Homogeneous catalysis acid-catalyzed exchange

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. 

As another example of novel catalysis employing montmorillonite, the clay was found to show excellent catalytic activity for the addition reaction of trimethylsilyl ketene acetal to a, -acetylenic esters (ynoates), which contrasted strikingly with the reactions induced by a homogeneous acid catalyst, trimethylsilyl triflate (TMSOTf), as well as the addition reactions of lithium enolates with ynoates [Eq. (17)] (89). Table XXIII summarizes the results of the reactions of the silicon and lithium enolates of methyl propionate (21) with ynoates (22a-c). Except for the reaction of 22c, ferric ion-exchanged montmorillonite (Fe-Mont), which is more acidic than Al-Mont, catalyzed exclusive 1,2-additions of trimethylsilyl ketene acetal to 22a and 22b to give 23 in... 

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