[stewie griffin]

Anyone ever used magtrieve (CrO2)?
Tetrahedron Letters Volume 38, Issue 22, 2 June 1997, Pages 3857-3860
It's supposed to work just like MnO2 but you can recover the spent oxidant with a magnet and recycle it by heating in an oven. I don't need to use it but I had never heard of it till a coworker told me about it yesterday. Sounds cool. Curious if folks have ever used it and if they liked it.

[nox]

What the deuce?

[Dan]

Yeah, I've used it to oxidise benzophenone hydrazone to diphenyldiazomethane (the other method I used was HgO/KOH, guess which was nicer...).

Nice, clean, non-toxic reactions with very simple workup. You have to use a large excess (around 15 - 20 eq) because only the surface reacts (so the core is still magnetic CrO₂ after the reaction is complete), which also makes it impossible to calculate your stoichimetric ratios - but if the reaction is clean then that's no big deal, and as you say it can be recycled.

Stirring the mixture is a bit of a pain - you either need an overhead or you have to accept the fact that you'll never use that magnetic stirrer bar for any other reaction. If you go for the magnetic stirrer approach the Magtrieve doesn't move - you just need to have the solution moving vigorously and you get sufficient flow over the surface of the solid. It also gets all over magnetic retrievers and is very difficult to remove. Also, put a sheet of paper on your stirrer plate so you can lift off any stray Magtrieve particles easily - otherwise they'll probably be stuck there forever. All minor annoyances though, it's definitely worth a shot and it's especially good for larger scale work.

As I say, in my case the choice was a no-brainer - method A involved filtering off >100g of liquid mercury, method B you just wave a magnet at it...

It would be cool to have other reagents immobilised on magnetic beads!

[Honclbrif]

I've used magnetic streptavidin beads. They're pretty sweet.

[ufTiger]

I've used it to oxidize primary alcohols to aldehydes.  It gives ~100% conversion with no acid formation.  Also it's very easy to remove the chromium.  Cons:  must use large excess of CrO2, takes a few days, need mechanical stirrer.