1/13/2007

Mystery vials revealed!

I'm sure you're all DYING to know what are in the Mystery Vials, so here's the reaction I was doing:



The purple stuff was tetraphenylcyclopentadienone, and the yellow stuff was hexaphenylbenzene. This doozy of a reaction is a fascinating one. The first step is a Diels-Alder reaction where tetraphenylcyclopentadienone reacts with diphenylacetylene in your typical [4+2] cycloaddition. Then, to regain aromaticity, the intermediate loses carbon monoxide (another pericyclic reaction!) to generate the central benzene ring:



This particular reaction requires high temperatures, so diphenyl ether, with a boiling point around 260°C, is the ideal solvent for this particular transformation. This reaction has been known since the 60's, but recently the king of making Ridiculous Aromatic Compounds (RACs) has been Klaus Müllen. He utilizes these hexaphenylbenzenes to make very large polycyclic aromatic hydrocarbons, which, owing to their large π-systems and aggregation behavior[1], have some very interesting electronic and liquid crystalline properties. The Müllen group generates these large systems using intramolecular Scholl reactions that link the peripheral phenyl groups together.



Although the Scholl reaction is generally pretty inefficent for intermolecular crosslinking, intramolecular Scholls are quite efficient and go in typically high yield.

An interesting thing to note: cyclopentadienone is considered semi-antiaromatic (you can make a resonance structure and generate a cyclic 4-electron system) and is too unstable at most temperatures. However, slap on four phenyl groups and you get a lovely, black crystalline solid that's quite stable and persistent. All in all, a fascinating reaction!

[1] Aggregation behavior- these things clump together like nobody's business (like most large aromatics) and are pretty much insoluble in everything, unless you hinder the aggregation or add solublizing groups.

7 comments:

Ψ*Ψ said...

Are the HBCs you're making insoluble, or have you gotten that far? If they are, I feel your pain...

milkshake said...

Ph2O is well above smoking point of silicone bath - how do you heat this reaction? Sandbath or heating mantle?

Excimer said...

i've used both. I prefer the heating mantle, though- bit easier to control the temperature and less messy.

een of andere vent said...

I always use Drysyn heating matles nowadays. they are great.

http://www.drysyn.com/

No oil mess, no metal-bath, easy temperature control and you can boil Ph2O very easy.

Felix said...

a clean organic synthesis, that's nice

no scary looking molecules ;-)

milkshake said...

On slightly unrelated subject - I was just drying 3A powdered sieves in a 1L flask tonight, on highvac using a heating mantle. I did not even crank it up on the maximum power - it was 85% power - and I though I was getting the temperature around 280-300C. At least that's what the thermometer was showing - I stuck the thermometer in from side in between the flask and mantle - but I did not realize that the temperature at the bottom must have been MUCH higher.

So imagine my suprise when I came back about 20 min later and found the flask bottom completely sucked in and concave, with the heating mantle glass textile neatly melted onto the flask surface...

The sieves amazingly survived the ordeal and the flask will be used as a piece of artwork (or to scare the onlookers)

Russ said...

I actually did that exact reaction as part of the lab component of one of my undergrad courses, many years ago! (The cycloaddition-cheletropic elimination, not the Scholl reaction)