From Prof. Robert H. Grubbs' group at CalTech
Recently, Prof. Grubbs has reported new RCM catalysts specifically efficient towards RCM to form tetrasubstituted olefins. This has appeared in Org Lett ASAP this month.
Traditional Ruthenium-based RCM catalysts (2-4) have better functional group tolerance than Schrock's Mo-based catalyst (1). However, Schrock's catalyst performs better in the formation of tetrasubstitued olefin. They have used diethyl dimethallylmalonate (5) as their test substrate and have found that catalysts 2 and 3 both failed miserably. 
However, in their recent work on Ru-catalyzed enantioselective metathesis on triene 7, they discovered that they could isolate alkene 9 when catalyst 4 was used, whereas only alkene 8 was obtained when catalyst 3 was utilized.
As catalyst 4 has only one ortho-group on each of the N-bound aryl rings, this led them to consider reducing the bulkiness at these positions, thus leading to the advent of catalysts 10-12. Their suspicion was found to be correct when they found that catalysts 10-12 worked much better with substrate 5 as shown by the graph below of the reaction at 30C.
Other substrates were also studied as shown in the table below.
From the table, although all catalysts (10-12) were found to work better than catalyst 3, catalyst 12 in particular was found to worked better than other catalysts in all cases where the reactions did proceed. But according to the graph above, the "S" profile of the curve corresponding to 12 gave some concern of the catalyst induction period where it takes some time for the catalyst to be reactive for RCM. This initiation period is more obvious in the less hindered substrates (see SI of the paper for other graphs) and is believed to be responsible for the need of higher reaction temperature and longer reaction time.
It is well-known in RCM catalyst design that in order to overcome the induction period, changes need to be made to the ligands trans to the NHC group. But this has been unsuccessful. However, the unsaturated analogs of the NHC ligand are able to accommodate this change better. This led to the consideration of catalysts 20-22 where ligands trans to the unsaturated NHC could be modified. They found that in general the unsaturated NHC ligand led to slower reaction as compared to the saturated analog of comparable structure (20 is slower than 12).
Graph below demonstrates the reaction profile of catalysts 20-22 compared to 12 when used with substrate 5 at 30C. It was found that catalysts 20-22 no longer had initiation period. Catalyst 21 show greater reactivity but soon died due to instability (after 43% conversion). But when the catalyst lacking phosphine ligand, ie 22, is used, the reaction reached 90% conversion in only 1.5 h.
This study has led to the discovery of the more promising catalyst 12 and later catalyst 22 was found to be extremely promising for the preparation of tetrasubstitued olefins.
Recently, Prof. Grubbs has reported new RCM catalysts specifically efficient towards RCM to form tetrasubstituted olefins. This has appeared in Org Lett ASAP this month.
Traditional Ruthenium-based RCM catalysts (2-4) have better functional group tolerance than Schrock's Mo-based catalyst (1). However, Schrock's catalyst performs better in the formation of tetrasubstitued olefin. They have used diethyl dimethallylmalonate (5) as their test substrate and have found that catalysts 2 and 3 both failed miserably. However, in their recent work on Ru-catalyzed enantioselective metathesis on triene 7, they discovered that they could isolate alkene 9 when catalyst 4 was used, whereas only alkene 8 was obtained when catalyst 3 was utilized.As catalyst 4 has only one ortho-group on each of the N-bound aryl rings, this led them to consider reducing the bulkiness at these positions, thus leading to the advent of catalysts 10-12. Their suspicion was found to be correct when they found that catalysts 10-12 worked much better with substrate 5 as shown by the graph below of the reaction at 30C.Other substrates were also studied as shown in the table below.From the table, although all catalysts (10-12) were found to work better than catalyst 3, catalyst 12 in particular was found to worked better than other catalysts in all cases where the reactions did proceed. But according to the graph above, the "S" profile of the curve corresponding to 12 gave some concern of the catalyst induction period where it takes some time for the catalyst to be reactive for RCM. This initiation period is more obvious in the less hindered substrates (see SI of the paper for other graphs) and is believed to be responsible for the need of higher reaction temperature and longer reaction time.It is well-known in RCM catalyst design that in order to overcome the induction period, changes need to be made to the ligands trans to the NHC group. But this has been unsuccessful. However, the unsaturated analogs of the NHC ligand are able to accommodate this change better. This led to the consideration of catalysts 20-22 where ligands trans to the unsaturated NHC could be modified. They found that in general the unsaturated NHC ligand led to slower reaction as compared to the saturated analog of comparable structure (20 is slower than 12).
Graph below demonstrates the reaction profile of catalysts 20-22 compared to 12 when used with substrate 5 at 30C. It was found that catalysts 20-22 no longer had initiation period. Catalyst 21 show greater reactivity but soon died due to instability (after 43% conversion). But when the catalyst lacking phosphine ligand, ie 22, is used, the reaction reached 90% conversion in only 1.5 h.This study has led to the discovery of the more promising catalyst 12 and later catalyst 22 was found to be extremely promising for the preparation of tetrasubstitued olefins. 
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