[phil81]

I want to fully reduce an N-Alkinylsulfonamide, as in TsRN-C#C-Ar --> TsRN-CH2-CH2-Ar

The reduction to the alkene is easy (H2, Pd/C). However, the reaction seems to stop there. Even after 3 days at 5 bar H2 there is no trace of the alkane.

I have tried the following catalysts: Pd/C, PtO2, Pd(OH)2/C. All in MeOH at RT.

Has anybody ever done a similar reaction? I'm going to give a sample to the high pressure lab for hydrogenation at 150 bar. Still, I don't see why this reduction should be so difficult - is the alkene somehow too deactivated? Please help.

[Kryolith]

Welcome to the forum! 

hmm.. 5 bar is perhaps too low. Can't you raise the pressure to 20 bar (works with small glass autoclaves).

Has anybody ever done a similar reaction? I'm going to give a sample to the high pressure lab for hydrogenation at 150 bar.

150 bar will probably even hydrogenate the phenyl group.

[phil81]

Thanks for your answer!

It's not phenyl, but some heteroaromatic. Anyway, I will be quite happy if *something* happens to the alkene. I can still tune the pressure to avoid side reactions.

[phil81]

Update: After 2 days at 150 bar, I get the (E)-alkene + some unreacted starting material. The (Z)-olefin must have isomerised under the reaction conditions. No alkane at all 

[Kryolith]

That's really nasty. Homogeneous catalysis might be an alternative. Raney-Ni will probably hydrogenate your aryl component. Just a few thoughts for the moment...

P.S.
Nice avatar. ChemDraw calls it: 

2-(4-((3,5-di(pent-1-ynyl)phenyl)ethynyl)-2,5-bis(3,3-dimethylbut-1-ynyl)phenyl)-1,3-dioxolane

[sjh9]

"N-Alkinylsulfonamide"


Could the sulfonamide be poisnoning the catalyst?  I realize that not all sulfur containing molecules poison catalysts, I just can't remember if sulfonamides do or not.

[sjb]

"N-Alkinylsulfonamide"


Could the sulfonamide be poisnoning the catalyst?  I realize that not all sulfur containing molecules poison catalysts, I just can't remember if sulfonamides do or not.

As far as I remember, it's only S(II) that poisons these sort of things. I've carried out a couple of alkene reductions in the presence of sulfonylureas that went uneventfully to the alkane. How hindered is the aryl part of your molecule?

[phil81]

Thank you all for your suggestions!

The aryl part is somewhat hindered, it's 2,6-substituted. But the alkene itself would still be not too hindered, wouldn't it? Catalyst poisoning might be an issue, though, because one substituent is an amino group.

I'm gonna try some homogeneous catalysts next.

[Mitch]

From the double bond, can't you just brominate. Then make the corresponding grigard and add water. Seems a bit lengthy, but might be the easiest route to go since nothing else worked.