Once again, I'm finally back into the blogging mood. I know its a little late, but let me just say, HAPPY 2008! All the stress, coursework, labwork & family issues have taken its toll on me, not to mention the time needed for me to recuperate from these event. Well, all I can say is that I'm finally back & I will try to be as productive as possible.
A while ago, when I started my Honours labwork around the Sept-Oct period, I had to do a Claisen rearrangement. For those of you that do not know what this is, the Claisen rearrangement is a [3,3]-sigmatropic rearrangement of an allyl vinyl ether bond, accompanied by the rearomatisation of the ring to form an allyl phenol under THERMAL conditions. Why did I emphasise thermal conditions? Well, sigmatropic rearrangment occurs under 2 distinctly different conditions; thermal and photochemical.
In the example below, 2-allylphenol is formed via an aromatic claisen rearrangement. Let me say that whilst the "normal" Claisen rearrangement had been repeated in literature countless of times, aromatic Claisen rearrangements were hard to find in literature. Many failed attempts have been reported on aromatic Claisen rearrangements.
As I was saying, very early on in my Honours degree, I was trying to get the reaction to work. Well, we did lean about it in undergrad and the reaction seemed fairly simple; heat the compound up neat at 300 degrees C for 2 hours. What could go wrong?
Well, first off, your compound turns black. Secondly, it becomes all viscous and isomers would form if your molecule has other substituents in it. In my case, I had 2 Cl substituents meta and para to the phenol group. Okay, I had a crappy reference (it was from a patent, the only one i could find on the formation of 2-allyl-3,4-dichlorophenol).
Further investigation prompt the use of an organic solvent, in this case, N,N-diethylaniline. After refluxing for about 5 days at 300 degrees C, some of the SM actually turned to the TM, with a 5:3 ratio of the TM to the unwanted isomer. Still, that wasn't good enough as I had only done the reaction on a very small scale (bout 100 mg). Purification would be a pain in the buttocks.
I then looked up some articles and found another interesting solvent used for the Claisen rearrangement carried out in the microwave, m-xylene. This however, favours the formation of the unwanted isomer, so this was scrapped.
We then had a microwave demonstration (not the household version, you can't make instant noodles in this one because its too small) & I decided to carry it out again under different conditions; 600W at 250degrees C for 15 mins. The nice yellow fluid turned black (as expected) when it was done. The NMR also suggested that the majority of the compound was the unwanted isomer.
Due to all the frustration, we decided to abandon this & try a different analogue. Of course, there are tons of ways to attach an alkyl group next to the phenol group. Then, sometime early February this year, I found an interesting article regarding the aromatic Claisen rearrangement with the use of 1,2-dichlorobenzene. I thought I would give it a go since there was nothing to lose form it & viola, I've got 2-allyl-3,4-dichlorophenol! The reflux took 3 days at 200 degrees C. The isomer was present in a 2:1 ratio [TM:isomer] but this time it was easier to purify.
I went on to make more since it was just a trial. So this time I had about 1.00 g of my compound in the solvent. Guess what happened after 3 days?
Here's Markus, one of the postdocs in my lab holding up my oil bath.... Yes, very high and continuous heat had caused the silicon oil to polymerise into a blob of slime. Hilarious if you ask me, since when I first saw it, I tought there was something wrong with my reaction. The temperature of the oil bath was at room temp, & the RBF looked as if it was sitting on top of a jelly. Still, the oil bath did retain its oiliness. I'm also supprised that the oil did not turn brown (or black in some case) even after going through such vigorous heat. I've heard from some of my postdocs that the oil they had used in the past were all brown and oily, so I guess I'm really lucky to have good quality oil for my oil baths (unlike those they use in the undergrad labs, which were either brown or black). Thats the end of my funny post for now.
KevK
