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

[Image: see text] 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: RuCl(2)(H(2)IMes)(PCy(3))(=CHPh); HII: RuCl(2)(H(2)IMes)(=CHAr), Ar = C(6)H(4)-2-O(i)Pr), slow salt metathesis is shown to arise from the low lability of the ancillary PCy(3) 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 RuI(2)(PCy(3))(=CHAr) HI-I(2) was prepared by treating “Grubbs I” (RuCl(2)(PCy(3))(2)(=CHPh), GI) with NaI, H(2)C=CHAr (1a), and a phosphine-scavenging Merrifield iodide (MF-I) 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 RuI(2)(H(2)IMes)(=CHAr′) (nG-I(2): Ar′ = C(6)H(3)-2-O(i)Pr-4-NO(2)) was instead accessed by sequential salt metathesis of GI with NaI, installation of H(2)IMes, and finally cross-metathesis with the nitrostyrenyl ether H(2)C=CHAr′ (1b), with MF-I as the phosphine scavenger. The bulky iodide ligands improve the selectivity for macrocyclization in ring-closing metathesis.