This is the same polymerization that happens to your MDP-2-P when you distill it. Because of the highly conjugated structure of the aldol condensation product, this polymerization is EXTREMELY energetically favorable. If enough of the motherfucker join together, it forms one entirely conjugated molecule. Talk about resonance stablility! The only reason that ketone doesn't spontaneously polymerize is that the rate is too slow. The reaction is constantly occuring, though, and that is why ketone degrades it left at room temperature. At 220C, however, this reaction is fast enough to turn a flask of ketone into tarry crap within a few minutes.
Also, remember from you organic class that aldol condensations are catalyzed by acids and bases. Therefore, if the pH of the ketone is not neutral, it will polymerize faster. I have actually experienced this before. If the extracts are not washed several times with distilled water or sodium bicarb soln, I end up with a lot of undistillable crap in the flask. If I do the proper washes, only about 1% of the initial mass does not distill out.
Another thing that I have noticed, is that when washing ketone with strong enough base, or acid, a lot of crap falls out of solution. I used to think "Great! I'm cleaning my ketone!" Little did I know, I was actually destroying my ketone, and what I thought was being removed by that wash was actually being formed by the wash! The moral of the story is NEVER, NEVER, NEVER expose ketone to extreme pH, ESPECIALLY if you plan to heat it up (i.e. distillation). Simple distilled water and sodium bicarb washed are sufficient.
I have one more point to make, which happens to be the most important one. Remember how I said that acid catalyzes ketone polymerization? Well guess what happens to be fairly acidic -- a wacker. While your ketone is being made, it is being destroyed at a rate relatative to the pH of the rxn mixture. This really doesn't amount to much, but there is something else that happens. Ever wonder why wackers never go to completion? According to equilibrium, it is an irreversible reaction, so it should. The reason lies in the precipitated solid found at the end of a wacker that appears just as the rate of oxidation starts to slow down. This precipitate is your catalyst; well kind of. Palladium forms pi-allylic complexes with certain olefins that can be very stable. One of these olefins is the dimer of MDP-2-P. As ketone is created, is dimerizes via aldol condensation due to the low pH of the rxn and subsequently forms a complex with the catalyst, thus deactivating it and precipitating it. The formation of this complex actually lowers the pH further, increasing the rate of dimerization and complex formation further; thus it is an autocatalytic process.
There are three solutions to this problem.
1) The first, is to increase the rate of safrole oxidation, thus increasing yield. This works b/c the dimerization/complexing occurs at a constant rate relative to pH and ketone concentration. It is somewhat like a timebomb - you have so much time to make ketone before the catalyst is all gone. The rate of dimerization, is proportional to the square of the ketone concentration, however, so once you build up a critical product concentration, the poisoning really kicks into high gear. Ways that you can speed up the oxidation are be raisin pH, using more catalyst, and lowering free chloride concentration. Another way to speed the rxn is to run it in methanol. The lower the alcohol (and the higher the dielectric constant), the faster the rxn runs.
2) Slow down the dimerization/complexation. This can be done in two ways. One is to operate at a higher pH. This can be tricky, though, as the catalyst reoxidation needs a low pH to occur. The other way is to keep a low ketone concentration. One way to do this would be to add enough water or glycol to the reaction mixture so that one a certain amount of ketone is soluble and the rest seperates out as a second phase.
3) Use an entirely different rxn mechanism. Many researchers have used a Pd(NH3)4Cl2 catalyst at neutral pH to oxidize other olefins at about the same rate as the acidic wacker does. In addition, selectivity is almost completely given to the methyl ketone (MDP-2-P) and isomerization is greatly inhibited. This reaction looks very promising and needs further research for making MDP-2-P.
Currently, I am researching all of these topics and am trying different rxn parameters along with TLC analysis to determine optimum conditions. Hopefully, I can come up with very good procedures for both the O2 and alkyl nitrite wackers soon.