Routes to High-Performing Ruthenium-Iodide Catalysts for Olefin Metathesis: Ligand Lability Is Key to Efficient Halide Exchange

Clean, high-yielding routes are described to ruthenium-diiodide catalysts that were recently shown to enable high productivity in olefin metathesis. For the second-generation Grubbs and Hoveyda catalysts (GII: RuCl2 (H(2)IMes)(PCy3)(=CHPh); HII: RuCl2 (H-2 IMes)(=CHAr), Ar = C6H4-2-(OPr)-Pr-i), slow salt metathesis is shown to arise from the low lability of the ancillary PCy3 or ether ligands, which retards access to the four-coordinate intermediate required for efficient halide exchange. To exploit the lability of the first-generation catalysts, the diiodide complex RuI2 (PCy3)(=CHAr) HI-I-2 was prepared by treating "Grubbs I" (RuCl2 (PCy3)(2) (=CHPh), GI) with NaI, H2C=CHAr (1a), and a phosphine-scavenging Merrifield iodide (MFI) resin. Subsequent installation of H(2)IMes or cyclic (alkyl)(amino)carbene (CAAC) ligands afforded the second-generation iodide catalysts in good to excellent yields. Given the incompatibility of the nitro group with a free carbene, the iodo-Grela catalyst RuI2 (H(2)IMes)(=CHAr') (nG-I-2 : Ar' = C6H3-2-(OPr)-Pr-i-4-NO2) was instead accessed by sequential salt metathesis of GI with NaL installation of H(2)IMes, and finally cross-metathesis with the nitrostyrenyl ether H2C=CHAr' (1b), with MF-I as the phosphine scavenger. The bulky iodide ligands improve the selectivity for macrocyclization in ring-closing metathesis.