstedt acid as the catalyst. Dubac and coworkers found a practical method using MW high-temperature conditions in which only 5-10 mol% FeCls (relative to the sulfonyl chloride) sufficed for complete reactions to occur [138], A number of arenes encompassing alkylbenzenes, anisole, and halobenzenes were sulfonylated by use of several different arylsulfonyl... 

Unlike the classical Biginelli reaction, which only tolerates open-chain jS-keto esters, cyclic j5-diketones can also participate in a related MGR [66], This procedure requires Meldrum s acid 29a (X = O and Z = CMc2) or barbituric acid derivatives 29b (X = NH or NMe and Z = GO), urea, and aldehydes (Scheme 17.21). The mixture was irradiated in a domestic MW oven for 4 min under solvent-free conditions. The reaction requires a Bronstedt acid as catalyst to give, selectively, a family of novel heterobicyclic compounds 30 in good yields (70-83%). 

Due to the richly hydroxylated surface of MMT, FA was chosen for its ability to compete with hydrogen bonding between platelets, and because it is bio-based. In addition, FA has water-Uke viscosity at room temperature, which facilitates penetration of FA into the interlayer galleries of MMT. The MMT surface contains an abundance of coordinativdy unsaturated Al edge sites which acT as Lewis acids and a surface high in Bronstedt acidity. Because the polymerization of FA to PFA... 

Figure 6.16 compares the FTIR spectra for CW-PFA with pure FA and with y-Al-PFA. In the 1300-1800cm spectral region, it can be seen that the growth of the y-diketone peak at 1710cm" with increasing polymerization is characteristic both in the y-Al-PFA system catalyzed by a Lewis acid (in this case, y-Al) and the CW-PFA system catalyzed by the sulfonic (Bronstedt) acid residues originating from the whiskers. 

The results of the experiment and model are shown in Fig. 12.4. Initially, there is a total uptake of all ammonia for about 10 min. This is followed by a breakthrough of ammonia until saturation is reached. After about 80 min, the ammonia is shut off and some weakly bound ammonia is released. Finally, the temperature is increased and a large ammonia desorption follows. The desorption peak is broad and contains a shoulder. Thus, there are different types of ammonia storage sites available in the catalyst. The model (see Fig. 12.4) describes this broad feature well by a coverage dependent heat of adsorption. It should be mentioned that S1, which is used in the global kinehc model is the total amount of sites that can adsorb ammonia, thus likely covering both Bronstedt acid sites as well as different copper sites. 

The (speculative) mechanism may first involve oxidative addition of an OH function to provide a Ir -H species which would behave as a Bronstedt acid towards the second alcohol and activate it for an nucleophilic attack at the C-O carbon atom. Displacement of water and release of the ether from the coordination sphere would regenerate the catalyst and close the cycle. 

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