After about four months of fucking around with it, I think I finally figured out why no one can get even half the yield that the patent claims for this method (+90%). For those of you who aren't that familiar with this method, here's a brief overview of it.
Basically, there are three different reactions going on here at the same time. The first one is the oxidation of safrole to the acetal and reduction of Pd+2 to Pd0. Now, the expensive Pd must be reoxidized in order to be catalytic, so Cu+2 is added. Two Cu+2 are both reduced to Cu+1 and Pd0 is reoxidized to Pd+2. Since a molar amount of Cu+2 is not very feasible, a secondary reoxidant is added to reoxidize the Cu+1 to Cu+2. The first substance used for this was molecular oxygen. While this method should work fine, there is another substance that is easier for the ghetto chemist to use. These are the alkyl nitrites. Alkyl nitrites are esters of nitrous acid and an aliphatic alcohol. Due to a very weak N-O bond, these molecules easily oxidize Cu+1 to Cu+2 by homolytic splitting, releasing NO gas and the aliphatic alcohol that the nitrite was made from.
As long as there is alkyl nitrite present, this cycle will continue until all of the safrole has been oxidized. If, however, the alkyl nitrite is depleted before all of the safrole is converted, the Cu+1 cannot be reoxidized. Since Cu+1 has very low solubility in the reaction mixture, it then precipitates and cannot be reoxidized, even if more alkyl nitrite is added. After all of the Cu+2 becomes depleted, there is nothing to reoxidize the Pd0. It does not necessarily precipitate, but does become inactive. To make the story short, the reaction is over.
Now, one would think that if he were to add 2.5 mol alkyl nitrite per mol safrole (2 mol is the stoichiometric amount) that the reaction would go to completion just fine. Well, your wrong. I did this about five times and never got more than 50% conversion. Supprisingly, upon hydrolyzing the reaction mixture at the end and heating to 50C for an hour, no ethyl nitrite (that is the nitrite that I was using) was evolved. I happen to know that if fresh ethyl nitrite is added to the reaction right before hydrolysis, it boils off during the heating. In other words, there was no ethyl nitrite in the post reaction mixture.
Now, I was getting really confused about this whole thing. If the ethyl nitrite wasn't in the post rxn mixture and it didn't all react, then where the hell did it go? Did it evaporate? Did it decompose? Well, last night, I finally discovered the answer to my question. Before starting the rxn, I weighed the entire flask. It weighed about 276 g. After the rxn was finished (before hydrolysis), I again weighed it. It was now about 260 g. Sixteen grams of something had left the flask in the exhaust gas. Doing some quick calculations, I figured that if all the safrole had been converted and 2 mol of NO gas was given off for every mol of safrole (I was using 20 g), then between 7 and 8 grams of NO gas would be given off. So what the fuck were the other 8 grams??? I had been running an ice cold gas condenser between the rxn flask and the exhaust hose, so no solvent or ethyl nitrite could have escaped. Or could it have? After consulting my thermodynamics book, I came to the following conclusion. Suppose that the vapor pressure of the ethyl nitrite is 200 mm Hg in the reaction mixture. That means that for every mol of NO produced, 0.36 mol of ethyl nitrite also comes off. That's assuming ideal gas/solution behavior. By no means are these numbers accurate; I'm just trying to prove a point. This goes to show that a very good amount of the original ethyl nitrite could end up being carried off with the NO gas stream. Of course some of the ethanol solvent could also be carried off, and that must be considered as well.
Putting all of this aside, the assumption that a lot of the ethyl nitrite escapes from the reaction mixture supports my observations very well. Here is an overview of them.
1) After about an hour, the copper co-catalyst precipitated and the reaction stopped, even though not half of the safrole had been converted.
2) While hydrolyzing, no ethyl nitrite escaped, even though not all of it had reacted.
3) The weight loss of the flask did not correspond to the amount of NO gas that would have been generated to produce the conversion that I obtained.
So why are none of us ghetto chemists able to pull this method off, yet over in Japan, they are getting +90% yields? Simple. They do their reaction in a closed vessel. All of the contents stay in the reaction vessel, and pressure builds up as the rxn progresses. They even hint at this, saying that after completion, the rxn contents can be distilled to obatain *NO*, Alcohol, Safrole, and Ketone.
Now that I've told you why it doesn't work, I guess I have to tell you how to make it work. Right now, I don't really know for sure, but if my theory is correct, this is how I would proceed. Start the reaction like you normally would, adding 2.5 mol EtONO per mol safrole. Since the EtONO seems to run out after about an hour, add 2.5 more mol equivalents after say, 45 minutes. You should have to do this again before the reaction is complete, after another 45 minutes (these are just guesses). DO NOT LET THE RXN RUN OUT OF NITRITE!!! Once the CuCl precipitates, it is OVER! Adding a kg of EtONO won't do a damn bit of good at that point. You will know when the rxn is done when it stops giving off NO gas.
I'll try this procedure soon and post the results. And another tip - PRE-STIR PdCl2 AND CuCl2 for AT LEAST SIX HOURS before adding anything else to the reaction. The catalyst solution should be transparent dark amber. If it is opaque or cloudy, or if you can see undissolved catalyst, stir it until it is completely dissolved. Otherwise, you can expect shit for yields.
For more information, check out my other two posts on the wacker, and the method that I am trying to follow.
Post 293619
(lab_bitch: "VERY interesting literature on the wacker", Chemistry Discourse) Literature on the wacker
Post 296918
(lab_bitch: "Wacker Mechanism", Chemistry Discourse) Wacker Mechanism
Post 270818
(potisgood: "Fuck Benzo! Ethyl Nitrite is the Shit", Methods Discourse) Procedure - This method doesn't work