Spotlight 2, Metathesis. The Schrock and Grubbs Catalysts

A cursory glance at the current synthetic literature attests to the explosion in the application of metathesis. The ring-closure metathesis (RCM) is the most widely used concept and the Grubbs ruthenium alkylidenes 1 and 2 and the Schrock molybdenum alkylidene 3 are the exclusive catalysts. All three catalysts are reactive with a diverse range of substrates, are tolerant of many functional groups (e.g. ROH, RCOR, RCHO, RCONR₂) and are effective in RCMs from simple 5-, 6-, 7-membered to 17-membered carbo-and hetero-cyclic arrays. Although the Schrock catalyst is the most reactive, it has a narrower functional group tolerance (e.g. incompatible with ROH and RCO₂H groups). In a further contrast, the Schrock catalyst promotes RCM of di-, tri- and even tetra-substituted alkenes whereas the Grubbs catalysta are successful for only disubstituted alkenes (Ene-yne RCm is an exception). Typcial RCM conditions are high dilution (10-50 mM) and slow substrate addition, especially when competitive polymerization is observed.