The 2010 Nobel Prize for Chemistry was awarded yesterday to Richard Heck, Ei-ichi Negishi and Akira Suzuki, who each developed reactions involving palladium. Why is that so important?
These reactions are collectively called cross-coupling reactions, and they are immensely important because they let chemists stitch together molecules to make brand new complicated ones, such as new drugs and plastics.
Carbon-based molecules are all around us: plastics, fuels and all things biological like DNA and hormones. Scientists have long known that sticking bits of these molecules together could make extremely useful products like more effective drugs. Unfortunately it's not that simple.
Carbon atoms themselves are pretty stubborn. Once they've found something they like, they don't let go easily. In chemical terms, that means they're reluctant to bond with things, especially other carbon atoms, but once they do, the bonds are very strong.
So encouraging a specific carbon atom in one molecule to bond with another carbon atoms in another molecule is quite a challenge. Fortunately chemists have a few nifty tricks, often involving metals.
Metals are essential to life, on a chemical level; haemoglobin, for example, has iron atoms in the active parts that transport oxygen. So it's not really a surprise that metals can be used with other kinds of organic (carbon-based) molecules.
But the foundations laid by yesterday's three laureates have had enormous impact. The Heck reaction, developed in 1968, involves an alkyl halide (a carbon with a Group 17 halogen element, such as chlorine or bromine) and a carbon-carbon double bond (when two carbon atoms share two pairs of electrons, a very strong kind of bond known as an alkene.) Usually when these two parts of molecules are mixed together, nothing happens.
The Heck reaction works with a metal catalyst, palladium. The organic halide and alkene stick to the metal surface, keeping them in close proximity and allowing the reaction to take place more quickly.
In 1977 Negishi developed a very precise reaction along the same principles, but instead of an alkene used a zinc-based organic molecule. The palladium holds the halide still and separates the halogen atom from the rest of the molecule. Then in 1979 Suzuki improved upon this again by using boron. This is a brilliant bit of chemistry (Nobel prize-winning, actually) but a little complicated, so bear with me while I explain the similar but more straightforward Negishi reaction.
The point of the reaction is to stitch two halves of a molecule together; let's call them R and R'. Chemists write R to mean an organic part of a molecule; to write them out in full gets a bit tiresome. These R groups would normally not react together, but we're aiming to make an R-R' compound, so we use catalytic tricks to convince them otherwise.
The Negishi reaction involves making two different specific molecules containing your desired R groups: R-X (where X is a Group 17 halogen, such as chlorine or bromine) and R'-Zn-X', where Zn is zinc and X' is another halogen. The trick here is that Zn isn't that stable when it's got one X and one R, but with two Xs it's much more stable. This is how almost all chemical reactions work: the reactants can rearrange themselves into something more stable.
But there's still the problem of breaking the bonds to begin with, especially the stable R-X bond. This is where the palladium (Pd) catalyst comes in. The R-X sticks to the Pd and the bonds rearrange: instead of it being R-X, it becomes R-Pd-X. This Pd-X bond is a lot less strong, so when a passing R'-Zn-X' molecule comes close enough, it jumps at the opportunity to nab the other X. As the X moves over to the Zn, the R' has to go somewhere else and so gets stuck to the Pd.
Suddenly you have your two carbon chains, R and R', sitting right next to each other looking for something to bond with, and as luck would have it carbon-carbon bonds are extremely strong, which is why we're going to all this trouble in the first place. The R and R' seize the moment, lose their association with Pd and join together as a stable R-R'. Now if that isn't ingenious and Nobel-worthy I don't know what is.
Being able to stitch these R groups together has meant chemists can build individual bits of very complex molecules and join them in controlled and precise ways. The three Nobel laureates used their new techniques to copy some of Nature's prized posessions, like a poison from a rare frog from Panama, and also some designer anti-cancer and anti-viral drugs far more effective than before.
And that's not all: to sequence DNA, which is to work out exactly how a specific strand of DNA is put together, you need to mark it with a fluorescent marker. To attach these markers researchers use variants of these cross-coupling reactions.
So why was the development of these reactions worth a Nobel prize for those three bright chemists? Their combined efforts gave chemists an array of precise new tools to build the drugs and materials of the future.