Allylic functions, hydrogenolysis

Dan and Henbesi (ii) demonslraied ihai ihe amount of salts remaining in platinum oxide catalysts had an important bearing on the hydrogenation-hydrogenolysis ratio of allylic functions. Hydrogenolysis is inhibited by salts remaining from the catalyst preparation or by salts such as sodium nitrite, cyanide, or hydroxide added later. 

The order for decreasing hydrogenolysis of allylic functions appears to be Pd > Pt Rh = Ru. Hydrogenolysis of allylic functions over palladium may... 

Steric hindrance around an allylic function will diminish its hydrogenolysis as access of the function to the catalyst surface is impeded. Reduction of 5-methylthebaine (32) proceeds smoothly over Pd-on-C in ethanol at 1 atm to afford 5-methyldihydrothebaine (33), whereas reduction of thebaine itself is less clean and gives dihydrothebainol, dihydrothebainone, and dihydro-thebaine (/b). 

The hydrogenation of unsaturated aldehydes IV can be a complex transformation, as depicted in Scheme 2. Although the desired reactions are normally either the formation of allylic alcohol V, or saturated aldehyde VI, by 1,2 addition of hydrogen across the functional group, 1,4-addition across the conjugated functions can provide the enol, VII. Over-hydrogenation can result either in further saturation or, for allylic alcohols, hydrogenolysis to the alkene VIII (which can, in turn, be further saturated). 

A frequent problem is selective reduction of an acetylene to the olefin in the presence of other easily reducible functions. Usually the reaction can be done without difficulty because of the relatively strong and preferential adsorption of the acetylenic function on the catalyst. Functions adjacent to the triple bond may cause special problems if the resulting allylic compound is itself susceptible to facile hydrogenolysis (18,23). The product composition is, as expected, sensitive to steric effects (82). 

The ability of Pd to avoid hydrogenolysis is attributed to the absence of allyl or homoallyl ether intermediate formation. A dihydro intennediate selectively hydrogenated to the enol ether, a function that is not susceptible to hydrogenolysis over Pd. ... 

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