Cis-addition of hydrogen

The stereoisomers of olefin saturation are often those derived by cis addition of hydrogen to the least hindered side of the molecule (99). But there are many exceptions and complications (97), among which is the difficulty of determining which side of the molecule is the least hindered. Double-bond isomerization frequently occurs, and the hydrogenation product is the resultant of a number of competing reactions. Experimentally, stereochemistry has been found to vary, sometimes to a marked degree, with olefin purity, reaction parameters, solvent, and catalyst 30,100). Generalizing, it is expedient, when unwanted products arise as a result of prior isomerization, to avoid those catalysts and conditions that are known to favor isomerization. 

These results, which correspond to stereospecific cis addition of hydrogen to the double bond, stand in direct contradiction to the report st> by Elving and coworkers that dimethylmaleic acid 112, R = H) affords dl-113 (R = H) exclusively and that dimethylfumaric acid affords only meso-113 (R = H). The latter results correspond of course to stereospecific tram addition of hydrogen across the double bond. Camilli, like Homer and R6der, however, observed c/s-addition of hydrogen, albeit in a slightly different system, 18> ... 

Evidence derived from a study of the stereochemistry of hydrogenation of 1,2-cyclononadiene and 1,2-cyclodecadiene led Moore (108) to conclude that allyl complexes like those postulated above must be intermediates. He noted that, of the four ways in which either allene could be adsorbed on a surface, two, a and b, would yield via cis addition of hydrogen the cis-cycloalkene and two, c and d, the tram isomer. Examination of... 

The reaction can also be used to produce m-2-substituted halocyclopropanes, l-chloro-2-phenylcydopropenes undergoing efficient hydrogenation using palladium on calcium carbonate by cis-addition of hydrogen with no evidence for competing hydrogenolysis of the C—Cl bond 27). 

Most catalysts also bring about the cis addition of hydrogen to alkynes. For example, [RhCl(PPh3)3] catalyzes the initial formation of (Z)-but-2-enoic acid from but-2-ynoic acid. 

Conformational analysis has been employed [46] with moderate success to explain the stereochemistry of the catalytic hydrogenation of steroidal olefins. In the hydrogenation of an unsaturated A/B trans steroid the steroid molecule will generally be adsorbed on the catalyst by its less hindered a-face, leading to cis addition of hydrogen from the a-direction. This pattern is followed for example by and A -olefins,... 

When substituted cyclopropenes are treated with HCo(CO)4 or HMn(CO)5, aldehydes, aside from hydrogenation products, can be obtained in a stoichiometric hydroformylation process. When cyclopropene 3 is dissolved in hexane and treated with a twofold excess of HCo(CO)4 under a carbon monoxide atmosphere, the only aldehyde product observed is methyl l,2-cfs-2,3-/ranj-2-formyl-2,3-diphenylcyclopropane-l-carboxylate (4). It can be separated from the hydrogenation products and isolated from the reaction mixture by crystallization or chromatography. The major amount of hydrogenation products (88 /o) arises from cis addition of hydrogen to the double bond. A similar reaction has been observed when HMnfCO), rather than HCo(CO)4, is used. Thus, in both hydroformylation and hydrogenation, products arising from cis addition are predominantly or even exclusively formed (see Table 1). ... 

In heterogeneous catalytic hydrogenations suprafacial (cis) addition of hydrogen would be expected, as the transfer of hydrogen atoms from the catalyst surface to the reactant is usually assumed. However, in some Pt catalyzed reactions antarafacial (trans) addition of hydrogen is also observed. The ratio of diastereomeric products formed is determined by the chemisorption equilibrium of the surface intermediates and by the relative rates of hydrogen entrance to the different unsaturated carbon sites. Both effects are influenced by steric factors. 

To establish beyond reasonable question that 9-octalin does indeed desorb, migrate, and readsorb on its other side, and finally (d) that no other mechanism is probable. Three kinds of experiments were needed. First, the production of octalins was studied as a fimction of conversion. Second, the yield of fm -decalin... 

There remains the question of whether the mechanism of formation of trans isomers is truly one in which the olefins desorb, turn over, and become readsorbed on the opposite face before saturation by cis addition of hydrogen. 

A mechanism of exclusively cis addition of hydrogen fits all of these observations, and could be pictured for the platinum catalyst as follows ... 

Dihydro intermediates were omitted from these road maps on the basis that they do not have any stereochemical significance over that possessed by the tetralins. Hexahydro intermediates are ignored as being too short lived to be desorbed in significant amounts. The second step is pictured as cis addition of hydrogen to the tetralins to form octalins with essentially randomly distributed double bonds. Only those octalins in which the double bond involves one or two of the three bridgehead positions are shown. The others would be stereo-ohemically significant only if double-bond isomerization is fast, as it is... 

Make a model of 2-butyne and the product of cis addition of hydrogen. 

With alkynes hydrocarbonylative coupling leads to unsaturated ketones with exclusive cis addition of hydrogen and the acyl group. Thus, rhodium carbonyl catalyzed carbonylation of acetylenes and ethene with carbon monoxide and hydrogen stereospecifically leads to a./l-un-saturated ethyl ketones by cross hydrocarbonylation4- 5, e.g., diphenylacetylene with ethene in the presence of Rh4(CO)l3 gives ( )-1.2-diphenyl-l-penten-3-one in 91 % yield5. 

Predicting the stereochemistry of reduction is, as shown above, difficult in many cases and identification of the less hindered face can also be difficult. As Rylander pointed out, adsorption may occur to minimize steric interaction but this may not be the step that controls the stereochemistry of the final product. The example of 345 seems to reinforce this statement. Stereochemistry can vary with the degree of isomerization, the catalyst, reaction conditions, and the purity of the alkene.368,369 Norbornene systems usually give exclusively exo-(cis)-addition of hydrogen unless there is significant steric encumbrance at the Cl position.370... 

The reaction proceeds hy cis addition of hydrogen and horon, probably via a four-center transition state such as 4. 

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