Dylan Stiles takes an organic chemist's tour round the periodic table
Dylan Stiles takes an organic chemist’s tour round the periodic table
It’s fair to say that organic chemists are most comfortable working within the confines of the top right quadrant of the periodic table. Carbon, nitrogen and oxygen comprise the meat of our intellectual diet. We are happy in this little box, venturing outside only when necessary, and usually with trepidation. But modern synthesis makes heavy use of metals, so we’re often forced to work with these shiny, strange elements. So let’s take a crash course guide to the rest of the periodic table, as seen through the eyes of an organic chemist.
The elements that flank both sides of the table belong to the main group. Sodium, potassium, zinc and magnesium have long been known in their elemental form, and chemists have made use of their reducing powers for centuries. The problem with these metals is that they usually only have one oxidation state, so they tend to be a little dull from a synthetic point of view. Magnesium(ii) is just a little too self-satisfied; there’s nothing you can do to reduce it easily back to magnesium(0), making a catalytic reaction. Grignard reagents are a one trick pony.
Lanthanides and actinides, the f-block elements that occupy the southern hemisphere of the periodic table, are a family of brothers and sisters all related in their affinity for the plus three oxidation state. Their salts are unique Lewis acids, often so similar to one another as to be interchangeable. But lanthanide chemistry is a niche market, and holmium is hardly a household name.
Things start getting interesting once you take a trip to the middle of the table, in the land of transition metals. The elements in groups 3-11 have d-electrons in their valence shell, which they are happy to give up. This means they can comfortably reside in a number of different oxidation states (manganese has six), and roam between them with impunity. Transition metals form coordination complexes with a variety of ligands, making their compounds soluble in organic solvents. Combine these two features and you have a recipe for catalysis.
Some transition metals have a preternatural gift at catalyzing very specific reactions. Want to dihydroxylate an olefin? Osmium tetroxide is the one and only solution. Epoxidizing an allylic alcohol? Vanadium is the metal for you. But these metals, however adept at their tasks, are limited in scope.
In the galaxy of transition metals that are used in synthesis, there’s one that stands out as a bright shining star: palladium. If you consider all the metal-catalyzed bond forming reactions that have been described over the years, palladium can do it all. Whether it’s a cross coupling reaction ? la Negishi, Suzuki, Sonogashira or Stille, or something more exotic, palladium is the workhorse of the d-block elements. Palladium complexes tend to be user friendly. They’re relatively non-toxic and unreactive towards air and moisture; perfect for the modern chemist too lazy to use a glove box. Palladium catalysts tolerate a range of unprotected functional groups like ketones and carboxylic acids, making for mild reaction conditions.
Many a fine dissertation has been written about palladium, and it would be futile to try and describe its versatility in any succinct manner. Perhaps the best way to appreciate it is to imagine a world without it. Suppose, through some cosmic stroke of bad luck, the Earth was completely devoid of palladium when it formed, and the periodic table had a gap in its teeth where element 46 should be. It’s not impossible - there are two metals below atomic number 92 that have essentially zero natural abundance (pop quiz - can you name them?). In this bleak world there would be no Wacker oxidation, no Heck reaction. It’s a terrible vision.
This is not to say that other metals are without their merit. The runners up for my list of desert island transition metals would be ruthenium and rhodium. These two have proven their worth in the realm of asymmetric hydrogenation, olefin metathesis, C-H insertion, and other feats where even palladium falls short. Critics also like to point out that as a rare earth metal, palladium is a tad expensive, fluctuating at the whim of the investment market.
One final quality of palladium that’s both an asset and a liability. The term ’noble’ means it’s quite content as a zerovalent metal. Veterans of palladium catalysis can attest to the heartbreak of having a reaction ’black out’, when the catalyst decides to precipitate out of solution. It’s an unfortunate fate for the darling of the d-10 series. Nobility comes at a price.
Dylan Stiles is a PhD student in California, US