Ethers Heterogeneous catalysis

It is in the very nature of the catalytic process that the intermediate compound formed between catalyst and reactant is of extreme lability therefore not many cases are on record where the isolation by chemical means, or identification by physical methods, of intermediate compounds has been achieved concomitant with the evidence that these compounds are true intermediaries and not products of side reactions or artifacts. The formation of ethyl sulfuric acid in ether formation, catalyzed by HjSO , and of alkyl phosphates in olefin polymerization, catalyzed by liquid phosphoric acid, are examples of established intermediate compound formation in homogeneous catalysis. With regard to heterogeneous catalysis, where catalyst and reactant are not in the same... 

Obviously, it is very desirable to substitute these modes of benzylic ether preparation by an heterogeneous catalysis process. Clays (50) and resins (51, 52) which were the first solid acid catalysts used have given low or moderate yields. The first experiments with zeolites were carried out by Rhodia (53, 54) on the etherification of vanillic alcohol (A) in a batch reactor over a HBEA zeolite with a Si/Al ratio of 12.5 ... 

We have shown that heterogeneous catalysis can be applied to reductive alkylation with success in reactions such as ether synthesis or N-alkylation of amides and anilines. Concerning the mechanism, several pathways are in competition depending on the structure of the substrate and of the alkylating agent. The important point is that both the product of addition (the hemiacetal or hemiaminal) and the product of elimination (imine, enamine or enolether)... 

We have shown that N-alkylation of amines by aleohols in the gas-phase over y-alumina is possible for a wide range of amines (aliphatic or aromatic) and alcohols (from methanol to hexanol). We have also proved that ethers can be used as alkylating agents. This last point is of practical interest because separation of the polar amine from the non-polar ether solvent is easier than from the polar alcohol. Above all, we have clearly demonstrated that chirality is compatible with gas-phase conditions and thus proposed a selective new method for mono-N-alkyla-tion and, in particular, for methylation of chiral amines. This opens new perspectives for gas-phase heterogeneous catalysis. 

Vidhani, D. V., Krafft, M. E., Alabugin, I. V. (2014). Rh(I)-Catalyzed Transformation of Propargyl Vinyl Ethers into , Z)-Dienals Stereoelectronic Role of trans Effect in a Metal-Mediated PericycUc Process and a Shift from Homogeneous to Heterogeneous Catalysis During a One-Pot Reaction. Journal of Organic Chemistry, 79, 352-364. 

The complex [Pt(CH3)2(l,5-cod)] (1,5-cod = l,S-cyclooctadiene) is widely used as a source of the Pt(CH3)2 moiety since the l,S-cyclooctadiene is readily substituted by phosphines, isocyanides, pyridines, and other ligands. Its volatility and clean thermal decomposition have led to its use in MOCVD experiments and as a source of finely dispersed platinum for heterogeneous catalysis. It can be made by treatment of suspensions of [PtCl2(l,S-cod)] or [Ptl2(l,5-cod)] in diethyl ether with MeLi, MeMgl, or LiCuMe2 in high... 

Among the industrially relevant cyclic ethers tetrahydrojuran (THF) is by far the most important. THF is produced by condensation of 1,4-butandiol that is obtained from several feedstock basis (acetylene, propene, butadiene, and butane, see Scheme 5.3.4). 1,4-Dioxane is obtained by condensation of diethylene glycol that itself is obtained by the addition of water to ethylene oxide. The reaction is catalyzed by sulfuric acid (homogeneous catalysis) or by acidic ion-exchange resins (heterogeneous catalysis) and proceeds at about 160 °C with continuous distillative removal of water. 

Clarke JKA, Darcy R, Hegarty BE, O Donoghue E, Amir Ebrahimi V, Rooney JJ. Eree-radicals in dimethyl ether on H-ZSM-5 zeohte—a Novel dimension of heterogeneous catalysis. J Chem Soc Chem Commun 1986 425. 

Acids that have weakly nucleophilic anions, e.g. HS04e from dilute aqueous H2S04, are chosen as catalysts, so that their anions will offer little competition to H20 any R0S03H formed will in any case be hydrolysed to ROH under the conditions of the reaction. Rearrangement of the carbocationic intermediate may take place, and electrophilic addition of it to as yet unprotonated alkene is also known (p. 185). The reaction is used on the large scale to convert cracked petroleum alkene fractions to alcohols by vapour phase hydration with steam over heterogeneous acid catalysts. Also under acid catalysis, ROH may be added to alkenes to yield ethers, and RCOzH to yield esters. 

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