Hydrogenation with tris chlororhodium

Various factors affect the susceptibility of olefins toward hydrogenation. Hydrogenation of prostaganglin PGEj, 9, with tris(triphenylphosphine)chlororhodium in a mixture of benzene and acetone gives mainly PGE, 10 (50% yield). The selectivity of the 5,6-double bond over the 13,14-double bond is not entirely because one is cis and the other trans, since 11 is also hydrogenated to 10 (as the major product) ... 

Previous work has shown that the electronic characteristics of the benzene substituent in triarylphosphine chlororhodium complexes have a marked influence on the rate of olefin hydrogenation catalyzed by these compounds. Thus, in the hydrogenation of cyclohexene using L3RhCl the rate decreased as L = tri-p-methoxyphenylphosphine > triphenylphosphine > tri-p-fluorophenylphosphine (14). In the hydrogenation of 1-hexene with catalysts prepared by treating dicyclooctene rhodium chloride with 2.2-2.5 equivalents of substituted triarylphosphines, the substituent effect on the rate was p-methoxy > p-methyl >> p-chloro (15). No mention could be found of any product stereochemistry studies using this type of catalyst. 

Valence tautomerizfttion. Tris(triphenylphosphine)chlororhodium (in the absence of hydrogen) catalyzes valence tautomerization.6 Thus, when exo-tricyclo[3.2.1.02-4]octene (1) is warmed at 90° with 1.3 moles % of the catalyst it is converted into a mixture of (2, 62%), (3, 32%), and (4, 6%). Rhodium on carbon... 

Asymmetric hydrogenation. Morrison et al. have reported on asymmetric hydrogenations catalyzed by rhodium(I) complexes of the Wilkinson type containing chiral ligands. This type of asymmetric synthesis had been carried out previously with relatively inaccessible phosphine ligands that are asymmetric at phosphorus. Phosphines that are asymmetric at carbon are more readily available and appear to be more efficient. Thus reduction of (E)- 3-methylcinnamic acid with prereduced tris(neomenthyldiphenylphosphine)chlororhodium in the presence of triethylamine leads to 3-phenylbutanoic acid, +34.5°, which contains 61% enantiomeric excess of the S-isomer. Hydrogenations of olefins exhibit a lower degree of asymmetric bias. 

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