Investigation of the mechanism of the conversion of diols-1,3 in the presence of acid catalysts communication 2. The conversion of pentadiol-2,4,2-methylpentanediol-2,4 and the mechanism of the formation of cleavage products

Abstract

1. Exhaustive dehydration of pentanediols-2,4 proceeds in stages and stereospecifically with respect to the formation of trans-pentadiene-1,3 (75–100%). Trans-piperylene can be obtained both through the step of the unsaturated alcohol and through the 2,4-dimethyl-β-oxide. 2. Together with dehydration, pentanediols-2,4 undergo cleavage to a substantial degree: on a phosphate catalyst acetone and isopropanol are formed, while on A12O3 acetaldehyde is formed as well. Carbonyl compounds can be formed by isomerization of the dehydration products of the β-oxide or unsaturated alcohol, as well as dealdolization of the intermediate ketol. 3. In the presence of Ca3(PO4)2, cleavage is due to the occurrence of a reaction of hydrogen transfer from the alcohol group of the diol to the carbonyl group of the ketone, resulting in the formation of a ketol, which then undergoes dealdolization. Hydrogen transfer and dealdolization proceed at temperatures (100–150°) at which dehydration of the diol does not take place. 4. In the presence of Al2O3, the degree of cleavage of the diol is lower than on Ca3(PO4)2. Cleavage through the ketol is inhibited by the water liberated as a result of poisoning of the catalyst with respect to the reaction of hydrogen transfer. 5. A general scheme was proposed for the conversions of diols-1,3 on acid catalysts, including reactions of dehydration, isomerization, cleavage, and hydrogen transfer.