“In tandem or alone: a remarkably selective transfer hydrogenation of alkenes catalyzed by ruthenium olefin metathesis catalysts” G. K. Zieli?ski, C. Samoj?owicz, T. Wdowik and K. Grela, Org. Biomol. Chem., 2015, 13, 2684 DOI: 10.1039/C4OB02480J
Chemists are always searching for ways to rapidly construct complex molecules from relatively simple starting materials and avoid tedious purification steps. Something I‘d rather avoid if I can, high pressure hydrogenations. Transition-metal-catalyzed tandem bond formations are powerful methods that present an entry point to the aforementioned problems. At Materia we often do olefin metathesis followed by other functional group transformations. So the putting the two together in one pot sounds pretty attractive.
Recently, the Grela group reported a tandem reaction involving olefin metathesis followed by transfer hydrogenation by using commercially available ruthenium olefin metathesis catalysts. Although not the first report of such a transformation, previously reported metathesis-hydrogenation sequences are typically conducted under high pressures of hydrogen, which is required to convert the ruthenium alkylidne complexes into a hydrogenation catalyst. In this report the transfer hydrogenation system is composed of a ruthenium metathesis catalyst and formic acid. The process is operationally simple and can be performed in a single pot.
The example shown in the scheme above starts with Ru-catalyzed double-RCM (ring closing metathesis) of tetraene 1 forming selectively product 2 bearing one five- and one six-membered unsaturated ring. Then, only one of the two seemingly very similar C-C double bonds present in 2 was reduced by Grubbs second generation catalyst and formic acid selectively due to the increased ring strain of the cyclopentene, yielding monosaturated product 3 in good yield. This process can easily be conducted in a one-pot fashion, converting 1 directly into 3 in 90% yield. It should be mentioned that the reaction conditions are capable of reducing 6-membered rings, albeit over much longer reaction times. A variety of olefins bearing an assortment of functional groups can also be reduced by this method. The highly selective nature of this system provides a useful addition to the still expanding collection of hydrogenation methods in conjunction with Materia’s favorite reaction, olefin metathesis!