The Stereochemistry of Hydrogenation of α,β-Unsaturated Ketones

This chapter discusses the stereochemistry of hydrogenation of αβ-unsaturated ketone. When the double bond is polarized by being in conjugation with an electron-withdrawing group as in an αβ-unsaturated ketone, other factors enter into product stereochemistry considerations. In the hydrogenation of these compounds, the stereochemistry of the product can be influenced by the nature of the solvent used in the reaction, hydrogen availability, and the nature of the catalyst. The product stereochemistry obtained on hydrogenation of αβ-unsaturated ketones is dependent on a number of reaction variables. The chapter discusses the effect of solvent on this hydrogenation of ketones. In neutral media the reaction occurs essentially by way of the classic Horiuti–Polanyi process that has been modified to incorporate both 1,2- and l,4-addition of hydrogen to the conjugated system. The relative amounts of 1,2- and 1,4-addition that take place are dependent on the nature of the solvent. In polar aprotic solvents, 1,4-addition predominates, whereas in nonpolar aprotic solvents the hydrogenation occurs primarily by a 1,2-addition sequence.