RACEMIZATION OF (R)-(-)-10-METHYL-DELTA(1(9))-OCTALIN - STEREOCHEMICAL REQUIREMENTS FOR DOUBLE-BOND MIGRATION (ISOMERIZATION) ON METAL-CATALYSTS

(R)-(-)-10-methyl-Delta(1(9))-octalin was synthesized in a mixture wit h cis- and trans-(R)-(-)-10-methyl-Delta(1(2))-octalin in a five step sequence. After removal of the cis- and trans-(R)-(-)-10-methyl-Delta( 1(2))-octalins by hydrogenation over 5% Pt/C, 5% Rh/ C, or 5% Pd/C, th e remaining (R)-(-)-10-methyl-Delta(1(9))-octalin was monitored for ra cemization (double bond migration) during continued hydrogenation. No racemization occurs over these catalysts. However, slow racemization d oes occur in increasing amounts during hydrogenations over a series of 1% Pd/SiO2 catalysts with dispersions increasing from 36%D to 84%D. T he ratio of cis- to trans-9-methyldecalin changes from 1.8 over 5% Rh/ C, 1.6 over 5% Pt/C, 1.1 over 5% Pd/C, to 1.0 over the 1% Pd/SiO2 cata lysts. Since double bond migration does not occur over the 5% catalyst s, it cannot be used to account for differences in the ratio of cis- t o trans- products. Congestion around the double bond inhibits addition , but double bond migration seems to require the allylic hydrogens to be nearly perpendicular to the planes of the double bond and the surfa ce. (C) 1998 Elsevier Science B.V. All rights reserved.