[johnnyd]

Hi guys I'm new to this forum but hope somebody can help me out.

The problem that I'm concerned with is reduction of a vinyl bromide via catalytic hydrogenation using palladium. Is it at all reasonable to hope that this reaction might yield the alkyl bromide? ie reduction of the olefin moiety without insertion of palladium into the carbon bromine bond which leads to cleavage under these hydrogenation conditions.

I'm not looking for suggested ways to do this transformation! I've already done this reaction and I'm wondering if I put it into a thesis will I be killed?! Can I justify the attempt? Because right now it looks like a ridiculous thing to have done!

Any help appreciated

[Doc Oc]

If you've already done it then why are you asking if it's okay?  Seems that the empirical evidence should speak for itself.

[stewie griffin]

So are you saying you tried the reaction b/c you wanted the alkyl bromide, but the reaction either didn't work or you weren't able to tell what happened? If the reaction worked then of course put it in the thesis.

[g-bones]

Its important to think about what types of palladium species are susceptible to oxidative addition by vinyl halides and what type of catalyst you are using to perform your hydrogenation.  I am assuming you are using something like palladium over carbon to hydrogenate (a heterogenous catalyst).  Would this likely participate in oxidative insertion chemistry?  The types of palladium species that do this are typically homogeneous palladium(0) catalyst (they may start as a Pd(II) precatalyst too, or more recently the active catalytic cycle may only proceed via palladium(II)/palladium (IV) catalyst).  this maybe a good place to start.  DEFINITELY put it in your thesis if you did it and it worked or else you were just wasting your own time.

[orgopete]

I would judge by whether you are making a contribution to science. If you tried a reaction that was already reported to succeed or fail, and obtained that same result, then skip it. If however, you have researched different factors in the success or failure of this reaction and believe you provide an insight not present in prior art, then you should put it in. You could do that whether the reaction failed or not because it would be the success or failure of the concept that you would be divulging.

The meaning of the word concept is significant. I would be anticipating some aspect of the reaction mechanism and not a variation of the reaction conditions. You should note that the commentary of g-bones focusses on what must be occurring in the reaction. That is an example of how this question should be answered. 

[johnnyd]

Appreciate the feedback folks thanks! I realise that my post doesn't read particularly well so apologies for that.

The reaction did not work. I obtained the undesired unsubstituted alkane instead of the alkyl bromide.

The reality is that this was a trial reaction i threw on 1 day in about 10 minutes! I later circumnavigated the problem so its potentially a tiny tiny part of a chapter. It could easily be ommitted. Basically, I just don't want to include it if this reaction is something that an intelligent organic chemist would never attempt because theoretically it does not make sense.

To g-bones: I hadn't considered the question of homo/heterogeneity of the catalyst. I used Pd/C so Pd(0) but it sounds like you're suggesting that the insertion process shouldn't take place? Is there some reason that this can't occur at the palladium-solvent interface? My knowledge of palladium chemistry is poor enough so any more insight would be appreciated.

[johnnyd]

Wait a minute, is Pd/C Pd(0) It cant be right? It'd be bastard reactive. Surely it would just oxidise immediately? Now I'm confused. Is Pd on carbon actually palladium oxide? Anyone?

[Smrt guy]

Pd/C is Pd(0).  You can also have Pd(OH)2/C though.

[orgopete]

I don't know the answer, but it strikes me as a good question. After reduction, the Pd/C is Pd(0). This is the so called "bastard" reactive. Adding Pd/C to a reaction is sufficiently reactive to ignite solvents if one is not careful, but it strikes me that it is also not reactive as the reduced form after reduction. You can generally weight it out without it igniting.

Since PdO will reduce to Pd(0) + H₂O, I shouldn't be surprised if some of the Pd/C is really in an oxidized state. The more exposure your catalyst has to air, the more it may oxidize. But, after a short(?) initiation/reduction process, it shouldn't make any difference in the long run.

[johnnyd]

Ok this is good stuff. Sometimes when you open a new batch of Pd/C it will ignite reaction solvent, probably because it is mostly Pd(0). I find this usually only happens for the first one or two reactions. Presumably after this time, the Pd/C has been somewhat oxidised but it is reduced back to Pd(0) by the reductive environment of the hydrogenation in situ. Really, thanks guys. Thats cleared it up for me.

Back to the question of homo/heterogeneity of the Pd? Anyone have any ideas on that? I could be wrong but I think G-bones was suggesting that the hydrogenation reaction can occur at the phase interface while oxidative insertion of palladium into the carbon bromine bond requires the Pd to be in solution. I would like if this was true!!! Any comments?

[OC pro]

This dehalgenation is well-known. It also often occurs when one tries to hydrogenate compounds bearing a phenyl chloride or bromide moiety. You should find something in the literature about that.

[OC pro]

I have to add: actually, Pd/C is used in heterogenic coupling reactions instead of the traditional Pd(0)-catalysts. Therefore, insertion into your C-Br-bond can occur leading to the alkane. You should of course mention this in your thesis.

[g-bones]

the mechanism by which heterogeneous catalysis proceeds is not well known.  some suggest radical pathways are involved.  It is true that hydrogenolysis can compete with hydrogenation and there are specific catalyst designed to try and avoid this convergence.  check out adam's catalyst vs pearlman's catalyst.  might make a neat little discussion in your thesis.

[orgopete]

I don't know about putting this in your thesis. Are you desperate for experiments to include? From the description so far, it does not sound as though this were a well reasoned experiment designed to extend the science. If you put it in there, your committee, if diligent enough, should grill you on it as well. I would concentrate on your main topic. Keep the detours to a minimum except as dictated by your main thesis.

If one of your committee is knowledgeable, he or she will know the literature and this will count as a negative. The only way I would put it in is if I have other experiments and literature citations to back this up. That has not been shown to this forum to this point.

[johnnyd]

Thanks orgopete and everyone else! To go back to where this started, what I really wanted to know was if its possible or any way reasonable to expect to obtain the bromoalkane under the conditions described. I didn't research the reaction before trying it. My bad. Guilty! If the answer to this question is no, then obviously it doesn't go in.