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 a Grubbs' catalyst, yielding the cycloalkene and a volatile alkene, in this example ethene.

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Many metathesis reactions with ruthenium catalysts are hampered by unwanted isomerization of the newly formed double bond and it is believed that ruthenium hydrides are responsible that form as a side reaction. In one study ^[1] 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 as TEMPO or phenol as an additive show the same picture but with additives such as 1,4-benzoquinone or acetic 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 occurring cyclophane floresolide (Scheme 3, R=H) ^[2].

References

• ^  Prevention of Undesirable Isomerization during Olefin Metathesis Soon Hyeok Hong, Daniel P. Sanders, Choon Woo Lee, and Robert H. Grubbs J. Am. Chem. Soc.; 2005; 127(49) pp 17160 - 17161; (Communication) DOI: 10.1021/ja052939w Abstract
• ^  Total synthesis of floresolide B and 6,7-Z-floresolide B K. C. Nicolaou and Hao Xu Chemical Communications, 2006, 600 - 602 Abstract. In this reaction step the phenol protective group is a nitrobenzoate group which can be removed in the final step by potassium carbonate. The reaction product is a mixture of E and Z isomers. Although one prochiral center is present the product is racemic. Floresolide is an atropisomer as the new ring forms (due to steric constrains in the transition 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.