One challenge of mechanistic studies of reversible reactions is that it’s hard to be sure whether there are reversible steps going on that you’re not monitoring. In the case of ring closing metathesis (RCM), one such step is a degenerate process where the substrate undergoes a metathesis reaction with a M=CH2 species as shown below to give back a new substrate and metal carbene that are indistinguishable from the starting point.
This recent report from the Grubbs group used isotopically labeled RCM substrates to try to get a handle on how often a substrate spins its wheels before heading down the path to product. When the di-deuterated RCM substrate shown below underwent the degenerate process, it produced a mixture of un-deuterated, di-deuterated, and tetra-deuterated material. The researchers monitored this non-product forming event during the course of an RCM reaction by mass spectroscopy. Several ruthenium catalysts were studied, and there were marked differences in the ratio of productive catalyst turnovers (as measured by GC yield) to the non-productive turnovers depending on the catalyst used.
The ratio of productive events to non-productive events depended primarily on the size of the ligand. In general, smaller NHC ligands gave fewer non-productive turnovers. The curious results are that all the non-symmetrical ligands examined gave more non-productive turnovers. Nailing down why this is the case may not only help in the next round of catalyst design, but could also provide insight into the mechanism of catalyst-olefin coordination.