Metal carbenes (species with a formal carbon-metal double bond) are intermediates in all olefin metathesis reactions. This is not the only reactivity that metal carbenes exhibit, however. With some metals (including ruthenium), an alkynyl carbene can undergo a bond reorganization to give a new alkynyl carbene (termed a metallotropic shift).
If a molecule is set up properly, the new alkynyl carbene can perform downstream chemistry. Daesung Lee’s group has demonstrated this nicely in a series of papers to make some interesting structures. In a 2005 report, they reported a polycyclization cascade initiated by a ring closing metathesis reaction.1 The dienetetrayne shown below was treated with the second generation Grubbs catalyst. This first performed an enyne metathesis reaction to give an intermediate ruthenium carbene, which isn’t well set up for any further metathesis chemistry, but can instead undergo a metallotropic shift to generate a new ruthenium carbene. Now this carbene is five atoms away from another alkyne, so the cycle can continue. In the example shown below, where three rings are formed, the whole process takes place in 86% yield. Lee and coworker report the same can process to form up to six rings in one step and reasonable yields (>30%).
Recently Lee and coworkers have reported isolating and fully characterizing an alkynyl-tethered ruthenium carbene via a similar process (enyne metathesis followed by a series of metallotropic shifts).2 Furthermore, they showed that the ruthenium diyne complex is a competent catalyst, albeit slow to initiate. Along with the thrill of making interesting looking compounds, the isolation of this ruthenium complex and its demonstrated competence as a catalytic species serve to support the mechanism that Lee proposes for this type of reaction.
1 Kim, M.; Lee, D. J. Am. Chem. Soc., 2005, 127, 18024-18025.
2 Yun, S. Y.; Kim, M.; Lee, D.; Wink, D. J. J. Am. Chem. Soc., 2009, 131, 24-25