As the collection of functional group tolerant olefin metathesis catalysts continues to grow, a basic recipe has predominated catalyst design: ruthenium, an alkylidene, Lewis basic donor ligands (e.g. phosphines, N-heterocyclic carbenes (NHCs)), and anionic ligands (e.g. chloride). Many different combinations of catalysts have been made by tuning the ligand environment around ruthenium to modulate activity, selectivity, etc. The development of novel donor ligands holds great promise to achieve further enhancements.
The Bielawski group has most recently isolated and reported the first examples of acyclic diaminocarbene (ADC) bearing metathesis catalysts and their preliminary catalytic activity. These carbenes are easily made from readily available formamidine precursors and are special in that they possess a wider N-C-N angle, are stronger σ-donors, and offer greater conformational variation and control than related NHCs. Interestingly, mixed NHC-ADC bis-carbene catalysts showed a preference to dissociate the more donating ADC ligand to initiate catalysis.
The catalytic activity of these new catalysts was determined by representative ring closing metathesis (RCM) reactions. Elevated temperatures were required to reach full conversions of diethyl diallylmalonate to the RCM product, but these catalysts were remarkably effective at the RCM of diethyl bis(2-methylallyl)malonate to make the sterically congested tetra-substituted product (quantitative conversion in less than 1 hour with 5 mol% catalyst 1 or 2 at 100 °C).
Interestingly, catalysts 1 and 2 provided lower cross metathesis E/Z ratios than the corresponding NHC catalysts. This led the researchers to perform catalyst spiking studies which showed that isomerization, e.g., via secondary metathesis, from cis to trans olefin products is slow with these catalysts compared to their NHC relatives.