Amberlyst dehydration

Cation-exchange resins are used as catalysts in the produdion of MTBE (methyl tertiary-butyl ether, 2-methoxy-2-methylpropane) and various other oxygenates and, lately, also in the dimerization of isobutene [30]. Other commercial applications of the cation-exchange resins indude dehydration of alcohols, alkylation of phenols, condensation readions, alkene hydration, purification of phenol, ester hydrolysis and other reactions [31]. The major producers of ion-exchange resins are Sybron Chemicals Incorporated [32] (Lewatit resins), Dow Chemical Company [33] (DOWEX resins), Purolite [28] (Purolite resins), and Rohm and Haas Company [27] (Amberlyst resins). 

TBA and isobutene have been compared as the etherifying agent at 60 °C. The initial molar ratio of isobutene to glycerol was 4.0 with Amberlyst A35 as the catalyst. The conversion of glycerol is lower when etherified with TBA than when etherified with isobutene. More hydrocarbons are formed with isobutene than with TBA. But, with TBA, mainly monoethers are formed and valuable triethers are formed only in small amounts. In addition, TBA dehydrates to water, which has an inhibition effect on ion-exchange resin catalysts [23],... 

Konig and co-workers also reported that Amberlyst 15 can promote the dehydration of carbohydrates to HMF using safe concentrated low melting mixtures consisting of choline chloride (ChCl) and about 50 wt% of carbohydrates. From fructose, glucose, sucrose, and inulin, HMF was produced with 40, 9, 27, and 54%, respectively within 1 h of reaction at a temperature around 100°C. Montmorillonite has also been used as a solid acid catalyst affording HMF with 49, 7, 35, and 7% yield from fructose, glucose, sucrose, and inulin, respectively [97]. 

Dehydration of alcohols to ethers, acetals, and ketals Deloxan ASP, Amberlyst Selectivity enhancement Gray et al. (1999)... 

Bregeault and co-workers have reported supporting [HP04 W0(02)2 2]2 species on resins and silica (Table 4.6).64 Amberlyst A26 was the macro-reticular resin used. The PW2 species was supported onto dehydrated porous silica. The catalysts were found to be highly selective for the epoxidation of limonene by hydrogen peroxide. 

There are limitations to the use of DCC yields are variable and A -acylureas are side products. Many other dehydrating agents " have been used, including DCC and an aminopyridine, Amberlyst-15, chlorosilanes, MeS02Cl-Et3N, and AA -carbonyldiimidazole(99). In the latter case, imidazolides (100) are intermediates that readily react with alcohols. 

These zirconium phosphate materials are being developed as replacements for ion exchange resin catalysts. The arylsulfonic acid MELS have been evaluated for butene isomerization, methanol dehydration, MTBE synthesis as well as cracking, and for the alkylation of aromatics. In the synthesis of MTBE this catalyst appears to out-perform the ion exchange resins, Amberlyst 15. 

In the examples shown in Schemes 2.19 and 2.20, neither the substrates nor the products were sensitive to elevated temperatures or acid and so dehydration was accomplished by azeotropic distillation. However, acetalisation of acid-sensitive substrates can be a major challenge. For example, a synthesis of the Crambescidin family of guanidine alkaloids was impeded by problems with the acid sensitivity of the ketone 2L1 [Scheme 2.21]. The triethylsilyl group made the desired acetalisation sluggish the firee alcohol reacted faster but it also underwent very easy -elimination. Conditions that accomplished removal of the triethylsilyl group and acetalisation without competing dehydration entailed reaction of 21.1 with the cyclic ortho ester 213 and 1,3-propanediol in the presence of Amberlyst-15 at room temperature in acetonitrile. The required hydro-... 

Hermans provided a number of insights into the reaction mechanism (Scheme 15.4). One key observation is that benzyl nitrite, formed via the reaction between HNO2 and benzyl alcohol, appears and decays over the reaction time course. Under anaerobic conditions with catalytic amberlyst-15 but without NO, benzyl nitrite fully converts to benzaldehyde and HNO, implicating its role as an intermediate (Scheme 15.4, steps I and II). In the anaerobic reaction, N2O was detected as a by-product. N2O can form through dimerization and then dehydration of HNO. Because N2O is inert and cannot be converted back into active NO species, it is important to inhibit its formation. The presence of NO2 gas was shown to reduce N2O formation, leading to the proposal that NO2 is an intermediate for the oxidation of HNO back to HNO2 (Scheme 15.4, step III). 

Fructose, one of the most common ketohexoses, readily dehydrates to afford HMF in the presence of Br0nsted acids in polar solvents. A variety of aprotic polar solvents, including DMSO, DMF, N,N-dimethylacetamide (DMA), and sulfolane, are used for these liquid-phase reaction because of the solubility of carbohydrates. A variety of solid acids, such as ion-exchange resins [156], zeolites [157, 158], metal oxides, and heteropoly acid salts, have been examined for HMF production from fructose [159,160]. Niobic acid, niobium phosphate, vanadium phosphate, sulfated zirconia, Amberlyst-15, and acid-functionalized mesoporous silicas are also found to exhibit high catalytic activity for fructose dehydration [161-167]. Moreover, soHd acid catalysts have also been examined in ionic liquids [168-175]. 

A more traditional reaction is the condensation of nitroalkanes with carbonyl compounds (Henry reaction). Nitroaldol products can be isolated but are more commonly oxidised or dehydrated. The nitroaldol products formed from methyl-8-nitrooctanoate and aldehydes, in the presence of Amberlyst A21 resin, were oxidised with... 

Dehydration and Cydization. Amberlyst 15 has been used in the cyclodehydration of pentane 1,5 diol to tetrahydropyran and in the synthesis of a library of substituted benzofurans (eq 7)... 

Dehydration and reduction of lactols into the corresponding cyiclic ethers can be carried out in high yields using Amberlyst 15 and triethylsilane (eq 9). ... 

Keywords Aldehydes, bromoniromethane, solvent-free, carbonate on silica, room temperature, Henry reaction, a-bromonitroalcohols, Amberlyst I5/AC2O, heterogeneous catalysis, dehydration, diastereoselectivity, (Z)-a-bromonitroalkenes... 

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