Encouraged by Vitus' idea, the following was attempted:
Preparation of Isodillapiole pseudonitrosite.
In a 1l, three-neck roundbottom flask, vented out of doors through a hose, 0.1 moles (22.8g) of Isodillapiole was dissolved with stirring in 190mls Et2O and chilled to 3C in an ice/water bath. To this was added 0.5 moles (34.5g) NaNO2 dissolved in 100mls dH2O, forming a lower layer. Stirring was continued at the lowest setting and 0.25 mole of H2SO4 (24.5g) dissolved in 100mls ice cold dH20 was added dropwise over 4 hours, and allowed to sit an additional 3 hours with no stirring. Ice bath was maintained throughout. After shaking rxn vessel, entire contents were decanted into a 2l beaker containing 0.5l dH20. After some swirling, the etheral layer with light yellow precipitate was decanted and vacuum filtered. The filtercake was washed with water, then IPA then ether and allowed to dry for twenty minutes.
Yield: 79% (25g)
Preparation of the Nitropropene
The psuedonitrosite was immediately transferred to a 250ml flask and 100ml of chloroform was added. The flask was put in a water bath at 40C and stirred vigorously. Some of the psuedonitrosite dissolved to form a slightly green/yellow tinted solution, but most if it stayed in suspension. 20g of Acidic acivated (gamma) Alumina of Activity grade I was added to the flask (stirring was stopped for this). The alumina immediately turned orange. A reflux condenser was attached and stirring was resumed whilst holding the temperature at 40C for the next 1.5 hours. After this time, the alumina was still orange, but the majority of the psuedonitrosite was still in suspension in the slightly green/yellow tinted solution. It seemed clear that the catalyst is unable to convert the pseudonitrosite to the nitropropene in any great amount at 40C. Accordingly the temperature of the water bath was raised to 55C for the next three hours, after which time the solution was a deep orange color. This was left to stand overnight.
The following morning, the chloroform was removed on a rotovap to leave around 20g of a red oil that was very similar in appearance to the crude nitropopene prepared by digesting the pseudonitrosite with KOH/EtOH. Attempts at recrystallizing this oil from IPA, EtOAC, IPA/EtOAc all failed as the product was too impure to isolate successfully. Attempts to form seed crystals using dry ice also failed.
I have always found that with Isodillapiole, the formation of the pseudonitrosite works just as in Antibody's description on Rhodium's page, but that the subsequent conversion to the nitropropene using EtOH/KOH produces a lot of side products that lower the yields even when the temperature is kept below 30C. When the temperature is kept low, the reaction can take 24hrs, due to the extremely poor solubility of this particular pseudonitrosite in ethanol. The pseudonitrosite is also seems far less soluble that that of Asarone in chlorinated solvents.
Probably the problem with the above trial is just that acidic alumina in chloroform is just not the way to go. It requires too high a temperature - one at which the pseudonitrosite decomposes to a variety of products without catalyst. I was hoping that maybe the catalyst would encourage the formation of the nitropropene in preference to the side reactions (as it appears to help convert other substrates in the patent above), but it looks like this may have been wrong.
However, maybe it's not worth writing this off completely. Its just possible that the reason it is not working well is due to the low solubility of the pseudonitrosite in chloroform, meaning that not much substrate is getting in contact with the catalyst. Maybe it would be worth giving it a go in ethyl acetate or with a different pseudonitrosite (I have always found asarone better yielding than isodillapiole even with the traditional technique of converting the psuedonitrosite to the nitropropene; isodillapiole pseudonitrosite seems to make very dirty nitropropene on digestion).
Wish I could find a reliable, high yielding process for converting Isodillapiole pseudonitrosite to the nitropropene. Maybe H2SO4 would work? (works for styrene pseudonitrosite I believe).
Oh well, another 2 days wasted.