Recent studies in olefin metathesis have focused on the synthesis of catalysts that can overcome the inherent thermodynamic preference of an olefin, providing synthetically useful quantities of a kinetically favored olefin isomer. Advancements in stereoretentive olefin metathesis using tungsten, ruthenium, and molybdenum catalysts are presented. The orientation of the substituents on the metallacyclobutane intermediate is critical for determining stereoselectivity; thus, controlling the geometry of this intermediate is necessary for achieving both stereoselective and stereoretentive metathesis.
In one study  it was found that isomerization is suppressed in the RCM reaction of diallyl ether with specific additives capable of removing these hydrides.
Without an additive, the reaction product is 2,3-dihydrofuran and not the expected 2,5-dihydrofuran together with the formation of ethylene gas. Radical scavengers, such as TEMPO or phenol , do not suppress isomerization ; however, additives such as 1,4-benzoquinone or acetic acid successfully prevent unwanted isomerization.
Both additives are able to oxidize the ruthenium hydrides which may explain their behavior. Another common problem associated with RCM is the risk of catalyst degradation due to the high dilution required for some cyclizations.
High dilution is also a limiting factor in industrial applications due to the large amount of waste generated from large-scale reactions at a low concentration. The following examples are only representative of the broad utility of RCM, as there are numerous possibilities.
For additional examples see the many review articles. One example is its use in the formation of the membered ring in the synthesis of the naturally occurring cyclophane floresolide.
Floresolide B was isolated from an ascidian of the genus Apidium and showed cytotoxicity against KB tumor cells. In , K. Although one prochiral center is present the product is racemic. Floresolide is an atropisomer as the new ring forms due to steric constraints in the transition state passing through the front of the carbonyl group in and not the back.
The carbonyl group then locks the ring permanently in place. The hydrogen bond stabilized the macrocycle precursor placing both dienes in close proximity, primed for metathesis. Only the S,S,S diastereomer was reactive illustrating the configuration needed for ring-closing to be possible. All for free. Most of the presentations and slideshows on PowerShow.
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A kinetically selective process is required to overcome this thermodynamic preference for olefin geometry.
These catalysts relied on successfully controlling the orientation of the metallacyclobutane substituents as well. Stereoselectivity is dependent on the catalyst, ring strain, and starting diene.
Floresolide is an atropisomer as the new ring forms due to steric constraints in the transition state passing through the front of the carbonyl group in and not the back. In one study, the addition of aluminum tris 2,6-diphenylphenoxide ATPH was added to form a 7-membered lactone.
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Another common problem associated with RCM is the risk of catalyst degradation due to the high dilution required for some cyclizations.
Martin and others reported the step synthesis of manzamine A with two ring-closing metathesis steps to access the polycyclic alkaloid. At the time, no previous membered ring had been formed through RCM, and previous syntheses were often lengthy, involving a macrolactonization to form the decanolide.
The ruthenium catalysts are not sensitive to air and moisture, unlike the molybdenum catalysts. There is truly something for everyone! Manzamine is a good target due to its potential as an antitumor compound.
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