[dtop]

Hello all,
I've been trying to do several oxidations on various scales from 30mg to 200 mg using both Silver (II) Oxide as well as Manganese Dioxide.  I typically dissolve up my compound in some solvent (ether or dichloromethane typically) and then proceed to throw in roughly 3-10 molar equivalents of one of the oxidants mentioned.  My typical reaction workup just involves me filtering off the oxidant through a short column packed with either celite or magnesium sulfate with ether/ethyl acetate.  The issue I am having is that I am unable to recover my material as it seems to be adsorbing onto the oxidants.  Has anyone else run into this issue and found a way around it?  I've tried many other oxidatants and none have been successful so I'm pretty much stuck with these two guys.

My mass recovery goes from 200 mg to starting to 50mg product, and all I've lost in mass are a couple of hydrogen atoms [verified by NMR].  I can't even do small scale reactions [30mg] as all my product seems to vanish.

[Honclbrif]

Stronger solvent to strip the product from the oxidant?

Worst comes to worst, they're metallic oxides. You could probably dissolve them with an appropriate acid or coordinating agent, so long as your compound isn't labile in them.

[discodermolide]

Hello all,
I've been trying to do several oxidations on various scales from 30mg to 200 mg using both Silver (II) Oxide as well as Manganese Dioxide.  I typically dissolve up my compound in some solvent (ether or dichloromethane typically) and then proceed to throw in roughly 3-10 molar equivalents of one of the oxidants mentioned.  My typical reaction workup just involves me filtering off the oxidant through a short column packed with either celite or magnesium sulfate with ether/ethyl acetate.  The issue I am having is that I am unable to recover my material as it seems to be adsorbing onto the oxidants.  Has anyone else run into this issue and found a way around it?  I've tried many other oxidatants and none have been successful so I'm pretty much stuck with these two guys.

My mass recovery goes from 200 mg to starting to 50mg product, and all I've lost in mass are a couple of hydrogen atoms [verified by NMR].  I can't even do small scale reactions [30mg] as all my product seems to vanish.

Why not filter it normally and do an extractive work-up, I doubt the product is sticking to the oxidants, if this is really so, use less or try and displace them with another aldehyde

[OC pro]

With MnO2 you will get normally quantitative yield in a 200mg batch. Try refluxing in dichloromethane. 10 equiv will surely do the job. The quality of MnO2 is also important. I recommend MnO2 "very active" available from Merck.
 
From another forum:
"The "active" MnO2 used for the selective oxidation of unsaturated alcohols is normally prepared by reaction of manganese sulfate with NaOH in hot water. This results in a precipitate of MnO2, that is filtered, washed with plenty of water and dried. Minor modifications of this procedure lead to MnO2 possessing diverse activity. For example, optimum activity results from drying at 105-125 ºC during 2-3 days. Interestingly, while water, and polar solvents, are known to hinder MnO2 oxidations, over-drying of MnO2 results in less active material. In fact, a greater activity is found in MnO2 possessing 4-8% of water. See, Quat.Rev. 61 (1959)."

[dtop]

With MnO2 you will get normally quantitative yield in a 200mg batch. Try refluxing in dichloromethane. 10 equiv will surely do the job. The quality of MnO2 is also important. I recommend MnO2 "very active" available from Merck.
 
From another forum:
"The "active" MnO2 used for the selective oxidation of unsaturated alcohols is normally prepared by reaction of manganese sulfate with NaOH in hot water. This results in a precipitate of MnO2, that is filtered, washed with plenty of water and dried. Minor modifications of this procedure lead to MnO2 possessing diverse activity. For example, optimum activity results from drying at 105-125 ºC during 2-3 days. Interestingly, while water, and polar solvents, are known to hinder MnO2 oxidations, over-drying of MnO2 results in less active material. In fact, a greater activity is found in MnO2 possessing 4-8% of water. See, Quat.Rev. 61 (1959)."

Stronger solvent to strip the product from the oxidant?

Worst comes to worst, they're metallic oxides. You could probably dissolve them with an appropriate acid or coordinating agent, so long as your compound isn't labile in them.

I tried more polar solvents [Methanol, DCM, ether, ethyl acetate, Acetone have all been tried], I still haven't been able to recover close to my starting mass.  I've been using freshly preprared Ag2O and "Active MnO2" from sigma Aldrich which I've played with at various dryness, and my best results have been through the use of storing it in a 140 C oven.  I will try refluxing in DCM though, that sounds like a good idea!

Why not filter it normally and do an extractive work-up, I doubt the product is sticking to the oxidants, if this is really so, use less or try and displace them with another aldehyde

Using less seems to not convert all my material to product, it seems I have to use somewhere between 3-10 equiv to convert all my material to product [tested visually by TLC].  I'll have to try this idea of displacing with another aldehyde!  I'll report back when I can!

Thank you guys/gals very much for taking the time to respond!  I appreciate it!

[nox]

Only used MnO₂ before, it's the 85% pure "activated" stuff from Sigma. If I remember I use 5-7 equivalents in chloroform, let it stir overnight at room temperature, then just filter off all the manganese gunk with a medium frit, washing with some DCM. Yields are around 65% after a column with propargylic systems.

[dtop]

Looks like I had a typo in my first post!  It's Silver(I) Oxide not Silver (II). 

Only used MnO₂ before, it's the 85% pure "activated" stuff from Sigma. If I remember I use 5-7 equivalents in chloroform, let it stir overnight at room temperature, then just filter off all the manganese gunk with a medium frit, washing with some DCM. Yields are around 65% after a column with propargylic systems.

Yeah, that's interesting I tried something similar, although my solvent system was DCM.  Were you able to recover a crude mass similar to your starting mass?

[nox]

I don't remember (and TBH I don't usually take a crude mass for those messy reactions), but I imagine I couldn't have lost too much mass if I got a 65% yield after a column.

[OC pro]

You should filter the reaction through a glas frit with Celite on top. Then do proper washing of the solid cake to recover all of your product. I have always >95% reisolation with this procedure.