Grubbs' ruthenium-carbenes beyond the metathesis reaction: less conventional non-metathetic utility.

ed via -hydride elimination, collapsing into the 1,3diene derivative 419. Alternatively, the formation of this product might be interpreted as a result of the direct thermal ene-type reaction of allenenes 418. To understand the latter process, compound 418b was refluxed in CH2Cl2 without the catalyst Ru-2 for a prolonged time, but no reaction took place. In addition, when heated at higher temperature (refluxing in toluene or xylene), the thermal [2 + 2]-cycloaddition between the terminal olefin and the distal double bond of the allenyl moiety furnished the corresponding cyclobutane derivative. When trienyne 421 was subjected to ring-closing metathesis reaction conditions using the second-generation Grubbs’ catalyst Ru-2, an unusual nonmetathetic activity was observed, with the cycloisomerization product being obtained as the major product. The main fraction of the reaction of trienyne 421 was found to be a mixture of the metathesis product 422 and the cycloisomerization adduct 423 (1:2 ratio) in 45% combined yield. Besides 422 and 423, a minor product 424 was isolated (11%) along with starting material 421 (6.5%). When the Hoveyda-Grubbs’ catalyst Ru-3 was used, further cycloisomerization occurred and all that was isolated was a mixture of 422 and 423 in a 1:4 ratio (45% yield) and 421 (6.5%) (Scheme 132).109 This result suggested that the alkyne was more reactive toward the ruthenium-carbene complex than the terminal alkene for trienyne 421. Therefore, initial coordination preferentially occurred at the alkyne to give the metallacyclopentene 425 (path a). Scheme 138. Vinylcyclopropane Epimerization Catalyzed by Carbene Ru-1 Scheme 139. Tandem RCM-Dehydration Catalyzed by Ru-1a 3854 Chemical Reviews, 2009, Vol. 109, No. 8 Alcaide et al.