HOMOGENEOUS HYDROGENATION OF DIOLEFINS CATALYZED BY TRICARBONYL CHROMIUM COMPLEXES .I. STEREOSELECTIVE 1,4 ADDITION OF HYDROGEN

Arene-Cr(CO)3 complexes catalyze selectively the homogeneous hydrogenation of conjugated dienes by 1,4 addition to give predominantly cis monoenes. With 1,4-dienes, conjugation to the 1,3-dienes precedes addition. With cis-1,3-dienes, isomerization by a 1,5-hydrogen shift during hydrogenation is indicated. Reduction rates are decreased by methyl substituents on C-1 and C-4 of the 1,3-diene system. Although 1,5-hexadiene is not reduced or isomerized, 1,5-cyclooctadiene is readily isomerized to 1,4- and 1,3-dienes and reduced to cyclooctene. Kinetic studies with computer simulation techniques show that, in a mixture, 1,3-cyclohexadieneis reduced twice as fast as 1,4-cyclohexadiene with methyl benzoate-Cr(CO)3. A mixture of conjugated methyl linoleate (9,11- and 10,12-octadecadiene) is reduced 22 times faster than methyl linoleate (9,12-diene). The arene-Cr(CO)3 catalysts are highly stereoselective for trans,Jmns-conjugated dienes (relative rates: cis,cis, 1.0; cis,trans, 8.0; and trans,trans, 25). The mechanism advanced for conjugate addition involves H2Cr(CO)3 and its 1,3-diene adduct, which undergoes 1,4-hydrogen insertion across a cisoid 1,3-diene system. © 1969, American Chemical Society. All rights reserved.