Reduction of Ketals and Thioketals

Ketals of acetone and cyclohexanone with methyl, butyl, isopropyl and cyclohexyl alcohols are hydrogenolyzed to ethers and alcohols by catalytic hydrogenation. While platinum and ruthenium are inactive and palladium only partly active, 5% rhodium on alumina proves to be the best catalyst which, in the presence of a mineral acid, converts the ketals to ethers and alcohols in yields of 70-100% [933]. 

The reduction is believed to be preceded by an acid-catalyzed reversible cleavage of the ketals to alcohols and unsaturated ethers which are subsequently hydrogenated. Mineral acid is essential. Best yields and fastest reductions are found with ketals of secondary alcohols. The hydrogenation proceeds at 2.5-4 atm at room temperature with ketals of secondary alcohols, and at 50-80° with ketals of primary alcohols. Acetone and cyclohexanone diisopropyl ketals gave 75% and 90% yields of diisopropyl and cyclohexyl isopropyl ether at room temperature after 1 and 2.5 hours, respectively [933]. 

The reagent of choice for the reduction of ketals to ethers is alone prepared in situ from lithium aluminum hydride and aluminum chloride in ether. At room temperature ethers are obtained in 61-92% yields [792, 934]. Cyclic ketals prepared from ketones and 1,2- or 1,3-diols afford on hydrogenolysis by alanes alkyl P- or y-hydroxyalkyl ethers in 83-92% yields [792]. 

Cyclic hve-membered monothioketals are desulfurized by Raney nickel mainly to their parent ketones (53-55% yields) and several by-products [935]. After stirring for 2 hours at 25° in benzene with W-2 Raney nickel, 4-tert-butylcyclohexanone ethylene monothioketal afforded 53-55% of 4-/err-bu-tylcyclohexanone, 12-14% of cw-4-/ert-butylcyclohexyl ethyl ether, 6-11% of 4-rert-butylcyclohexanone diethyl ketal, 16% of 4-rert-butylcyclohex-l-enyl ethyl ether, and 7-12% of 4-terr-butylcyclohexene [935]. 

Alone formed by the reaction of lithium aluminum hydride and aluminum chloride in ether cleaves exclusively the carbon-oxygen bond in cyclic monothioketals derived from ketones and mercaptoethanol, and on refluxing in 100% excess for 2 hours produces -hydroxyethyl sulfides (yields 66-91%) on prolonged heating with the reagent these -hydroxyethyl sulfides are further reduced to the corresponding ethyl sulfides (thioethers) (yields 28-81%) [936]. 

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