Beilstein Journal of Organic Chemistry started a thematic series last year on olefin metathesis, including several review articles and research papers with Karol Grela as the guest editor.
I enjoyed reading Benjamin Davis’ review on the allylic chalcogen effect in olefin metathesis. It’s been known for a while that allylic hydroxy groups activate olefins for metathesis reactions. More recently, the activating effect of allyl sulfides and selenides was discovered. The examples included in the review showed that the effect of the allylic chalcogen was a general phenomena in metathesis chemistry and an additional tool to better control metathesis reactions. The activation by sulfides was recently used for functionalization of proteins by cross-metathesis (CM) in water.
Another very useful summary included in the series is on ene-yne CM with ruthenium carbene catalysts. En-yne metathesis is a powerful strategy to access conjugated 1,3-dienes. The examples included reactions of alkynes with ethylene, terminal olefins and cyclic olefins. The beneficial effect of excess ethylene on the reactivity or stereoselectivity of en-yne CM was reviewed as well. A limitation of the reaction is that it doesn’t work with internal olefins other than some cyclic substrates.
The number of efficient catalysts for alkyne metathesis has been growing in recent years. The history and the latest developments in this area were reviewed by Matthias Tamm. The main focus was on two recent systems: a modified Schrock system containing imidazolin-2-iminato ligands and silanoate-supported molybdenum and tungsten complexes.
The subject of olefin metathesis in nano-sized systems, including dendritic nano-catalysts and construction and functionalization of dendrimers was reviewed. Metathesis in water in the presence of a dendrimer containing triethylene glycol termini was described. The dendrimer served as a molecular micelle to solubilize the hydrophobic substrate and catalyst and made possible the use of relatively low (for aqueous metathesis) catalyst loadings (down to 0.04 mol % for ring closing metathesis).
Imagine having a catalyst which just sits in your reaction mixture until you tell it to go. This kind of catalyst can be very useful for some applications, especially in materials chemistry. Even better, you can turn the catalyst off when you want to. The Lemcoff group has developed a latent sulfur-chelated Hoveyda type catalyst, which was activated by light and deactivated by heat. Illumination with UV light generated an active isomer while subsequent heating regenerated the more thermodynamically stable inactive isomer.
If you haven’t checked the series out yet, don’t miss it! It’s interesting and free!