dormouse (Member)
04-19-00 11:40
No 122750
      Some Alternative Wacker-type Methods -Strike  Bookmark   

Author Topic:   Some Alternative Wacker-type Methods
Administrator posted 09-13-98 01:50 PM          
Here's some more methods for making a familiar terminal olefin (safrole), into a methyl ketone (MD-P2P). These are Wacker-type reactions but use different reaction mechanisms and different catalysts. More expensive catalysts. But the results are rather interesting.

This first one below is not that interesting. The catalyst sounds like a nightmare and the reaction products are strange. But read it if you like.

J. Am. Chem. Soc., 104, p6782-6784 (1982) -
"Novel Catalytic Oxidations of Terminal Olefins by Cobalt(II)-Schiff Base Complexes: CoSalMDPT [a.k.a. Cobalt(II) bis(salicylidene-gamma-iminopropyl)methylamine] catalyzes the oxidation of terminal olefins to the methyl ketone and 2-alcohol........

Example reaction: In a Parr pressure bottle submerged in an oil bath is placed 45ml of either methanol or ethanol, 39mmol olefin, and 0.73mmol CoSalMDPT. The reaction is heated at 70oC for three hours at an O2 pressure of between 30-90psi. The psi used doesn't seem to matter."

That was quick! But the yield are strange and low. Out of 39mmol of olefin one gets a max of 6.7mmol ketone. But one also gets around 4mmol P2Pol as well. Consistantly. H2O2 can be substituted. Strike has no idea where to get the catalyst. Strike is sure it can be made. Probably from Cobalt Chloride. Don't ask.

This second method is a lot better. It makes quantitative ketone out of terminal olefin (100% conversion). Uses a rhodium catalyst. The ‘cocatylyst' that acts as the reoxidizer is either Cu(NO3)2(HMPA)4, Cu(ClO4)2(HMPA)4, or Fe(NO3)3(HMPA)4. There are a couple of others which you can read about in the paper. Expplanations are given about the preparation of the cocatalysts.00

JACS, 100, p5437 (1978)
"Reaction conditions: temp, 40oC; P(O2), 860mmHg [simply oxygen at or near atmospheric pressure, a ballon will be perfect]; reaction time 4h; olefin, 2.5M; solvent ethanol [or methanol, or isopropanol]; catalyst RhCl3.3H2O, 0.02M; Cu(ClO4)2(HMPA)4, 0.04M; water (coming from reactants), 0.25M"

The yields are extremely high. And you will notice that the catalyst's ratios respective to the amount of olefin are extremely low. With the reaction being mild and short, this method could be a backup for the wacker oxidation in the future (or now if one wishes)

drone 342
Member posted 09-22-98 11:24 AM          
While Rhodium is even more expensive than palladium, perhaps such a small amount would be effective. Thought the yields were low, that cobalt reagent looks promising. The ligands give me the impression that they were a little concerned over chirality or something, and th a simple cobalt reagent could be used in its place, nu? I'll give these articles a look-through, and see what else I can come up with.

-drone #342

drone 342
Member posted 10-14-98 02:53 PM          
The use of other transition metals for oxidations is a VERY interesting prospect -- especially when many of the alternative reagents would only cost a fraction the price.

As everybody knows, Pd forms a partial bond across the pi-orbital of the olefin (in this case, safrole). The Pd thus weaken the pi-bond, and sets it up for an attack by the electronetgative oxidizing agent. This all happens so nicely because the d pi-p pi overlap between the Pd and carbon-carbon double bond is such a nice shape.

Now, like I said in another discussion, there's nothing sacred whatsoever about Pd and its orbitals; other transition metals have this same effect on olefins (though few do it as nicely and efficiently as Pd; some will catalyze the reaction, but yield an epoxide as the final product.)

The article that's recently got me hot-and-bothered about catalysts is Synlett(8):876, 1997 -- a lovely little article from a bunch of Indian scientists. I love Indian chemistry; its so simple, yet so directly practical to everyday needs, like MDMA synthesis! Anyways, they use CoCl2 as a catalyst. At less than 100th the price of its palladium counterpart, this method has a lot going for it.

I have a list of about 70-80 different journal articles using various non-palladium transition metal compounds as catalysts fore olefin oxidation, if anybody is curious. All of it is recent research, so most of the stuff is overly complicated for our chemical interests. Still, the theories behind it are quite applicable, and much of it can be simplified (once you strip out the asymetric synthetic details and protecting group esoterata, its cake.)

Just say the word,

-drone #342

Member posted 10-14-98 06:21 PM          
Please, Rev. Drone, give us more details.

drone 342
Member posted 10-14-98 09:47 PM          

That's all the invitation I need! Let's get down to business (I'm not going to get too carried away here; this is "serious chemists only", and I get chastized for being too off-the-wall as it is.) There really are more variations that I can describe, but what looks best to me are the cobalt-based versions of this reaction. Co2+, coupled with either an imine or an aldehyde, or perhaps even a ketone, will act quite effectively at epoxidizing olefins (epoxides are readily isomerized in this case to the corresponding acetone.) In addition to the Synlett reference, as well as Strike's there are many, MANY more. E-mail me, and I'll forward you a huge-ass list.

-drone #342