[jammydodger]

Hi all,

How can you add H2 across the alkene part of an a,b-unsaturated ketone, to make simply the ketone? I take it that you cannot use your standard hydrogenation with H2/ catalyst; would that be because it can add to the carbonyl as well, to produce the alcohol? I assume that conjugate addition has to occur instead, but I'm stuck trying to think of an appropriate soft nucleophile which would attack at the b-carbon to add H. Or perhaps the mechanism is longer and more roundabout than that.

Any help would be much appreciated! It would be great just to have a starting point.

[KritikalMass]

The carbonyl will not be reduced if you use H₂/PdC in ethanol (is that what you are referring to as "standard H₂/catalyst"?) Thus, the final result will be your saturated ketone.

Edit:

is this on paper or in the lab. nice, straightforward, consistent conditions for 1,4-reduction

So you really can't reduce an enone to a ketone with H₂/PdC in ethanol? I was under the impression that you could 

[azmanam]

is this on paper or in the lab.  if it's in the lab, try the two conditions at the end of this thread.  nice, straightforward, consistent conditions for 1,4-reduction

http://www.chemicalforums.com/index.php?topic=31329.0

[jammydodger]

Thanks for clearing that up. The reason why I thought we couldn't add H2 like that was because this was part of a questionsheet on which nearly all the other questions were in some way related to conjugate addition... You know when you think something cannot possibly be that simple!

Thanks also for the interesting link.

[movies]

So you really can't reduce an enone to a ketone with H₂/PdC in ethanol? I was under the impression that you could 

You can definitely reduce enones to ketones with H₂ & Pd/C!  Ketone reduction is also possible with these conditions, but only under much more forcing conditions.

[azmanam]

agreed.  I was just pointing out some other conditions if op actually needed to physically do the rxn and was having trouble for whatever reason.