High-Yield Synthesis of a Long-Sought, Labile Ru-NHC Complex and Its Application to the Concise Synthesis of Second-Generation Olefin Metathesis Catalysts

The controlled reaction of [RuCl2(p-cymene)]2 with H2IMes generates the previously challenging precatalyst and Ru synthon RuCl2(p-cymene)­(H2IMes) (Ru-2) in 96% isolated yield. Critical to success is inhibiting premature p-cymene displacement. This is achieved by carrying out the synthesis at ambient temperatures, protected from light, and at sufficient dilutions (25 mM in THF) to enable stoichiometric control and inhibit bimolecular decomposition. The ease with which p-cymene loss can be deliberately induced, however, is key to the utility of Ru-2 in both catalysis and catalyst synthesis. The transformation of Ru-2 into two second-generation olefin metathesis catalysts is described. RuCl2(H2IMes)­(CH­(o-C6H4-OiPr)) (HII) and RuCl2(H2IMes)­(PPh3)­(CHPh) GII′ (a desirable, faster-initiating analogue of GII) are accessible in ca. 80% yield over two steps from commercially available [RuCl2(p-cymene)]2. Synthesis from RuCl2(PPh3)3, in comparison, requires three or four steps for HII or GII′, respectively, and proceeds in lower yields.