COLLOIDAL RHODIUM - A NEW CATALYTIC-SYSTEM FOR THE REDUCTION OF DIBENZO-18-CROWN-6 ETHER

The stereoselective hydrogenation of dibenzo-18-crown-6 ether in the c is-syn-cis isomer of dicyclohexano-18-crown-6 ether has been investiga ted using either supported metal catalysts or colloidal metal particle s prepared under phase transfer catalysis conditions. Supported cataly sts generally lead to a mixture of isomers with a poor stereoselectivi ty in cis-syn-cis. In contrast, when phase transfer agents like tertia ry amines and ammonium salts were used in a biphasic system, a good se lectivity was achieved under mild conditions. The effect of different factors such as the nature and concentration of the phase transfer age nts upon the selectivity has been investigated. An increase of the hyd rogen pressure to 5 MPa leads to a cis-syn-cis/cis-anti-cis isomer rat io of 95/5. Strong evidence for the heterogeneous nature of the cataly sis involved is presented. A mechanism involving the phase transfer ag ent as a modifier on the surface of the colloidal rhodium particles is proposed. (C) 1994 Academic Press, Inc.