- Ring-closing metathesis
Ring-closing metathesis or RCM is a variation on
olefin metathesis that allows the closing of previously hard to make rings (7-8 member rings in particular). RCM is simply an intramolecular olefin metathesis with aGrubbs' catalyst , yielding thecycloalkene and a volatile alkene, in this exampleethene .:
Many metathesis reactions with
ruthenium catalysts are hampered by unwantedisomerization of the newly formed double bond and it is believed that ruthenium hydrides are responsible that form as a side reaction. In one study [cite journal | title = Prevention of Undesirable Isomerization during Olefin Metathesis | author = Soon Hyeok Hong, Daniel P. Sanders, Choon Woo Lee, andRobert H. Grubbs | journal =J. Am. Chem. Soc. | year = 2005 | volume = 127 | issue = 49 | pages = 17160–17161 | format = Communication | doi = 10.1021/ja052939w] it is found that isomerization is suppressed in the RCM reaction of diallyl ether with specific additives capable of removing these hydrides ("scheme 2"). 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 asTEMPO orphenol as an additive show the same picture but with additives such as 1,4-benzoquinone oracetic acid on the other hand isomerization is absent. Both additives are able to oxidize the ruthenium hydrides which may explain their behaviour.:
Ring closing metathesis is important in the
total synthesis . One example is found in the synthesis of the naturally occurringcyclophane "floresolide" ("Scheme 3", R=H) [cite journal | title = Total synthesis of floresolide B and 6,7-Z-floresolide B | author =K. C. Nicolaou and Hao Xu | journal =Chemical Communications | year = 2006 | pages = 600–602 | doi = 10.1039/b517385jIn this reaction step the phenolprotective group is a nitrobenzoate group which can be removed in the final step bypotassium carbonate . The reaction product is a mixture of E and Z isomers. Although oneprochiral center is present the product isracemic . Floresolide is anatropisomer as the new ring forms (due to steric constrains in thetransition state ) passing through the front of the carbonyl group in (blue) and not the back. The carbonyl group then locks the ring permanently in place.] .References
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