Ring closing metathesis is not the only type of olefin metathesis to profit from microwave irradiation. Several examples of cross metathesis demonstrate the power of microwave irradiation to transform yields and rates as well. The review coauthored by Coquerel and Rodriguez provides some examples.1
In an alkyne/ethylene cross metathesis for the preparation of enantiomeric 2-(1-acetylamino-1-arylmethyl)-1,3-butadienes under classical conditions with 1 atm of ethylene, no diene product could be detected. However, using microwave irradiation, the desired dienes could be isolated with retention of their optical activity.2
Sometimes the challenge for cross metathesis reactions is the generation of a homodimer rather than the heterodimer. This was the case for dinucleosides derived from thymidine. Using classical conditions, the homodimer prevailed as the major product. Microwave irradiation allowed better selectivity for the heterodimer without affecting the stereochemistry.3
Other cited examples demonstrating beneficial effects of microwave irradiation include the generation of peptidomimetics via the cross metathesis of amino acids with terminal olefin groups and the cross metathesis of ethyl acrylate with a variety of functionalized olefins. A recent example involving the cross metathesis of homoallyl bromide and allylsilane showcased the rate acceleration effect – being quite slow under classical conditions (90°C, 0.8 mol% catalyst, 8 hr, 6% yield) but much improved with microwave irradiation (60°C, 0.15 mol%, 69% yield) – along with an impressive turnover number (TON) of 460 (which is usually only 10 to 50). Given these examples, it is reasonable to expect microwave irradiation to make waves in the future as it improves the rate and ambit of olefin metathesis reactions.
1 Coquerel, Y.; Rodriguez, J. Eur. J. Org. Chem. 2008, 1125?1132.
2 Castagnolo, D.; Renzulli, M.L.; Galletti, E.; Corelli, F.; Botta, M. Tetrahedron: Asymmetry 2005, 16, 2893?2896.
3 Colombeau, L.; Zerrouki, R.; Krausz, P.; Champavier, Y. Lett. Org. Chem. 2005, 2, 613?615.
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