Despite the considerable scope of olefin metathesis in organic chemistry, some substrates are still hesitant to undergo efficient metathesis reactions under classical thermal conditions. Microwave irradiation has been proposed as an alternative activation mode for olefin metathesis. It has afforded success with otherwise unproductive metathesis reactions and also abbreviated reaction times. A great review of microwave olefin metathesis coauthored by Coquerel and Rodriguez provides several intriguing examples.1
The first report of microwave assisted ring closing olefin metathesis detailed the preparation of a poly(ethylene glycol)-supported cyclic amino derivative.2 This conversion under classical conditions required 24 hours, with microwave irradiation it was complete in 10 minutes. This initial foray set the stage for further developments, including rate acceleration using ionic liquids, the formation of methoxycarbonyl-substituted dihydropyrroles, dihydrofurans, and cyclopentenes with significantly reduced catalyst loadings, cyclization of peptides, as well as ring closing metathesis of olefin substrates incorporating a free amine. A particularly stunning example of the power of microwave irradiation can be found in the total synthesis of guaianolide as reported by Reiser and coworkers, achieving a tetrasubstituted double bond.3
While the principal reason for rate enhancement in microwave assisted metathesis is a thermal effect, simply put, the higher the reaction temperature, the higher the rate, there are some additional factors to consider. Higher turnover numbers in microwave irradiation could be attributed to the catalyst decomposing faster on contact with the reaction vessel walls as opposed to the homogeneously directly heated solution. There is also some debate concerning non-thermal effects such as the electrostatic polar effect. Either way, these examples certainly suggest a powerful role for microwave assisted olefin metathesis in the future!
1 Coquerel, Y.; Rodriguez, J. Eur. J. Org. Chem. 2008, 1125-1132.
2 Varray, S.; Gauzy, C.; Lamaty, F.; Lazaro, R.; Martinez, J. J. Org Chem. 2000, 65, 6787-6790.
3 Nosse, B.; Schall, A.; Jeong, W.B.; Reiser, O. Adv. Synth Catal. 2005, 347, 1869-1874.