This is a lab that gives students a chance to use a catalyst to perform "green" chemistry. Specifically, they will make one of the new generation of palladium cross-coupling catalysts called PEPPSI, which is a highly active, water soluble catalyst and use it in a Suzuki coupling reaction. Student groups of 2 or 3 will synthesize an NHC ligand, incorporate it into a metal compound, and then perform catalysis in an aqueous microwave reaction using a variety of different reaction conditions in order to determine the optimal catalytic system.
A student should develop the skills to synthesize imidazolium salts for use in N-heterocyclic carbene ligands.
A student should learn some of the tenents of green chemistry, and be able to identify what qualifies a reaction as "green".
A student should be able to process a large set of catalytic reaction data in order to calculate turnover number, turnover frequency, and identify the optimum set of catalytic conditions.
Vacuum line (optional, but very useful)
Microwave (a fancy microwave synthesis instrument is not necessary; we use an old acid digestion system, and it works just fine)
Microwave reaction vessels (Glass or Teflon works, but they need to be able to withstand ~150 PSI)
Glyoxal (40 wt% in water)
Hydrochloric acid (4.0 M in 1,4-dioxane)
Tetraethylammonium chloride (or Tetrabutylammonium chloride)
Sodium dodecyl sulfate
Students tend to get poor yields on the synthesis of the imidazolium salt, and sometimes do not have enough to make the metal compound. I keep a backup stash of IiPr*HCl that they can dip into if needed (usually with a penalty to their lab practical score).
The original prep for PEPPSI synthesis calls for vacuum distillation to recover the 3-chloropyridine. I've found that we can get this done much more quickly and easily if we adjust the rotavap so that it is as close to horizontal as possible and use it as a sort of poor-man's Kugelrohr (Aldrich stopped selling their model, and the Buchi instrument is quite expensive). For my class, the biggest bottleneck is waiting for the rotary evaporator, as our class meets at the same time as organic lab. Also, since we are reclaiming the 3-chloropyridine solvent/reagent in Week 3, I really only trust them to use my personal rotavap, which is the only one in the department that isn't covered in stains.
It is usually a lot easier (and cheaper) to have the group that finishes their PEPPSI synthesis first take an NMR of the precatalyst and either print or email it to the entire class.
The catalysis itself works extremely well. All students were able to get at least some product in one or both of their reactions, and some of the iterations gave quantitative yields.
Since a few of the reactions have a long stirring step that the class reaches at the same time, I use that opportunity to give a brief overview of what TON and TOF are, and how to calculate them. By this point in the course, they should have seen at least one palladium catalyzed cross-coupling mechanism in lecture. I leave it up to them to look up the difference between % conversion and % yield.