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 (: RuCl(HIMes)(PCy)(=CHPh); : RuCl(HIMes)(=CHAr), Ar = CH-2-O Pr), slow salt metathesis is shown to arise from the low lability of the ancillary PCy 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(PCy)(=CHAr) was prepared by treating "Grubbs I" (RuCl(PCy)(=CHPh), ) with NaI, HC=CHAr (), and a phosphine-scavenging Merrifield iodide () resin. Subsequent installation of HIMes 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(HIMes)(=CHAr') ( : Ar' = CH-2-O Pr-4-NO) was instead accessed by sequential salt metathesis of with NaI, installation of HIMes, and finally cross-metathesis with the nitrostyrenyl ether HC=CHAr' (), with as the phosphine scavenger. The bulky iodide ligands improve the selectivity for macrocyclization in ring-closing metathesis.